L6599AF datasheet(25/32 Pages) STMICROELECTRONICS

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L6599AF

Application information

kind. If the function is not used, the pin must be connected to a voltage greater than 1.24 V

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

6.7

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 (DBOOT, Figure 14 a) to

charge the bootstrap capacitor CBOOT. In the L6599AF 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 14 b.

Figure 14. Bootstrap supply: a) standard circuit; b) internal bootstrap synchronous diode

The diode prevents any current being able to flow from the VBOOT pin back to VCC, in case

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 14 b).

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.

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