CS5172GD8 datasheet(14/21 Pages) ONSEMI

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Summing up, the output voltage peak−peak ripple can be

calculated by:

VOUT(RIPPLE) +

(IIN * IOUT)(1 * D)

(COUT)(f)

)

IOUTD

(COUT)(f)

) IIN

ESR

The equation can be expressed more conveniently in

terms of VCC, VOUT and IOUT for design purposes as

follows:

VOUT(RIPPLE) +

IOUT(VOUT * VCC)

(COUT)(f)

)

(IOUT)(VOUT)(ESR)

VCC

The capacitor RMS ripple current is:

IRIPPLE + (IIN * IOUT)2(1 * D))(IOUT)2(D)

+ IOUT

VOUT * VCC

VCC

Although the above equations apply only for boost

circuits, similar equations can be derived for flyback

circuits.

Reducing the Current Limit

In some applications, the designer may prefer a lower

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

be connected between the VC pin and ground to reduce its

clamp voltage. Consequently, the current limit of the

internal power transistor current is reduced from its nominal

value.

The voltage on the VC pin can be evaluated with the

equation

VC + ISWREAV

where:

RE = .063

W, the value of the internal emitter resistor;

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

Since RE and AV cannot be changed by the end user, the

only available method for limiting switch current below

1.5 A is to clamp the VC 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 Figure 37, clamps the

VC voltage to a diode drop above the voltage on resistor R3.

Unfortunately, such a simple circuit is not generally

acceptable if VIN is loosely regulated.

Figure 37. Current Limiting using a Diode Clamp

VCC

VIN

Another solution to the current limiting problem is to

externally measure the current through the switch using a

sense resistor. Such a circuit is illustrated in Figure 38.

Figure 38. Current Limiting using a Current Sense

Resistor

RSENSE

VCC

VIN

Output

Ground

PGND

AGND

The switch current is limited to

ISWITCH(PEAK) +

VBE(Q1)

RSENSE

where:

VBE(Q1) = the base−emitter voltage drop of Q1, typically

0.65 V.

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