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产品型号MAX845ESA的概述

芯片MAX845ESA的概述 MAX845ESA是一款高效的电源管理芯片，专为低电压应用设计。该芯片支持宽范围的输入电压，使其能够与多种电源及应用相适配。MAX845ESA的核心功能是提供稳定的输出电压，确保连接的负载能够获得所需的电源支持。这一特性使得它在消费电子、通信设备、汽车电子等领域得到了广泛的应用。 MAX845ESA的设计围绕着低功耗和高效率展开，使得其成为许多便携式和电池驱动设备的理想选择。此外，该芯片具有过压保护、过流保护等多重保护功能，确保在各种工作环境下都能可靠运行。所有这些特性共同构成了MAX845ESA的优越性能和可靠性，使其在市场上保持了一定的竞争力。芯片MAX845ESA的详细参数在了解MAX845ESA的整体特性后，为了深入理解其性能，必须关注其详细参数。MAX845ESA的输入电压范围为2.7V至12V，能够适应多种电源配置。输出电压可以设定为固定值...

产品型号MAX845ESA的Datasheet PDF文件预览

19-0372; Rev 4; 10/97

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

_______________Ge n e ra l De s c rip t io n

____________________________Fe a t u re s

The MAX845 provides an isolated power supply small

enough to fit in thin PCMCIA cards and space-sensitive

applications. It drives a low-profile center-tapped trans-

former primary from a 5V or 3.3V DC power supply. The

secondary can be wound to provide any isolated posi-

tive or negative voltage at powers up to 750mW.

♦ Transformer Driver for Ultra-Thin 5V-µs Transformers

♦ Isolated DC-to-DC Power Supply for PCMCIA

Applications

♦ 450kHz Minimum Switching Frequency

♦ Ultra-Low Input Supply Current Ripple

♦ Single +5V or +3.3V Supply

The MAX845 consists of an oscillator followed by a tog-

gle flip-flop. The flip-flop generates two 50% duty-cycle

square waves, which are complementary at half the

oscillator frequency (450kHz, min). These two signals

drive the ground-referenced N-channel power switch-

es. Internal circuitry ensures break-before-make action

between the two switches.

♦ 5µW Low-Power Shutdown Mode

♦ 8-Pin SO and µMAX Packages

♦ Low Output Ripple Permits Miniature Output

Capacitors

A low-power shutdown disables both the switches and

the oscillator, reducing power consumption. An evalua-

tion kit (MAX845EVKIT-MM) is available to evaluate low-

profile 5V 40mA and 5V 100mA applications.

________________________Ap p lic a t io n s

_______________Ord e rin g In fo rm a t io n

PCMCIA Modem Cards

PART

TEMP. RANGE

0°C to +70°C

PIN-PACKAGE

Dice*

Isolated Data Acquisition

Isolated Interface Power Supply

Noise-Immunity Communications Interface

Bridging Ground Differences

Medical Equipment

MAX845C/D

MAX845ESA

MAX845EUA

-40°C to +85°C

8 SO

8 µMAX

*Contact factory for dice specifications.

Process Control

Low-Power LAN Networks

___________________P in Co n fig u ra t io n

__________Typ ic a l Op e ra t in g Circ u it

TOP VIEW

ON / OFF

OUTPUT

CR1

5V @ 150mA

GND1

GND2

MAX845

N.C.

FREQUENCY

SELECT

GND1

GND2

SO/µMAX

CR2

________________________________________________________________ Maxim Integrated Products

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.

For small orders, phone 408-737-7600 ext. 3468.

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

ABSOLUTE MAXIMUM RATINGS

Supply Voltage (V ) ...............................................-0.3V to +7V

Operating Temperature Range ...........................-40°C to +85°C

Storage Temperature Range .............................-65°C to +160°C

Junction Temperature ......................................................+150°C

Lead Temperature (soldering, 10sec) .............................+300°C

Control Input Voltage (SD, FS) ...................-0.3V to (V + 0.3V)

Peak Output Switch Current (D1, D2)......................................1A

Output Switch Voltage (D1, D2).............................................12V

Average Output Switch Current (D1, D2) .........................200mA

Continuous Power Dissipation (T = +70°C)

SO (derate 5.88mW/°C above +70°C).........................471mW

µMAX (derate 4.10mW/°C above +70°C) ....................330mW

MAX845

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

ELECTRICAL CHARACTERISTICS

(V = 5V ±10%, T = T

to T , unless otherwise noted. Typical values are at T = +25°C.)

