5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

6

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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.

MAX1649

MAX1651

V+

CS

FB

GND

5

6

28

3

C2

330

μF

7

1

EXT

OUT

SHDN

4

C3

0.1

μF

C4

0.1

μF

C1

100

μF

R1

0.05

Ω

D1

NSQ03A02L

L1

47

P1

Si9430

*

OUTPUT

@ 1.5A

REF

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, the OUT pin 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