5V/3.3V/3V or Adjustable, High-Efficiency,

_______________________________________________________________________________________

7

_______________Detailed Description

The MAX649/MAX651/MAX652 are BiCMOS, step-

down, switch-mode power-supply controllers that pro-

vide fixed outputs of 5V, 3.3V, and 3V, respectively.

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 MAX649/MAX651/MAX652 offer three main

improvements over prior solutions:

1) The converters operate with tiny (less than 9mm

diameter) 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 (1.0mA to 1.5A).

3) The maximum supply current is only 100µA.

PFM Control Scheme

The MAX649/MAX651/MAX652 use a proprietary, cur-

rent-limited PFM control scheme. As with traditional

PFM converters, 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 (16µs) and

minimum switch off-time (2.3µs). Once off, the minimum

off-time one-shot holds the switch off for 2.3µs. 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 MAX649/MAX651/MAX652 also limit the peak induc-

tor current, which allows them to run in continuous-con-

duction mode and maintain high efficiency with heavy loads

(Figure 3a). 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 (16µs later), or 2) the current limit is reached.

To increase light-load efficiency, the current limit for

the first two pulses is set to half the peak current limit.

If those pulses bring the output voltage into regulation,

the voltage comparator holds the MOSFET off and the

current limit remains at half its peak. If the output vol-

tage is still out of regulation after two pulses, the

current limit for the next pulse is raised to its peak (Figure

3b). Calculate the peak current limit by dividing the

Current-Limit Trip Level (see

Electrical Characteristics)

by the value of the current-sense resistor.

Shutdown Mode

When SHDN is high, the MAX649/MAX651/MAX652 enter

shutdown mode. In this mode, the internal biasing circuit-

ry is turned off (including the reference) and the supply

current drops to less than 5µA. EXT goes high, turning off

the external MOSFET. SHDN is a TTL/CMOS logic-level

input. Connect SHDN to GND for normal operation.

Quiescent Current

In normal operation, the quiescent current is less than

100µA. However, this current is measured by forcing

the external transistor switch off. In an actual applica-

tion, even with no load, additional current is drawn to

supply external feedback resistors (if used) and the

diode and capacitor leakage currents. In the circuit of

quiescent current is 90µA.

EXT Drive Voltage Range

EXT swings from V+ to GND and provides the drive out-

put for an external P-channel power MOSFET.

Modes of Operation

When delivering high output currents, the MAX649/

MAX651/MAX652 operate in continuous-conduction

mode (CCM). In this mode, current always flows in the

MAX649

MAX651

MAX652

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

0.1

Ω

D1

NSQ03A02L

L1

22

P1

Si9430

*

OUTPUT

@ 1.5A

REF

Figure 1. Test Circuit