MAX1881 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1881

Quad-Output TFT LCD DC-DC Converters with Buffer

Maxim Integrated 

Quad-Output TFT LCD DC-DC

Converters with Buffer

Main Step-up Controller

During normal pulse-width modulation (PWM) opera-

tion, the MAX1778/MAX1880–MAX1885 main step-up

controllers switch at a constant frequency of 500kHz or

1MHz (see Selector Guide), allowing the use of low-

profile inductors and output capacitors. Depending on

the input-to-output voltage ratio, the controller regulates

the output voltage and controls the power transfer by

modulating the duty cycle (D) of each switching cycle:

D ≈ VMAIN - VIN

VMAIN

On the rising edge of the internal clock, the controller

sets a flip-flop when the output voltage is too low, which

turns on the N-channel MOSFET (Figure 2). The induc-

tor current ramps up linearly, storing energy in a mag-

netic field. Once the sum of the feedback voltage error

amplifier, slope-compensation, and current-feedback

signals trip the multi-input comparator, the MOSFET

turns off, the flip-flop resets, and the diode (D1) turns

on. This forces the current through the inductor to ramp

back down, transferring the energy stored in the mag-

netic field to the output capacitor and load. The MOS-

FET remains off for the rest of the clock cycle. Changes

in the feedback voltage-error signal shift the switch-cur-

rent trip level, consequently modulating the MOSFET

duty cycle.

Under very light loads, an inherent switchover to pulse-

skipping takes place (Figure 3). When this occurs, the

controller skips most of the oscillator pulses in order to

reduce the switching frequency and gate charge loss-

es. When pulse-skipping, the step-up controller initiates

a new switching cycle only when the output voltage

drops too low. The N-channel MOSFET turns on, allow-

ing the inductor current to ramp up until the multi-input

comparator trips. Then, the MOSFET turns off and the

diode turns on, forcing the inductor current to ramp

down. When the inductor current reaches zero, the

diode turns off, so the inductor stops conducting cur-

rent. This forces the threshold between pulse-skipping

and PWM operation to coincide with the boundary

between continuous and discontinuous inductor-cur-

rent operation:

ILOAD(CROSSOVER)

≈

1

2





VIN

VMAIN





2





VMAIN -

fOSCL

VIN





MAX1778

MAX1880

MAX1881

MAX1882

MAX1883

MAX1884

MAX1885

PWM

COMPARATOR

OSC

(80% DUTY)

S

RQ

ILIM

ILIM

COMPARATOR

gm

ERROR

AMPLIFIER

INTG

CINTG

L1

LX

D1

VIN

(2.7V TO 5.5V)

CIN

VMAIN

(UP TO 13V)

COUT

PGND

FB

REF

VREF

1.25V

GND

R1

RCOMP

(OPTIONAL)

CCOMP

(OPTIONAL)

R2

CREF

( ) VMAIN =

1 + R1

R2

VREF

VREF = 1.25V

Figure 2. Main Step-Up Converter block Diagram

20 ______________________________________________________________________________________

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