L6668 datasheet(14/23 Pages) STMICROELECTRONICS

14 / 23 page

L6668

14/23

At converter power-down the system will lose regulation as soon as the input voltage is so low that either

peak current or maximum duty cycle limitation is tripped. Vcc will then drop and stop IC activity as it falls

below the UVLO threshold (8.7V typ.).

The Vcc_OK signal is de-asserted as the Vcc voltage goes below a threshold Vccrestart located at about

5V. The HV generator can now restart but, if Vin < Vinstart, as shown in figure 36, HV_EN is de-asserted

too and the HV generator is disabled.

This prevents converter's re-start attempts and ensures monotonic output voltage decay at power-down.

The low restart threshold Vccrestart ensures that, during short circuits, the restart attempts of the L6668 will

have a very low repetition rate, as shown in the timing diagram of figure 37, and that the converter will

work safely with extremely low power throughput.

4.2 Frequency Foldback Block and operation at medium/light load

At heavy load, namely as the voltage on pin COMP (VCOMP) is higher than 3V, the device works at a fixed

frequency like a standard current mode PWM controller.

As the load is reduced, and the VCOMP voltage falls below 3V (approximately corresponding to 50% of the

maximum load in a fully DCM system), the oscillator frequency can be made dependent on converter's

load conditions - the lower the load, the lower the frequency and vice versa.

Figure 38. Frequency foldback function: oscillator frequency is a function of COMP voltage

This is done by adding an external resistor RSTBY between pins RCT (#16) and STBY (#13), which acti-

vates the circuit shown in figure 38.

When VCOMP is below 3 V (oscillator's peak voltage) the voltage on the STBY pin, which is internally con-

nected to a current sink, tracks VCOMP and then some of the current that charges CT is diverted to ground

through the STBY pin.

In this way the rate of rise of the voltage across CT is slowed down and the oscillator frequency decreased,

the lower VCOMP the lower the frequency. Instead, when VCOMP is greater than 3 V the STBY pin features

high impedance and the oscillator frequency fosc will be determined by RT and CT. These components can

be then calculated as it is usually done with this type of oscillator:

VCOMP

fosc

1.4

3.0

4.4

RSTBY

3.0V

RSTBY

L6668

Vref

STBY

COMP

RCT

OSCILLATOR

VCOMP

fosc

1.4

3.0

4.4

RSTBY

3.0V

RSTBY

L6668

Vref

STBY

COMP

RCT

OSCILLATOR

T CT

1.4

osc

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