5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
______________________________________________________________P in De s c rip t io n
PIN NAME
FUNCTION
Sense Input for fixed 5V or 3.3V output operation. OUT is internally connected to the on-chip voltage divider.
Although it is connected to the output of the circuit, the OUT pin does not supply current. Leave OUT unconnected
for adjustable-output operation.
OUT
1
Feedback Input. Connect to GND for fixed-output operation. Connect a resistor divider between OUT, FB, and
GND for adjustable-output operation. See Setting the Output Voltage section.
2
3
FB
Active-High Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the refer-
ence, output, and external MOSFET are turned off. Connect to GND for normal operation.
SHDN
4
5
REF
V+
1.5V Reference Output that can source 100µA. Bypass with 0.1µF.
Positive Power-Supply Input
Current-Sense Input. Connect current-sense resistor between V+ and CS. When the voltage across the resistor
equals the current-limit trip level, the external MOSFET is turned off.
6
CS
7
8
EXT
Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.
Ground
GND
49/MAX651
The MAX1649/MAX1651 offer four main improvements
over prior solutions:
V
IN
C4
C1
1) The converters operate with miniature surface-mount
inductors, due to their 300kHz switching frequency.
0.1µF 100µF
5
V+
2) The c urre nt-limite d PFM c ontrol s c he me a llows
greater than 90% efficiencies over a wide range of
load currents (10mA to 1.5A).
R1
0.05Ω
MAX1649
MAX1651
6
CS
3) Dropout volta ge ha s be e n re duc e d to le ss tha n
300mV for many applications.
P1
Si9430*
OUTPUT
@ 1.5A
3
4
7
1
SHDN
EXT
4) The quiescent supply current is only 100µA.
L1
47µH**
P FM Co n t ro l S c h e m e
The MAX1649/MAX1651 use a proprietary, current-limit-
ed PFM control scheme. As with traditional PFM con-
verters, the external power MOSFET is turned on when
the voltage comparator senses that the output is out of
regulation. However, unlike traditional PFM converters,
switching is accomplished through the combination of a
peak current limit and a pair of one-shots that set the
maximum switch on-time (32µs) and minimum switch
off-time (1.1µs). Once off, the off-time one-shot holds
the switch off for 1.1µs. After this minimum time, the
switch either 1) stays off if the output is in regulation, or
2) turns on again if the output is out of regulation.
REF
OUT
FB
GND
2
8
C3
0.1µF
D1
NSQ03A02L
C2
330µF
*SILICONIX SURFACE-MOUNT MOSFET
**SUMIDA CDRH125-470
Figure 1. Test Circuit
_______________De t a ile d De s c rip t io n
The MAX1649/MAX1651 a re BiCMOS, s te p -d own,
s witc h-mod e p owe r-s up p ly c ontrolle rs tha t p rovid e
adjustable and fixed outputs of 5V and 3.3V, respec-
tive ly. The ir uniq ue c ontrol s c he me c omb ine s the
advantages of pulse-frequency-modulation (low supply
current) and pulse-width-modulation (high efficiency at
high loads). An external P-channel power MOSFET
allows peak currents in excess of 3A, increasing the
output current capability over previous PFM devices.
Figure 2 is the block diagram.
The MAX1649/MAX1651 also limit the peak inductor cur-
rent, which allows them to run in continuous-conduction
mode and maintain high efficiency with heavy loads
(Figure 3). This current-limiting feature is a key compo-
nent of the control circuitry. Once turned on, the switch
stays on until either 1) the maximum on-time one-shot
turns it off (32µs later), or 2) the current limit is reached.
EXT swings from V+ to GND and provides the drive out-
put for an external P-channel power MOSFET.
6
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
