MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
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_______________Detailed Description
The MAX1649/MAX1651 are BiCMOS, step-down,
switch-mode power-supply controllers that provide
adjustable and fixed outputs of 5V and 3.3V, respec-
tively. Their unique control scheme combines 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 offer four main improvements
over prior solutions:
1) The converters operate with miniature surface-mount
inductors, due to their 300kHz switching frequency.
2) The current-limited PFM control scheme allows
greater than 90% efficiencies over a wide range of
load currents (10mA to 1.5A).
3) Dropout voltage has been reduced to less than
300mV for many applications.
4) The quiescent supply current is only 100µA.
PFM Control Scheme
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.
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.
Figure 1. Typical Application Circuit
______________________________________________________________Pin Description
NAME FUNCTION
1
OUT
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, OUT does not supply current. Leave OUT unconnected for
adjustable-output operation.
2 FB
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.
PIN
3 SHDN
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.
4 REF 1.5V Reference Output that can source 100µA. Bypass with 0.1µF.
8 GND Ground
7 EXT Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.
6 CS
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.
5 V+ Positive Power-Supply Input
