L6599AD datasheet(28/35 Pages) STMICROELECTRONICS

28 / 35 page

Application information

L6599A

28/35

Doc ID 15308 Rev 7

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).

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 (DBOOT, Figure 31a) to

charge the bootstrap capacitor CBOOT. In the L6599A 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 31b.

Figure 31.

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 31b).

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.

This concern applies to converters designed with a high resonance frequency (indicatively,

> 150 kHz), so that they run at high frequency also at full load. Otherwise, the converter runs

at high frequency at light load, where the current flowing in the MOSFETs of the half bridge

leg is low, so that, generally, an R(DS)ON rise is not an issue. However, it is wise to check this

point anyway and the following equation is useful to compute the drop on the bootstrap

driver:

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