Detailed Description

The MAX1844 buck controller is targeted for low-voltage

power supplies for notebook computers. Maxim’s propri-

etary Quick-PWM pulse-width modulator in the MAX1844

is specifically designed for handling fast load steps while

maintaining a relatively constant operating frequency

and inductor operating point over a wide range of input

voltages. The Quick-PWM architecture circumvents the

poor load-transient timing problems of fixed-frequency

current-mode PWMs while also avoiding the problems

caused by widely varying switching frequencies in con-

ventional constant-on-time and constant-off-time PWM

schemes.

The MAX1844 requires an external 5V bias supply in

addition to the battery. Typically, this 5V bias supply is

the notebook’s 95% efficient 5V system supply. Keeping

the bias supply external to the IC improves efficiency

and eliminates the cost associated with the 5V linear reg-

ulator that would otherwise be needed to supply the

PWM circuit and gate drivers. If stand-alone capability is

needed, the 5V supply can be generated with an exter-

nal linear regulator such as the MAX1615.

The battery and 5V bias inputs can be connected

together if the input source is a fixed 4.5V to 5.5V supply.

If the 5V bias supply is powered up prior to the

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

10

______________________________________________________________________________________

UVP

7V TO 20V

5V

BIAS SUPPLY

C2

330

µF

SEE TABLE 1 FOR OTHER COMPONENT SELECTIONS.

POWER-GOOD

INDICATOR

L1

4.7

µH

2.5V

SHDN

V+

D2

CMPSH-3

C6

3.3

µF

C7

0.1

µF

C4

0.22

µF

270k

Ω

130k

Ω

Q1

D1

R2

100k

Ω

Q2

15m

Ω

C5

4.7

µF

R1

20

Ω

SKIP

ILIM

ON/OFF

CONTROL

LOW-NOISE

CONTROL

DL

LX

BST

DH

CS

OUT

FB

LATCH

OVP

PGOOD

MAX1844

5V

TON

REF

GND

C1

10

µF

Figure 1. Standard Application Circuit