MAX A

MIN

PARAMETER

CONDITIONS

MIN

TYP

1.5

MAX

4.0

UNITS

Switch On-Resistance

Switch Frequency

D1, D2; 100mA

FS = V = 4.5V

Ω

450

550

675

860

500

575

1.1

900

FS = V = 5.5V

1100

kHz

FS = 0V, V = 4.5V

FS = 0V, V = 5.5V

Operating Supply Current (Note 1)

Shutdown Supply Current (Note 2)

No load, SD = 0V, FS = V

5.0

0.8

µA

SD = V

0.4

High

Low

2.4

Shutdown Input Threshold

Shutdown Input Leakage Current

FS Input Threshold

High

Low

0.8

FS = 0V

FS Input Current

µA

FS = V

Minimum Start-Up Voltage

2.5

2.2

Note 1: Operating supply current is the current used by the MAX845 only. Load current is not included.

Note 2: Shutdown supply current includes output switch leakage currents.

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s

(Typical Operating Circuit, V = 5V, C1 = 0.1µF, C2 = C3 = 0.33µF, T1 = Halo TGM-010P3, CR1 = CR2 = MBR0520, FS = V

= +25°C, unless otherwise noted.)

SHUTDOWN SUPPLY CURRENT

vs. TEMPERATURE

OUTPUT RESISTANCE vs. TEMPERATURE

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

FIGURE 11b

FIGURE 11c

SD = V

= 4.5V

= 5.5V

-40 -20

20 40

80 100

-40 -20

60 80

100

-40 -20

80 100

TEMPERATURE (°C)

D1, D2 FREQUENCY vs. TEMPERATURE

SUPPLY CURRENT vs. TEMPERATURE

D1, D2 FREQUENCY vs. TEMPERATURE

1000

950

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

850

800

750

= 5.0V

= 6.0V

900

= 6.0V

FS HIGH

850

800

750

700

650

600

= 5.5V

700

650

600

550

500

= 5.0V

= 4.5V

FS LOW

-40 -20

80 100

-40 -20

80 100

-40

-20

0 20 40

TEMPERATURE (°C)

80 100

TEMPERATURE (°C)

EFFICIENCY vs. LOAD CURRENT

OUTPUT VOLTAGE vs. LOAD CURRENT

100

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

TRANSFORMERS

USED IN FIGURE 11b

TRANSFORMERS

USED IN FIGURE 11c

FIGURE 11b

FIGURE 11c

TGM-030P3

TGM-010P3

TGM-020P3

TGM-010P3

TGM-020P3

40 60 80

LOAD CURRENT (mA)

100 120 140 160

20 40 60 80 100 120 140 160

LOAD CURRENT (mA)

20 40 60 80 100 120 140 160

LOAD CURRENT (mA)

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )

(Typical Operating Circuit, V = 5V, C1 = 0.1µF, C2 = C3 = 0.33µF, T1 = Halo TGM-010P3, CR1 = CR2 = MBR0520, FS = V

= +25°C, unless otherwise noted.)

SWITCHING WAVEFORM

(BREAK-BEFORE-MAKE)

SWITCHING WAVEFORMS

(TWO CYCLES)

D1OFF

D2OFF

MAX845

500mV/div

5V/div

CIRCUIT

OF FIG. 1

D2ON

D1ON

200ns/div

400ns/div

TIME FROM SHUTDOWN TO POWER-UP

2V/div

OUTPUT

5µs/div

_____________________P in De s c rip t io n

NAME

FUNCTION

0.1µF

50Ω

Open Drain of N-Channel Transformer Drive 1

Ground. Connect both GND1 and GND2 to

ground.

GND1

ON / OFF

Frequency Select (internal pull-up). If FS =

50Ω

or open, switch frequency = 725kHz; if

MAX845

FS = 0V, switch frequency = 535kHz.

Shutdown. Ground for normal operation,

connect to V for shutdown.

FREQUENCY

SELECT

GND1

GND2

N.C.

No Connect. Not internally connected.

+5V Supply Voltage

Ground. Connect both GND1 and GND2 to

ground.

