L6599D datasheet(29/36 Pages) STMICROELECTRONICS

29 / 36 page

L6599

Application information

29/36

voltage on the pin exceeds 7V the device is shutdown. If its supply voltage is always above

the UVLO threshold, the IC will restart as the voltage falls below 7V.

The LINE pin, while the device is operating, is a high impedance input connected to high

value resistors, thus it is prone to pick up noise, which might alter the OFF threshold or give

origin to undesired switch-off of the IC during ESD tests. It is possible to bypass the pin to

ground with a small film capacitor (e.g. 1-10 nF) to prevent any malfunctioning of this kind. If

the function is not used the pin has to be connected to a voltage greater than 1.25V but

lower than 6V (worst-case value of the 7V threshold).

7.7

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 to charge the bootstrap

capacitor CBOOT. In the L6599 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 33.

Figure 33. Bootstrap supply: internal bootstrap synchronous diode

The diode prevents any current can flow from the VBOOT pin back to VCC in case that the

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

To drive the synchronous DMOS it is necessary a voltage higher than the supply voltage

VCC. This voltage is obtained by means of an internal charge pump (Figure 33).

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 r(DS)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 R(DS)ON of the

external high-side MOSFET and then its conductive loss.

L6599

OUT

VBOOT

Vcc

LVG

CBOOT

L6599D 데이터시트(PDF) 29 Page - STMicroelectronics