L6599ATN datasheet(25/31 Pages) STMICROELECTRONICS

25 / 31 page

L6599AT

Application information

Doc ID 15534 Rev 3

25/31

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 bootstrap circuitry. This

solution normally requires a high voltage fast recovery diode (DBOOT, Figure 14a) to charge

the bootstrap capacitor CBOOT. In the L6599AT, a patented integrated structure replaces this

external diode. It is achieved 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 14b.

Figure 14.

Bootstrap supply: a) standard circuit;

b) internal bootstrap synchronous diode

The diode prevents any current flowing 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 14b).

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 MOS. 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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