Rainbow-electronics MAX1801 Manual de usuario descargar pdf (Pagina 8)

MAX1801

Digital Camera Step-Up Slave

DC-DC Controller

8 _______________________________________________________________________________________

Oscillator

The MAX1801 requires a 0 to 1.25V sawtooth oscillator

signal that is obtained from the MAX1800 or the

MAX1802 (at OSC). The 100kHz to 1MHz oscillator sig-

nal sets the converter switching frequency, and it is

used to control pulse-width modulation and maximum

duty cycle.

Maximum Duty Cycle

The MAX1801 uses the master-generated oscillator sig-

nal at OSC, the voltage at DCON, and an internal com-

parator to limit its maximum switching duty cycle (see

Setting the Maximum Duty Cycle). Limiting the duty

cycle can prevent saturation in some magnetic compo-

nents. A low maximum duty cycle can also force the

converter to operate in discontinuous current mode,

simplifying design stability at the cost of a slight reduc-

tion in efficiency.

Soft-Start

The MAX1801 features a soft-start function that limits

inrush current and prevents excessive battery loading

at startup by ramping the output voltage to the regula-

tion voltage. This is achieved by increasing the internal

reference to the transconductance amplifier from 0 to

the 1.25V reference voltage over 1024 oscillator cycles

when initial power is applied or when the part is taken

out of shutdown or sleep mode.

CLK

REFI

FAULT

PROTECTION

ENABLE

LEVEL

SHIFT

REF

COMP

DCON

OSC

0.4V

1.1V

2.35V

SOFT-

START*

*SOFT-START RAMPS REFI FROM 0 TO V

REF

IN 1024 CLK CYCLES.

POWER

POWER-ON

Figure 2. PWM Controller Block Diagram

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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Rainbow-electronics MAX1801 Manual de usuario Pagina 8

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