FC3255 datasheet(10/18 Pages) FCI

10 / 18 page

The situation is different in a flyback circuit. The input

current is discontinuous and a significant pulse cur-

rent is see by the input capacitors. Therefore, there

are two requirements for capacitors in a flyback regu-

lator: energy storage and filtering. To maintain a sta-

ble voltage supply to the chip, a storage capacitor

larger than 20µF with low ESR is required. To reduce

the noise generated by the inductor, insert a 1.0µF ce-

ramic capacitor between V

and ground as close as

possible to the chip.

When the power switch is turned on, I

is shunted to

ground and I

discharges the output capacitor.

OUT

When the I ripple is small enough, I can be treated

as a constant and is equal to input current I ,

Summing up, the output voltage peak-peak ripple can

be calculated by:

The equation can be expressed more conveniently in

terms of V

, V

and I

for design purposes as

OUT

follows:

The capacitor RMS ripple current is:

Although the above equations apply only for boost cir-

cuits, similar equations can be derived for flyback cir-

cuits.

Figure 11. Boost Circuit Effective Input Filter

VCC

CIN

RESR

IIN

Reducing the Current Limit

In some applications, the designer may prefer a lower

limit on the switch current than 1.5A. An external shunt

can be connected between the V pin and ground to re-

duce its clamp voltage. Consequently, the current limit

of the internal power transistor current is reduced from

its nominal value.

The voltage on the V

pin can be evaluated with the

equation

where:

= 0.063 W, the value of the internal emitter resistor;

= 5V/ V, the gain of the current sense amplifier

Since R

and A

cannot be changed by the end user,

the only available method for limiting switch current be-

low 1.5A is to clamp the V pin at a lower voltage. If

the maximum switch or inductor current is substituted

into the equation above, the desired clamp voltage will

result.

A simple diode clamp, as shown in Figure12, clamps

the V voltage to a diode drop above the voltage on re-

sistor R3. Unfortunately, such a simple circuit is not

generally acceptable if V

is loosely regulated.

OUT(RIPPLE)

+ I X ESR

(I - I

) (1 - D)

OUT

)(f)

OUT

)(f)

OUT

OUT(RIPPLE)

I(V

- V

)

OUT

)(f)

OUT

)(V

)(ESR)

OUT

)(f)

OUT

- I

) (1 - D) + (I

) (D)

OUT

- V

OUT

RIPPLE

= I

OUT

= I

R A

CSW E V

Figure 12. Current Limiting using a Diode Clamp

VCC

V IN

FC3255 1.5A PWM Switching Regulators

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