CS51033GD8 datasheet(6/10 Pages) ONSEMI

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CS51033

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Buck Regulator Operation

A block diagram of a typical buck regulator is shown in

Figure 4. If we assume that the output transistor is initially

off, and the system is in discontinuous operation, the

inductor current IL is zero and the output voltage is at its

nominal value. The current drawn by the load is supplied by

the output capacitor CO. When the voltage across CO drops

below the threshold established by the feedback resistors R1

and R2 and the reference voltage VREF, the power transistor

Q1 switches on and current flows through the inductor to the

output. The inductor current rises at a rate determined by

(VIN − VOUT)/Load. The duty cycle (or “on” time) for the

CS51033 is limited to 80%. If output voltage remains higher

than nominal during the entire COSC change time, the Q1

does not turn on, skipping the pulse.

Figure 4. Buck Regulator Block Diagram

RLOAD

Feedback

Control

CIN

VIN

Charge Pump Circuit

(Refer to the CS51033 Application Diagram on page 2).

An external charge pump circuit is necessary when the VC

input voltage is below 5.0 V to ensure that there is suffifient

gate drive voltage for the external FET. When VIN is applied,

capacitors C1 and C2 will be charged to a diodes drop below

VIN via diodes D2 and D4, respectively. When the P−Ch

FET turns on, it’s drain voltage will be approximately equal

to VIN. Since the voltage across C1 can not change

instantaneously, D2 is reverse biased and the anode voltage

rises to approximately 2.0

× 3.3 V − VD2. C1 transfers some

of its stored charge C2 via D3. After several cycles there is

sufficient gate drive voltage.

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