CS51033 datasheet(4/8 Pages) CHERRY

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Block Diagram

COSC

VCC

VGATE

PGnd

VFB

Gnd

2.5V

1.5V

1.25V

1.15V

VCC

2.4V

2.5V

1.5V

7IC

VCC

Fault

Comp

0.7V

2.3V

Slow Discharge

Comparator

Slow Discharge

Flip-Flop

CS Charge

Sense

Comparator

Oscillator

Comparator

VFB

Comparator

VGATE

Flip-Flop

Hold

Off

Comp

Control Scheme

The CS51033 monitors the output voltage to determine

when to turn on the PFET. If VFB falls below the internal ref-

erence voltage of 1.25V during the oscillator’s charge cycle,

the PFET is turned on and remains on for the duration of the

charge time. The PFET gets turned off and remains off dur-

ing the oscillator’s discharge cycle time with the maximum

duty cycle to 80%. It requires 7mV typical, and 20mV maxi-

mum ripple on the VFB pin is required to operate. This

method of control does not require any loop stability com-

pensation.

Startup

The CS51033 has an externally programmable soft start fea-

ture that allows the output voltage to come up slowly, pre-

venting voltage overshoot on the output.

At startup, the voltage on all pins is zero. As VCC rises, the

VC voltage along with the internal resistor RG keeps the

PFET off. As VCC and VC continue to rise, the oscillator

capacitor (COSC ) and the Soft start/Fault Timing capacitor

(CS) charges via internal current sources. COSC gets charged

by the current source IC and CS gets charged by the IT

source combination described by:

ICS = IT -

( + )

The internal Holdoff Comparator ensures that the external

PFET is off until VCS > 0.7V preventing the GATE flip-flop

(F2) from being set. This allows the oscillator to reach its

operating frequency before enabling the drive output. Soft

start is obtained by clamping the VFB comparator’s (A6) ref-

erence input to approximately 1/2 of the voltage at the CS

pin during startup, permitting the control loop and the out-

put voltage to slowly increase. Once the CS pin charges

above the Holdoff Comparator trip point of 0.7V, the low

Theory of Operation

Circuit Description

Figure 1: Block Diagram for CS51033

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