Rainbow-electronics MAX1844 Manual de usuario descargar pdf (Pagina 11)

battery supply, the enable signal (SHDN) must be

delayed until the battery voltage is present in order to

ensure startup. The 5V bias supply provides V

and

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

BIAS

= I

+ f (Q

+ Q

) = 5mA to 30mA (typ)

where I

is 550µA (typ), f is the switching frequency,

and Q

are the MOSFET data sheet total

gate-charge specification limits at V

= 5V.

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

MAX1844

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________ 11

Figure 2. MAX1844 Functional Diagram

REF

-10%

FROM

OUT

REF

ERROR

AMP

TOFF

TON

REF

+10%

FEEDBACK

MUX

(SEE FIGURE 6)

CHIP

SUPPLY

1.0V

0.1V

POR

OVP

ILIM

- 1V

SHDN

PGOOD

ON-TIME

COMPUTE

TON

1-SHOT

TRIG

2V TO 28V

TRIG

REF

OUTPUT

GND

ZERO CROSSING

CURRENT

LIMIT

BST

+5V

SKIP

TON

LATCH

V+

MAX1844

0.7V

1.14V

UVP

20ms

TIMER

OVP/UVP

LATCH

0.1V

OUT

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 22 23

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Rainbow-electronics MAX1844 Manual de usuario Pagina 11

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