P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1664/1664A, LX1665/65A
DUAL OUTPUT PWM CONTROLLERS WITH 5-BIT DAC
P R O D U C T I O N D A T A S H E E T
USING THE LX1664/65 DEVICES
LINEAR REGULATOR
LINEAR REGULATOR (continued)
Referring to the front page Product Highlight, a schematic is
presented which uses a MOSFET as a series pass element for a
linear regulator. The MOSFET is driven by the LX1664 controller,
and down-converts a +5V or +3.3V supply to the desired VOUT
level, between 1.5 & 3.5V, as determined by the feedback
resistors.
The current available from the Linear regulator is dictated by
the supply capability, as well as the MOSFET ratings, and will
typically lie in the 3-5 ampere range. This output is well suited
for I/O buffers, memory, chipset and other components. Using
3.3V supply to convert to 1.5V for GTL+ Bus will significantly
reduce heat dissipation in the MOSFET.
MOSFET Comments
Heatsinking the MOSFET becomes important, since the linear
stage output current could approach 5 amperes in some applica-
tions. Since there are no switching losses, power dissipation in
the MOSFET is simply defined by PD = (VIN - VOUT
)
I output
*
current. This means that a +5VIN to +3.3VOUT at 5A will require that
the MOSFET dissipate (5-3.3) 5 = 8.5 watts. This amount of
*
FIGURE 13 — Typical Transient Response
power in a MOSFET calls for a heatsink, which will be the same
physical size as that required for a monolithic LDO, such as the
LX8384 device.
Channel 2 = Linear Regulator Output.
Set point = 3.3V @ 2A (20mV/div.)
Channel 4 = Switching Regulator Output.
VCC_CORE set point = 2.8V
Thedropoutvoltageforthelinearregulatorstageistheproduct
of RDS ON
IOUT. Using a 2SK1388 device at 5A, the dropout
*
voltage will be (worst case) 37 milliohms x 5A = 185mV.
Note that the RDS ON of the (linear regulator) MOSFET does not
affect heat dissipation, only dropout voltage. For reasons of
economy, a FET with a higher resistance can be chosen for the
linear regulator, e.g. 2SK1388 or IRLZ44.
Channel 3 = Switching Regulator Load Current
Transient 0 - 13A
Output Voltage Setting
As shown in Application Information Figures 6-9, two resistors (R5
& R6) set the linear regulator stage output voltage:
TABLE 5 - Linear Regulator MOSFET Selection Guide
VOUT = 1.5 (R + R ) / R
*
6
Device
RDS(ON)
@
ID @
Max. Break-
5
6
10V (mΩ)
TC = 100°C
down Voltage
As an example, to set resistor magnitudes, assume a desired
VOUT of 3.3 volts:
IRFZ24N
IRL2703
IRLZ44N
70
40
22
12
17
29
55
30
55
1.5 (12.1k + 10k) / 10k = 3.3 volts (approximately)
*
In general, the divider resistor values should be in the vicinity
of 10-12k ohm for optimal noise performance. Please refer to
Table 6.
Avoiding Crosstalk
To avoid a load transient on the switching output affecting the
linear regulator, follow these guidelines:
1) Separate 5V supply traces to switching & linear FETs as
much as possible.
2) Place capacitor C9 as close to drain of Q4 as possible.
Typical transient response is shown in Figure 13.
Copyright © 1999
Rev. 1.2 11/99
15
MICROSEMI [ Microsemi ]