GND2

Open Drain of N-Channel Transformer Drive 2

Figure 1. Test Circuit

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

OUTPUT

5V @ 150mA

CR1

F / F

MAX845

OSC

FREQUENCY

SELECT

CR2

400kHz/

700kHz

ON / OFF

GND2

GND1

ISO

GND

Figure 2. Detailed Block Diagram

Th e o ry o f Op e ra t io n

_______________De t a ile d De s c rip t io n

Figure 2 shows the MAX845 driving both a TGM-010P3

transformer with a center-tapped primary, and a sec-

ondary with a voltage-doubler rectifier topology. All of the

transformers driven by the MAX845 must have a center

The MAX845 is a tra ns forme r d rive r s p e c ific a lly

designed to provide isolated power for PCMCIA and

other height- and/or space-sensitive applications. It

drives a center-tapped transformer primary from a 5V

or 3.3V DC p owe r s up p ly. The s e c ond a ry c a n b e

wound to provide any isolated DC voltage needed at

power levels up to 750mW.

tap with V applied. Whenever one of the MAX845 out-

puts (D1 or D2) goes low, the other goes to approximate-

ly double the supply voltage. A voltage is induced in the

secondary and the rectifier diodes steer the currents into

the a p propria te outp ut c a pa c itor. On a lte rna te ha lf

cycles, each capacitor is charged. The output voltage is

the sum of the voltages from each output capacitor. This

topology yields the simplest and smallest transformer

because the least number of secondary turns is required

for a given voltage.

The 450kHz minimum switching frequency allows the

use of very thin transformers, making the MAX845 ideal

for PCMCIA and other space-limited applications. The

MAX845 is designed to drive a single transformer less

than 0.09 inches (2.3mm) in height, including package.

Further reduction down to 0.050 inches (1.27mm) can

be achieved using a transformer without a package.

__________Ap p lic a t io n s In fo rm a t io n

The MAX845 consists of an RC oscillator driving a pair

of N-channel power switches. The oscillator runs at

double the output frequency, driving a toggle flip-flop

to e nsure 50% duty c yc le to e a c h of the switc he s.

Internal circuitry ensures break-before-make action

between the two switches.

With the MAX845 transformer driver, designers have

the a d va nta g e s of p us h/p ull c onve rte r top olog y in

space-sensitive applications. The push/pull DC-DC

converter topology allows isolated multiple outputs,

step-up/step-down or inverted outputs, easier filtering

on the input and the output, and lower overall noise.

A low-current shutdown mode disables all internal cir-

cuitry, including the oscillator and both power switches.

Drive the shutdown pin (SD) high to shut down the part;

drive SD low for normal operation. The SD pin has no

internal default condition and must not be allowed to

float.

Is o la t e d P o w e r fo r P CMCIA Ap p lic a t io n s

Medical instrumentation, modems, and LAN-interface

cards often require isolated power supplies. One of the

best switching-regulator topologies for this application

is the push/pull forward-converting DC-DC power sup-

ply shown in Figures 3 and 4. Because the transformer

works in the forward mode (rather than the flyback

mode), its core does not store energy and, therefore,

can be small. Input and output capacitors can be small

because of the high-frequency and continuous-current

waveforms.

Most MAX845 applications will operate at high frequen-

cies. The frequency-select pin (FS) is pulled high or left

open (FS is internally pulled up to V ) to operate at a

minimum of 450kHz. Pulling FS low selects the low-fre-

quency state.

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

0.01µF

1N4148

3.3V

SUPPLY

0.1µF

5V @ 150mA

ISO OUTPUT

MBR0520

1CT:1.3CT

0.33µF

ON / OFF

MAX845

ISO

GND

GND1 GND2

FREQUENCY

SELECT

MBR0520

GND1

GND2

SEE FIGURE 11

FOR RECTIFIER

CONFIGURATIONS

Figure 3. 5V to Isolated 5V Application Circuit

Figure 4. 3.3V Input to Isolated Output Application Circuit

The MAX845 is a versatile transformer driver, capable

of driving a center-tapped transformer primary from a

5V or 3.3V DC power supply (Figures 3 and 4). The

secondary can be wound to provide any isolated volt-

age needed at power levels up to 750mW with a 5V

supply or up to 500mW with a 3.3V supply. Figure 3

shows a typical 5V to isolated 5V application circuit that

delivers up to 150mA of isolated 5V power.

will be higher at 3.3V, so transformer winding resistance

will be more critical and efficiencies will be lower. The

MAX845 output current must still be limited to 200mA

(see Absolute Maximum Ratings), so the available out-

put power will be less than with a 5V power source.

