Miniature, Low-Voltage,

Precision Step-Down Controller

_______________________________________________________________________________________

7

MAX1637

0.1

µF

+5V

NOMINAL

0.1

µF

1

µF

470pF

C1

Q1

CMPSH-3

Q2

C2

L1

*

R1

R2

R3

OUTPUT

4.7

µF

*SEE

RECTIFIER CLAMP DIODE SECTION

**OPTIONAL RC NETWORK FOR POWER-ON-RESET

DL

PGND

LX

DH

BST

CSH

CSL

FB

1M

**

ON/OFF

CC

GND

SHDN

REF

SYNC

20

Ω

SKIP

0.01

Figure 1. Standard Application Circuit

______Standard Application Circuit

The basic MAX1637 buck converter (Figure 1) is easily

adapted to meet a wide range of applications where a

5V or lower supply is available. The components listed

in Table 1 represent a good set of trade-offs among

cost, size, and efficiency, while staying within the worst-

case specification limits for stress-related parameters

such as capacitor ripple current. Do not change the cir-

cuit’s switching frequency without first recalculating

component values (particularly inductance value at

maximum battery voltage).

The power Schottky diode across the synchronous rec-

tifier is optional because the MOSFETs chosen incorpo-

rate a high-speed silicon diode. However, installing the

Schottky will generally improve efficiency by about 1%.

If used, the Schottky diode DC current must be rated to

at least one-third of the maximum load current.

_______________Detailed Description

The MAX1637 is a BiCMOS, switch-mode power-supply

(SMPS) controller designed primarily for buck-topology

regulators in battery-powered applications where high

efficiency and low quiescent supply current are critical.

Light-load efficiency is enhanced by automatic idle-

mode operation—a variable-frequency, pulse-skipping

mode that reduces transition and gate-charge losses.

The step-down, power-switching circuit consists of two

N-channel MOSFETs, a rectifier, and an LC output filter.

Output voltage for this device is the average AC volt-

age at the switching node, which is regulated by

changing the duty cycle of the MOSFET switches. The

gate-drive signal to the high-side N-channel MOSFET,

which must exceed the battery voltage, is provided by

a flying-capacitor boost circuit that uses a 100nF

capacitor between BST and LX. Figure 2 shows the

major circuit blocks.