battery supply, the enable signal (SHDN) must be

delayed until the battery voltage is present in order to

gate-drive power, so the maximum current drawn is:

Free-Running, Constant-On-Time PWM

Controller with Input Feed-Forward

The Quick-PWM control architecture is a pseudo-fixed-fre-

quency, constant-on-time on-demand PWM with voltage

feed-forward (Figure 2). This architecture relies on the out-

put filter capacitor’s ESR to act as a current-sense resistor,

so the output ripple voltage provides the PWM ramp sig-

nal. The control algorithm is simple: the high-side switch

on-time is determined solely by a one-shot whose pulse

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________

11

Figure 2. MAX1844 Functional Diagram

REF

-10%

FROM

OUT

REF

FB

ERROR

AMP

TOFF

TON

REF

+10%

FEEDBACK

MUX

(SEE FIGURE 6)

CHIP

SUPPLY

x2

1.0V

R

0.1V

POR

OVP

9R

ILIM

SHDN

PGOOD

ON-TIME

COMPUTE

TON

1-SHOT

1-SHOT

TRIG

IN

2V TO 28V

TRIG

Q

Q

S

R

2V

REF

REF

5V

OUTPUT

DL

CS

GND

LX

ZERO CROSSING

CURRENT

LIMIT

DH

BST

5V

+5V

Q

SKIP

TON

LATCH

V+

Σ

MAX1844

S

R

Q

0.7V

1.14V

UVP

20ms

TIMER

OVP/UVP

LATCH

0.1V

OUT