L5993 datasheet(13/22 Pages) STMICROELECTRONICS

13 / 22 page

Pin 16. C-POWER (Constant Power Function).

An external capacitor connected between this pin

and SGND completes the peak-holding circuit that

detects the peak voltage of the synchronized os-

cillator. The circuit gets a DC voltage (which de-

creases as the synchronizing frequency fed into

pin 1 (SYNC) rises) used to clamp the error ampli-

fier output (VCOMP), as shown in the detailed inter-

nal schematic of fig. 30.

In this way the pulse-by-pulse setpoint is moved

downwards as the frequency rises (and vice versa for

a frequency decrease, due to the 47k

Ω dischargere-

sistor) and, as a result, the maximum power deliver-

ableto the load is held roughly constant.

The external capacitor must be large enough to

get a real DC voltage on the pin. Considering the

spread of the internal 47k

Ω resistor, the minimum

capacitance value (CCP) needed to have less than

1% ripple superimposed on the DC voltage is:

CCP

330

⋅ƒmin

where

ƒmin (Hz) is the minimum synchronizing fre-

quency.

When this function is not used, pin 16 has to be

connected directly to pin 4.

Considering the ordinary design criteria for the

transformer, the circuit usually works well without

any adjustment. Anyway, the variations of the

maximum power limit on varying the switching fre-

quency and/or the mains voltage can be mini-

mized by modifying one or more of the following

parameters:

- Primary inductance;

- Transformer turns ratio;

- Oscillator free-running frequency;

- Sense resistor.

A trial process is required, involving the parame-

ters that are more practicable to modify. In fact,

the optimum behavior is achieved for a specific

combination of the above parameters and de-

V15=GND

V5=V13=GND

V15=VREF

V5=V13=GND

V10

DX =

tc +td

DX =

2·tc +td

D97IN498

Figure 29. Half duty cycle option.

E/A

2.5V

VFB

COMP

30K

Ω

30K

Ω

VREF

C-POWER

CCP

BUFFER

47K

Ω

RCT

15K

Ω

TIMING

CLAMP

PWM

COMPARATOR

TO PWM

LATCH

L5993

113

SYNC

ISEN

D97IN768A

Figure 30. Constant Power circuit internal schematic

L5993

13/22

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