Lo w -No is e P o w e r S u p p ly

The MAX845 topology is inherently low noise, in that

either one or the other of the two power devices is on at

any given time. By alternating between two identical

states with one side on and the other off, the input cur-

rent is nearly constant and secondary output power is

available at all times. There is an intentional break-

before-make action to prevent any possibility of both

power switches conducting at the same time. During

this 100ns non-overlap interval, the input current goes

to zero. This adds a small high-frequency component

to the input current waveform. This ripple current can

easily be absorbed by a small input bypass capacitor

3 .3 V S u p p ly

Any of the application circuits shown may be converted

to 3.3V operation by changing the turns ratio of the trans-

former and operating the MAX845 from a boost supply,

as shown in Figure 4. In normal operation, whenever one

of the MAX845 outputs goes low, the other goes to

approximately double the supply voltage. Since the cir-

cuit is symmetrical, the two outputs can be combined

with diodes, lightly filtered, then used to power the

MAX845, and possibly other light loads as well.

(0.33µF) from V

noise bias supply using the MAX845 transformer driver.

to ground. Figure 5 shows a low-

The diodes on the primary side may be any fast-switch-

ing small-signal diodes, such as the 1N914, 1N4148, or

CMPD2838. The value of the primary filter capacitor is

not critical and can be very small, since it only needs to

supply current to the MAX845 during the break-before-

make interval.

Whe n us ing the two-d iod e p us h-p ull (Fig ure 11a )

rectifier or the four-diode bridge (Figure 11b), the out-

put voltage tends to be more constant than in most

alternative topologies. As described above, the circuit

alternates between two identical states that both pro-

vide power to the load. The only part of the cycle that

produces output ripple is the 100ns non-overlap inter-

val, which can easily be filtered by a small ceramic

output capacitor (0.33µF).

The transformer could be any of the same ones used for

5V operation, but for optimum performance it should

have fewer primary turns, as the ET product required is

now only 3.3V-µs. For a given power level, the currents

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

0.33µF

MBR0520L*

78L05

MAX845

N.C.

OUT

GND

0.33µF

HALO

TGM-030P3

-5V

100mA

GND1 GND2 N.C.

*1N914 POSSIBLE FOR LOWER CURRENTS

Figure 5. Low-Noise Supply

remains in the shutdown mode until the voltage is high

enough to allow proper operation.

Is o la t e d Da t a Co n ve rs io n

Almost any serial-interface device is a candidate for

operation across an isolation barrier; Figure 6 illustrates

one example. The MAX176 analog-to-digital converter

(ADC) operates from +5V and -12V supplies, provided

by the multiple-tapped secondary and linear regulators.

This circuit easily supplies several hundred milliwatts of

additional isolated power for signal conditioning, multi-

plexing, or sensors. A +12V supply can be generated

by adding two more diodes from the ends of the sec-

ondary, and a -5V supply can be generated by con-

Is o la t e d 4 m A t o 2 0 m A An a lo g In t e rfa c e

The 4mA to 20mA current loop is widely used in the

process-control industry for transducer and actuator

control signals. These signals are commonly referred to

a distant ground that may be at a considerably higher

voltage with respect to the local ground. The circuit in

Figure 8 generates an isolated 4mA to 20mA current

from a 5V supply.

necting additional diodes to the ⁄ and ⁄ tap points on

Is o la t e d RS -4 8 5 Da t a In t e rfa c e

The MAX845 power-supply transformer driver also pro-

vides isolated power for RS-485 data-interface applica-

tions. The application circuit of Figure 9 combines the

MAX845 with a low-dropout linear regulator, a trans-

former, several high-speed optocouplers, and a Maxim

RS-485 interface device.

the secondary. The MAX845 supplies sufficient power

for almost any Maxim ADC.

Te le p h o n e -S u b s c rib e r-Lin e P o w e r S u p p ly

The standard telephone system is placed in the “off

hook” state by placing a load on the line to signal the

central office that service is requested. Normally, most of

this power is wasted in a load resistor, but some systems

can benefit from utilizing this free power. Figure 7 shows

one way to transform the wasted telephone power to an

isolated, regulated 5V at currents up to 50mA.

Is o la t e d RS -2 3 2 Da t a In t e rfa c e

The MAX845 is ideal for isolated RS-232 data-interface

applications requiring more than four transceivers. Its

750mW output power capability enables it to drive 10

transceivers simultaneously. Figure 10 shows the typi-

cal application circuit for a complete 120kbps isolated

RS-232 data interface. This figure also shows how the

Sharp PC417 optocouplers can be replaced by the

lowe r-c os t Qua lity Te c hnolog ie s 4N25 d e vic e s to

achieve data transfer rates up to 19.2kbps.

