L6699D datasheet(32/38 Pages) STMICROELECTRONICS

32 / 38 page

Latched shutdown

L6699

32/38

Doc ID 022835 Rev 2

Latched shutdown

The L6699 is equipped with a comparator having the non-inverting input externally available

on pin 8 (DIS) and with the inverting input internally referenced to 1.85 V. As the voltage on

the pin exceeds the internal threshold, the IC is immediately shut down, the PFC_STOP pin

is asserted low and the quiescent consumption reduced to a low value. The information is

latched and it is necessary to let the voltage on the VCC pin go below the UVLO threshold to

reset the latch, de-assert the pin PFC_STOP, and restart the IC.

This function is useful to implement a latched overtemperature protection very easily by

biasing the pin with a divider from an external reference voltage (e.g. pin 4, RFmin), where

the upper resistor is an NTC physically located close to a heating element like the MOSFET,

or a secondary diode or the transformer.

An OVP can be implemented as well, e.g. by sensing the output voltage and transferring an

overvoltage condition via an optocoupler. A latch-mode OCP protection can be implemented

by connecting this pin to DELAY (pin 2).

Bootstrap section

The supply of the floating high-side section is obtained by means of a bootstrap circuitry.

This solution normally requires a high voltage fast-recovery diode (DBOOT, Figure 23) to

charge the bootstrap capacitor CBOOT. In the L6699 a patented integrated structure,

replaces this external diode. It is realized by means of a high voltage DMOS, working in the

third quadrant and driven synchronously with the low-side driver (LVG), with a diode in

series to the source, as shown in

Figure 24.

The diode prevents that any current can flow from the VBOOT pin back to VCC if the supply is

quickly turned off when the internal capacitor of the pump is not fully discharged. To drive

the synchronous DMOS, a voltage higher than the supply voltage VCC is necessary. This

voltage is obtained by means of an internal charge pump (

Figure 24).

The bootstrap structure introduces a voltage drop while recharging CBOOT (i.e. when the low

side driver is on), which increases with the operating frequency and with the size of the

external Power MOSFET. It is the sum of the drop across the RDS(on) and the forward drop

across the series diode. At low frequency this drop is very small and can be neglected but,

as the operating frequency increases, it must be taken into account. In fact, the drop

reduces the amplitude of the driving signal and can significantly increase the RDS(on) of the

external high-side MOSFET and then its conductive loss.

L6699D Datasheet(PDF) 32 Page - STMicroelectronics