CS5172GD8 datasheet(16/21 Pages) ONSEMI

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CS5171, CS5172, CS5173, CS5174

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This circuit, shown in Figure 40, requires a minimum

number of components and allows the Soft−Start circuitry to

activate any time the SS pin is used to restart the converter.

Figure 40. Soft Start

VCC

VIN

Resistor R1 and capacitors C1 and C2 form the

compensation network. At turn on, the voltage at the VC pin

starts to come up, charging capacitor C3 through Schottky

diode D2, clamping the voltage at the VC pin such that

switching begins when VC reaches the VC threshold,

typically 1.05 V (refer to graphs for detail over temperature).

VC + VF(D2))VC3

Therefore, C3 slows the startup of the circuit by limiting

the voltage on the VC pin. The Soft−Start time increases with

the size of C3.

Diode D1 discharges C3 when SS is low. If the shutdown

function is not used with this part, the cathode of D1 should

be connected to VIN.

Calculating Junction Temperature

To ensure safe operation of the CS5171/2/3/4, the

designer must calculate the on−chip power dissipation and

determine its expected junction temperature. Internal

thermal protection circuitry will turn the part off once the

junction temperature exceeds 180

°C ± 30°. However,

repeated operation at such high temperatures will ensure a

reduced operating life.

Calculation of the junction temperature is an imprecise

but simple task. First, the power losses must be quantified.

There are three major sources of power loss on the CS517x:

• biasing of internal control circuitry, PBIAS

• switch driver, PDRIVER

• switch saturation, PSAT

The internal control circuitry, including the oscillator and

linear regulator, requires a small amount of power even

when the switch is turned off. The specifications section of

this datasheet reveals that the typical operating current, IQ,

due to this circuitry is 5.5 mA. Additional guidance can be

found in the graph of operating current vs. temperature. This

graph shows that IQ is strongly dependent on input voltage,

VIN, and temperature. Then

PBIAS + VINIQ

Since the onboard switch is an NPN transistor, the base

drive current must be factored in as well. This current is

drawn from the VIN pin, in addition to the control circuitry

current. The base drive current is listed in the specifications

DICC/DISW, or switch transconductance. As before, the

designer will find additional guidance in the graphs. With

that information, the designer can calculate

PDRIVER + VINISW

ICC

DISW

where:

ISW = the current through the switch;

D = the duty cycle or percentage of switch on−time.

ISW and D are dependent on the type of converter. In a

boost converter,

ISW(AVG) ^ ILOAD

Efficiency

VOUT * VIN

VOUT

In a flyback converter,

ISW(AVG) ^

VOUTILOAD

VIN

Efficiency

VOUT

VOUT )

VIN

The switch saturation voltage, V(CE)SAT, is the last major

source

on−chip

power

loss.

V(CE)SAT is the

collector−emitter voltage of the internal NPN transistor

when it is driven into saturation by its base drive current. The

value for V(CE)SAT can be obtained from the specifications

or from the graphs, as “Switch Saturation Voltage.” Thus,

PSAT ^ V(CE)SATISW

Finally, the total on−chip power losses are

PD + PBIAS)PDRIVER)PSAT

Power dissipation in a semiconductor device results in the

generation of heat in the junctions at the surface of the chip.

This heat is transferred to the surface of the IC package, but

a thermal gradient exists due to the resistive properties of the

package molding compound. The magnitude of the thermal

gradient is expressed in manufacturers’ data sheets as

qJA,

or junction−to−ambient thermal resistance. The on−chip

junction temperature can be calculated if

qJA, the air

temperature near the surface of the IC, and the on−chip

power dissipation are known.

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