Be c a us e the te le p hone line is a hig h-imp e d a nc e

source, there can be a start-up problem with any DC-

to-DC converter; when the line voltage is low during

start-up, the frequency can be too low for the trans-

former, causing it to saturate. This excess saturation

current can keep the voltage from climbing to normal

operating levels. Thus the purpose of Q1, Q2, and the

a s s oc ia te d re s is tors is to e ns ure tha t the MAX845

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

ISOLATION

BARRIER

1CT : 1.5CT : 3CT

5V INPUT

MAX845

78L05

4 x 1N5817

10µF

ISO

MAX845

ON/OFF

79L12

ISO

GND1

GND2

10µF

-12V

74HC04

START

6N136

INPUT CLOCK

200Ω

SER

74HC595 QF

10µF

SCK

RCK

SCLR

MAX176

D11(MSB)

D10

0.1µF

6N136

200Ω

ANALOG

INPUT

AIN

CONVST

CLOCK

DATA

INPUT

VREF

GND

470Ω

0.1µF

74HC04

0.1µF

10µF

0.1µF 10µF

8.2k

′

SER

SIGNAL

GROUND

74HC595 QF

SCK

RCK

SCLR

INPUT

D0 (LSB)

INPUT

0.1µF

Figure 6. Typical Isolated Data-Conversion Application

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

TELEPHONE SUBSCRIBER LINE

ISOLATION

BARRIER

6.8V

1N5817

1:2:1

22k

IC2

TL431

5V @ 50mA

ISO OUTPUT

2N3906

0.1µF

100k

IC1

100k

ISO

GND

MAX845

1N5817

N.C.

100k

680k

GND1

GND2

2N3904

Figure 7. 5V from Telephone-Subscriber Line

ISOLATION

BARRIER

1CT:5CT

24V UNREGULATED

10µF

1N5817

78L05

MAX845

GND

OUT

GND1

GND2

OUT

1N5817

0k to 1k

4mA to 20mA

ISO

49.9k

2N3904

MAX480

IL300

0.1V to 0.5V

2N3904

MAX480

10k

49.9k

24.9Ω

Figure 8. Typical 4mA/20mA Application Circuit

_______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

ISOLATION

BARRIER

MAX845

0.1µF

1N5817

ICT:1.3CT

ISO 5V

OUT

0.1µF

2.2µF

MAX883

MAX845

ON / OFF

SET

GND SHDN

N.C.

GND1 GND2

3.3k

PC410 / 417

*74HC04

390Ω

3.3k

PC357T

390Ω

MAX481

MAX483

MAX485

MAX487

485

I/O

PC410 / 417

3.3k

*74HC04

390Ω

GND

*74HC04 OR EQUIVALENT

Figure 9. Typical RS-485 Application Circuit

10 ______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

ISOLATION

BARRIER

1CT:1.3CT

0.1µF

MBR0520

ISO 5V

OUT

N.C.

0.1µF

2.2µF

MAX883

MAX845

ON / OFF

SET GND SHDN

MBR0520

N.C.

GND1 GND2

5 x 3.3k

10 x PC417

27, 28

13, 14

*74HC04

GND

390Ω

OUT

74HC04

OUT

74HC04

OUT

74HC04

OUT

74HC04

OUT

5 x 3.3k

74HC04

MAX225

390Ω

OUT

74HC04

OUT

74HC04

OUT

74HC04

OUT

74HC04

OUT

*74HC04 OR EQUIVALENT

4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps)

1N5711

4N25

1N5711

4N25

3.3k

74HCO4

3.3k

390Ω

ISO

OUT

ISO

390Ω

ISO

*74HC04

ISO

GND

Figure 10. Typical RS-232 Application Circuit

______________________________________________________________________________________ 11

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

______________Co m p o n e n t S e le c t io n

An ungapped toroid core must never be allowed to sat-

Tra n s fo rm e r

The MAX845 drives any transformer that has a center-

tapped primary and a saturation rating of at least 5V-µs

(ET product) per side. The oscillator frequency varies

urate. An empirical way to measure a toroid’s ET prod-

uct is to wind 20 turns on the bare core and observe

the current waveform on an oscilloscope while driving

the winding with a function generator. Generate a 50%

duty-cycle square wave at a test frequency of 500kHz,

with no DC offset. Gradually increase the driving volt-

age until the waveform suddenly begins to draw more

current. At this point, the core is saturating, so reduce

the driving voltage until the core just barely stops satu-

rating. The ET product indicated is simply the maxi-

mum voltage that can be applied without saturation,

multiplied by 1µs (the time of half of the period of the

input signal). Because the ET product varies linearly

with the numb e r of turns , this te s t wind ing c a n b e

scaled up or down to act as a suitable primary for that

particular core.

