Digital Camera Step-Up

Power Supply

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9

Detailed Description

The MAX1800 typical application circuit is shown in

Figure 1. It features a main step-up DC-DC converter,

three auxiliary step-up DC-DC controllers, an uncom-

mitted gain block, a power-ready comparator, and con-

trol capability for multiple external MAX1801 slave

DC-DC controllers. The uncommitted gain block can be

used with an external P-Channel MOSFET to make a

linear regulator. The linear regulator can be used with

the main output for step-up/step-down functionality or

to make a separate stand-alone output voltage.

Together, these provide a complete high-efficiency

power-supply solution for digital still cameras. Figure 2

shows the MAX1800 functional diagram.

Master-Slave Configuration

The MAX1800 supports MAX1801 “slave” controllers

that obtain input power, a voltage reference, and an

oscillator signal directly from the MAX1800 “master”

DC-DC converter. The master-slave configuration

reduces system cost by eliminating redundant circuitry

and controlling the harmonic content of noise with syn-

chronized converter switching.

Main DC-DC Converter

The MAX1800 main step-up DC-DC switching convert-

er generates a 2.7V to 5.5V output voltage from a +0.7V

to +5.5V battery input voltage. An internal switch and

synchronous rectifier allow conversion efficiencies as

high as 95% while reducing both circuit size and the

number of external components. The converter oper-

ates in a low-noise, constant-frequency PWM mode to

regulate the voltage across the load. Switching har-

monics generated by fixed-frequency operation are

consistent and easily filtered.

The internal N-channel MOSFET switch turns on during

the first part of each cycle, allowing current to ramp up

in the inductor and store energy in a magnetic field.

During the second part of each cycle, the MOSFET

turns off and the voltage across the inductor reverses,

forcing current through the internal P-channel synchro-

nous rectifier to the output filter capacitor and load. As

the energy stored in the inductor is depleted, the cur-

rent ramps down. The synchronous rectifier turns off

when the inductor current approaches zero or at the

beginning of a cycle.

The current-mode PWM controller uses the voltage at

COMPM to program the inductor current and regulate

Pin Description (continued)

PIN

NAME

FUNCTION

24

ONM

Main Converter Enable Input. High level turns the main converter on. Connect ONM to POUT to automatically

start the main converter. When the main converter is off, all other outputs are disabled.

25

AI

Analog Gain Block Input. AI is the positive input to the gain block. The negative input is internally connected

to the 1.25V reference.

26

AO

Analog Gain Block Output. AO is a push-pull output driven between GND and POUT. The voltage gain of the

block is approximately 100.

27

ONA

Analog Gain Block Enable Input. Connect ONA to POUT to enable the gain block. When ONA is low, the AO

output is driven to POUT.

28

DCON3

Maximum Duty-Cycle Control Input for Auxiliary Controller 3. Connect to POUT to set the default maximum

duty cycle. Connect a resistive voltage-divider from REF to DCON3 to set the maximum duty cycle between

40% and 90%. Pull DCON3 below 400mV to turn the controller off.

29

COMP3

Compensation for Auxiliary Controller 3. Output of transconductance error amplifier. Connect a series resistor

and capacitor to GND to compensate the control loop. See Compensation Design.

30

FB3

Feedback Input for Auxiliary Controller 3. Connect a feedback resistive voltage-divider from the output to FB3

31

ON3

Enable Input for Auxiliary Controller 3. Connect ON2 to POUT to automatically start auxiliary controller 3.

32

DL3

External MOSFET Gate Drive Output for Auxiliary Controller 3. DL3 swings between POUT and GND with

typical 500mA drive current. Connect DL3 to the gate of the external switching N-channel MOSFET for

auxiliary controller 3.