CS5172GD8 datasheet(15/21 Pages) ONSEMI

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CS5171, CS5172, CS5173, CS5174

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The improved circuit does not require a regulated voltage

to operate properly. Unfortunately, a price must be paid for

this convenience in the overall efficiency of the circuit. The

designer should note that the input and output grounds are

no longer common. Also, the addition of the current sense

resistor, RSENSE, results in a considerable power loss which

increases with the duty cycle. Resistor R2 and capacitor C3

form a low−pass filter to remove noise.

Subharmonic Oscillation

Subharmonic oscillation (SHM) is a problem found in

current−mode control systems, where instability results

when duty cycle exceeds 50%. SHM only occurs in

switching regulators with a continuous inductor current.

This instability is not harmful to the converter and usually

does not affect the output voltage regulation. SHM will

increase the radiated EM noise from the converter and can

cause, under certain circumstances, the inductor to emit

high−frequency audible noise.

SHM is an easily remedied problem. The rising slope of

the inductor current is supplemented with internal “slope

compensation” to prevent any duty cycle instability from

carrying through to the next switching cycle. In the CS517x

family, slope compensation is added during the entire switch

on−time, typically in the amount of 180 mA/

ms.

In some cases, SHM can rear its ugly head despite the

presence of the onboard slope compensation. The simple

cure to this problem is more slope compensation to avoid the

unwanted oscillation. In that case, an external circuit, shown

in Figure 39, can be added to increase the amount of slope

compensation used. This circuit requires only a few

components and is “tacked on” to the compensation

network.

Figure 39. Technique for Increasing Slope

Compensation

VSW

The dashed box contains the normal compensation

circuitry to limit the bandwidth of the error amplifier.

Resistors R2 and R3 form a voltage divider off of the VSW

pin. In normal operation, VSW looks similar to a square

wave, and is dependent on the converter topology. Formulas

for calculating VSW in the boost and flyback topologies are

given in the section “VSW Voltage Limit.” The voltage on

VSW charges capacitor C3 when the switch is off, causing

the voltage at the VC pin to shift upwards. When the switch

turns on, C3 discharges through R3, producing a negative

slope at the VC pin. This negative slope provides the slope

compensation.

The amount of slope compensation added by this circuit

DT +

VSW

R2)R3

* e

*(1*D)

R3C3fSW

fSW

* D)REAV

where:

DI/DT = the amount of slope compensation added (A/s);

VSW = the voltage at the switch node when the transistor

is turned off (V);

fSW = the switching frequency, typically 280 kHz

(CS5171/3) or 560 kHz (CS5172/4) (Hz);

D = the duty cycle;

RE = 0.063

W, the value of the internal emitter resistor;

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

In selecting appropriate values for the slope compensation

network, the designer is advised to choose a convenient

capacitor, then select values for R2 and R3 such that the

amount of slope compensation added is 100 mA/

ms. Then

R2 may be increased or decreased as necessary. Of course,

the series combination of R2 and R3 should be large enough

to avoid drawing excessive current from VSW. Additionally,

to ensure that the control loop stability is improved, the time

constant formed by the additional components should be

chosen such that

R3C3 t

* D

fSW

Finally, it is worth mentioning that the added slope

compensation is a tradeoff between duty cycle stability and

transient response. The more slope compensation a designer

adds, the slower the transient response will be, due to the

external circuitry interfering with the proper operation of the

error amplifier.

Soft−Start

Through the addition of an external circuit, a Soft−Start

function can be added to the CS5171/2/3/4 family of

components. Soft−Start circuitry prevents the VC pin from

slamming high during startup, thereby inhibiting the

inductor current from rising at a high slope.

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