linearly with V . The transformer is most vulnerable to

s a tura tion a t the minimum fre q ue nc y, b e c a us e the

MAX845

switches are on for the longest period. At V = 4.5V,

the transformer must withstand at least:

—

4.5V x ———–——— x

450kHz min

= 5V-µs

And at V

= 5.5V, the transformer must withstand

at least:

5.5V x ———–——— x — = 5V-µs

550kHz min

Thus, the required ET product is constant over the

entire 5V ±10% range.

A gapped core, such as a bobbin or drum core, is not

limited by ET product, but rather by inductance and

winding resistance. The primary inductance must be

high enough to prevent excessive current flow under

light-load conditions, yet low enough that it can be

wound on the core. Good results can be achieved by

using a primary inductance between 50µH and 200µH.

Calculate the number of turns required by using the

Select either a toroid or a gapped core. Although some

applications will require custom transformers, many

can use standard transformer designs, such as those

listed in Table 1. Some of these manufacturers have

standard products designed for the MAX845, while

some have standard products that can be adapted for

specific customer requirements. Table 1 also lists some

suppliers of suitable magnetic cores.

manufacturer’s A (inductance per turn squared) value,

or measure a test winding with an inductance meter.

Inductance varies with the square of the number of turns.

Table 1. Transformer and Transformer-Core

Suppliers

While most MAX845 applications will use a toroid trans-

former for highest efficiency and lowest EMI, there may

be applications that can utilize less expensive trans-

formers, such as E, I, or U-shaped cores, magnetic

bobbins, or etched windings on a printed circuit board.

Table 1 lists some transformer and core suppliers who

can assist with your magnetics design.

TRANSFORMERS

TRANSFORMER CORES

Halo Electronics

Phone: (415) 969-7313

FAX: (415) 367-7158

Magnetics Inc.

Phone: (412) 282-8282

FAX: (412) 282-6955

Ask for MAX845 Transformer

Coilcraft

Fair-Rite Products

Phone: (914) 895-2055

FAX: (914) 895-2629

The secondary or secondaries can be scaled to produce

whatever output is required for the application at hand,

taking into account the rectifier topology to be used and

the forward voltage loss of the diodes selected.

Phone: (708) 639-6400

FAX: (708) 639-1469

Ask for MAX845 Transformer

BH Electronics

Philips Components

Phone: (401) 762-3800

FAX: (401) 762-3805, ext. 324

S t e p -b y-S t e p Tra n s fo rm e r

De s ig n P ro c e d u re

Before starting the design, determine the minimum and

maximum output voltage requirement, the minimum

a nd ma ximum loa d c urre nt, the p hys ic a l s ize c on-

straints, and the cost budget.

Phone: (612) 894-9590

FAX: (612) 894-9380

Ask for MAX845 Transformer

MMG (Magnetic Materials Group)

Phone: (201) 345-8900

FAX: (201) 345-1172

Sumida USA

Phone: (708) 956-0666

FAX: (708) 956-0702

1) Select an appropriate core shape and material from

core vendors’ data sheets; trade-off EMI vs. space

and cost. Since the MAX845’s output waveform is a

square wave, it is rich in harmonics, so choose a

material with low losses at up to several MHz.

Amidon Associates

Phone: (714) 850-4660

FAX: (714) 850-1163

12 ______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

2) Use a test winding to measure ET product (if using

an ungapped toroid) and/or A value for the core.

3) Determine the number of turns required for the pri-

mary winding. For an ungapped toroid, ET product

from center-tap to D1 must be at least 5V-µs. Other

core types must have sufficient inductance to limit

D1 and D2 output current under minimum load con-

ditions, and must not be allowed to saturate.

MAX845

GND1 GND2

4) Select a rectifier topology based on performance

requirements (ripple vs. loss, and space required

for secondary winding). Refer to Table 2, Rectifier

Topology Trade-Offs.

