Rainbow-electronics MAX1845 Manual de usuario descargar pdf (Pagina 14)

MAX1845

Dual, High-Efficiency, Step-Down

Controller with Accurate Current Limit

14 ______________________________________________________________________________________

FROM

OUT

REF

FROM ZERO-CROSSING

COMPARATOR

ERROR

AMP

TON

FEEDBACK

MUX

(SEE FIGURE 9)

TO DL DRIVER

SHUTDOWN

TO DH DRIVER

ON-TIME

COMPUTE

TON

1-SHOT

FROM ILIM

COMPARATOR

FROM

OPPOSITE

PWM

OPPOSITE

PWM

TOFF 1-SHOT

TRIG

FAULT

TIMER

TON

V+

TO PGOOD

OR-GATE

1.1V 0.9V

0.7V

0.1V

1.14V

OVP

- 1V

UVP

FB_

OUT_

Figure 3. PWM Controller (One Side Only)

include larger physical size and degraded load-tran-

sient response (especially at low input voltage levels).

DC output accuracy specifications refer to the threshold

of the error comparator. When the inductor is in continu-

ous conduction, the output voltage will have a DC regula-

tion higher than the trip level by 50% of the ripple. In

discontinuous conduction (SKIP = GND, light-load), the

output voltage will have a DC regulation higher than the

trip level by approximately 1.5% due to slope compensa-

tion.

Forced-PWM Mode (

SKIP

= High)

The low-noise, forced-PWM mode (SKIP = high) dis-

ables the zero-crossing comparator, which controls the

low-side switch on-time. This causes the low-side gate-

drive waveform to become the complement of the high-

side gate-drive waveform. This in turn causes the

inductor current to reverse at light loads as the PWM

loop strives to maintain a duty ratio of V

OUT

. The

benefit of forced-PWM mode is to keep the switching

frequency fairly constant, but it comes at a cost: The

no-load battery current can be 10mA to 40mA, depend-

ing on the external MOSFETs.

Forced-PWM mode is most useful for reducing audio-

frequency noise, improving load-transient response,

providing sink-current capability for dynamic output

voltage adjustment, and improving the cross-regulation

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Rainbow-electronics MAX1845 Manual de usuario Pagina 14

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