Rainbow-electronics MAX1800 Instrukcja Użytkownika Pobierz pdf (Strona 11)

MAX1800

Digital Camera Step-Up

Power Supply

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the output voltage. The controller forces the inductor cur-

rent to rise above the 300mA Idle Mode threshold to

ensure pulse skipping and improved efficiency at light

loads.

Auxiliary DC-DC Controllers

The MAX1800 auxiliary controllers operate in a low-noise,

fixed-frequency, PWM mode, with output power limited by

the external components. The controllers regulate their out-

put voltages by modulating the pulse width of the drive sig-

nal for an external N-channel MOSFET switch. The auxiliary

controllers are inactive until the main output has started.

Figure 3 shows a block diagram for a MAX1800 auxil-

iary PWM controller. A sawtooth oscillator signal at OSC

governs the internal timing. At the beginning of each

cycle, DL_ goes high to turn on the external MOSFET

switch. The MOSFET switch turns off when the internally

level-shifted sawtooth rises above COMP_ or when the

maximum duty cycle is exceeded. The switch remains

off until the beginning of the next cycle. An internal

transconductance amplifier establishes an integrated

error voltage at COMP_, thereby increasing the loop

gain for improved regulation accuracy.

EN Q

REF

0.9V

REF

RDY

OSC

COMP

AUX1

AUX2

AUX3

IC POWER

2.35V

1.25V

POUT

PGND

STARTUP

OSCILLATOR

MAIN PWM

CONTROLLER

REFERENCE

GAIN

BLOCK

DCON1

ONM

REF

GND

OSC

FBM

COMPM

FB1

COMP1

OUT

FB2

COMP2

DCON2

FB3

COMP3

DCON3

ON1

ON2

ON3

ONA

OSCILLATOR

UNDERVOLTAGE LOCKOUT

OSC

COMP

DUTY

OSC

COMP

DUTY

OSC

COMP

DUTY

DL1

DL2

DL3

RDYM

MAX1800

Figure 2. Simplified Functional Diagram

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 23 24

Digital Camera Step-Up 1
Power Supply 1

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