Figure 11a. 2-Diode Push-Pull

5) Work backward from V

requirements to deter-

OUT

mine the secondary to primary turns ratio. Include

losses in the rectifier diodes, and estimate resistive

losses in the windings. For load currents exceed-

ing 150mA, use a voltage step-down transformer to

step up the output current from the MAX845. Do

not exceed the MAX845’s absolute maximum out-

put current rating (200mA).

MAX845

GND1 GND2

6) Wind the transformer with the largest diameter wire

that will fit the winding area. Select a wire gauge to

fill the wind ing a p e rture a s muc h a s p os s ib le .

Larger diameter wire has lower resistance per unit

length. Doubling the wire diameter reduces resis-

tive losses by a factor of four.

Figure 11b. 4-Diode Bridge

Bobbin or drum cores suffer from low coupling between

windings. This usually requires bifilar winding for the

two halves of the primary.

MAX845

Due to the inhe re nt c omp le xity of ma g ne tic c irc uit

design, it will be necessary to build a prototype and re-

iterate the design. If necessary, adjust the design by

altering the number of primary or secondary turns, or the

wire gauge. If using a different core material or geome-

GND1 GND2

try, evaluate its ET product or A as described above.

Figure 11c. Voltage Doubler

Re c t ifie r To p o lo g y

Figure 11 shows various rectifier topologies. Refer to

Table 2 for selection criteria. The turns ratio of the trans-

former must be set to provide the minimum required out-

put voltage at the maximum anticipated load, with the

minimum expected input voltage. In addition, the calcu-

lations should allow for worst-case losses in the recti-

fiers. Since the turns ratio determined in this manner will

ordinarily produce a much higher voltage at the sec-

ondary under conditions of high input voltage and/or

light loading, be careful to prevent an overvoltage con-

dition from occurring (see the Output Voltage vs. Load

Current graph in the Typical Operating Characteristics).

Dio d e s

Use fast-switching diode rectifiers. Ordinary silicon sig-

nal diodes like the 1N914 or 1N4148 may be used for

low output current levels (less than 50mA), but Schottky

diodes have a lower forward voltage drop and should

b e us e d for hig he r-c urre nt a p p lic a tions . Ce ntra l

Semiconductor has low-current Schottky diodes as

d ua ls in SOT-23 p a c ka g e s (CMPSH-3 s e rie s ). The

Nihon SB05W05C is a common-cathode dual in a SOT-

23; it works well in the two-diode full-wave configura-

tion. The Motorola MBR0520 is an excellent choice for

all configurations.

______________________________________________________________________________________ 13

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

The simplest voltage regulator is the shunt zener shown

in Figure 12. The series resistor (R ) value should be as

high as possible to still deliver the maximum expected

load current with minimum input voltage. Be sure that no

ratings are exceeded at maximum input voltage and

minimum load current conditions; under such conditions,

the zener diode may have to dissipate much more power

than the load. Alternatively, start with the maximum allow-

able zener dissipation and select the series resistor

under light-load, high-line conditions. Then verify that

there is sufficient output current available with worst-

case low input voltage.

MAX845

SIMPLE SHUNT ZENER

22k

For better regulation than the simple shunt zener, con-

sider a shunt regulator IC such as the TL431. This

device behaves like a zener diode whose voltage can

be programmed by a resistor ratio. It can be used as a

stand-alone device or can be boosted above its 150mA

maximum rating without compromising its accuracy by

adding a discrete PNP transistor, as shown in Figure 12.

TL431

5V OUTPUT

PROGRAMMABLE-IC SHUNT REGULATOR (STAND ALONE)

The input power of a shunt regulator is nearly indepen-

dent of load, so efficiency at light loads tends to be

worse than it would be with a series regulator.

Ou t p u t Filt e r Ca p a c it o r

Ceramic capacitors can be used as output capacitors

because of the lower level of output ripple current. In

applications where output ripple is not critical, a 0.33µF

chip or ceramic capacitor is normally sufficient. Refer to

Table 3 for suggested capacitor suppliers.

22k

2N2907

5V OUTPUT

TL431

In applications sensitive to output-ripple noise, the out-

put filter capacitor (C2) should have a low equivalent

series resistance (ESR) and a low equivalent series

inductance (ESL), and its capacitance should remain

fairly constant over temperature.

PROGRAMMABLE-IC SHUNT REGULATOR WITH DISCRETE PNP

Sprague 595D surface-mount solid tantalum capacitors

and Sanyo OS-CON through-hole capacitors are recom-

mended, if space allows, due to their extremely low ESR.

Capacitor ESR usually rises at low temperatures, but OS-

CON capacitors provide very low ESR below 0°C.

Figure 12. Shunt-Regulator Circuits

Ou t p u t Re g u la t o r

Sinc e the outp ut volta g e is not re g ula te d a g a ins t

changes in the input voltage or load current, an output

voltage regulator may be needed. A series linear regu-

lator gives good performance and reasonably good

efficiency at low cost. A shunt regulator costs less,

occupies less space, and gives adequate performance

for some applications.

In p u t Byp a s s Ca p a c it o r

The input bypass capacitor (C1) is not critical. Unlike

switching regulators, the MAX845’s supply current is

fairly constant, and is therefore less dependent on the

inp ut b yp a s s c a p a c itor. A low-c os t 0.33µF c hip or

c e ra mic c a p a c itor is norma lly s uffic ie nt for inp ut

bypassing.

Se rie s re g ula tors s uc h a s the MAX666, MAX667,

MAX882/MAX883/MAX884, or MAX603/MAX604 simpli-

fy designs. Just select one with the desired output volt-

age and current capability, and connect it.

14 ______________________________________________________________________________________

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

MAX845

Table 2. Rectifier Topology Trade-Offs

Table 3. Suggested Capacitor Suppliers

CAPACITOR

SUPPLIER

Matsuo

USA Phone: (714) 969-2491

FAX: (714) 960-6492

TOPOLOGY

ADVANTAGE

DISADVANTAGE

Low-ESR 267 Series

Murata Erie

USA Phone: (800) 831-9172

FAX: (404) 436-3030

• Only 3 external

components

Ceramic

2-Diode

Push/Pull

(Figure 11a)

• More turns on

transformer

• Low output ripple

• Single diode drop

Sprague Electric Co.

USA Phone: (603) 224-1961

FAX: (603) 224-1430

Very Low-ESR 595D/293D

Series

• 5 external

components

• Simpler transformer

winding requirements

4-Diode

Bridge

(Figure 11b)

• Higher cost

• Low output ripple

• 2 diode drops

• 4 external

components

Voltage

Doubler

(Figure 11c)

• Fewest turns on

transformer

• Higher output

ripple

• 2 diode drops

___________________Ch ip To p o g ra p h y

0. 085"

(2. 159mm)

GND1

GND2

0. 058"

(1. 4732mm)

SUBSTRATE CONNECTED TO V

TRANSISTOR COUNT: 31

______________________________________________________________________________________ 15

Is o la t e d Tra n s fo rm e r Drive r

fo r P CMCIA Ap p lic a t io n s

________________________________________________________P a c k a g e In fo rm a t io n

MAX845

16 ______________________________________________________________________________________

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MAX845ESA产品参数

型号:	MAX845ESA
是否无铅：	含铅
是否Rohs认证：	不符合
生命周期：	Obsolete
零件包装代码：	SOIC
包装说明：	0.150 INCH, MS-012A, SOIC-8
针数：	8
Reach Compliance Code：	not_compliant
ECCN代码：	EAR99
HTS代码：	8542.39.00.01
风险等级：	5.06
接口集成电路类型：	BUFFER OR INVERTER BASED PERIPHERAL DRIVER
JESD-30 代码：	R-PDSO-G8
JESD-609代码：	e0
长度：	4.9 mm
湿度敏感等级：	1
功能数量：	1
端子数量：	8
最高工作温度：	85 °C
最低工作温度：	-40 °C
输出电流流向：	SOURCE AND SINK
标称输出峰值电流：	1 A
封装主体材料：	PLASTIC/EPOXY
封装代码：	SOP
封装等效代码：	SOP8,.25
封装形状：	RECTANGULAR
封装形式：	SMALL OUTLINE
峰值回流温度（摄氏度）：	245
电源：	5 V
认证状态：	Not Qualified
座面最大高度：	1.75 mm
子类别：	Other Interface ICs
最大供电电压：	5.5 V
最小供电电压：	4.5 V
标称供电电压：	5 V
表面贴装：	YES
技术：	CMOS
温度等级：	INDUSTRIAL
端子面层：	Tin/Lead (Sn/Pb)
端子形式：	GULL WING
端子节距：	1.27 mm
端子位置：	DUAL
处于峰值回流温度下的最长时间：	NOT SPECIFIED
宽度：	3.9 mm
Base Number Matches：	1

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