AN2785 datasheet(11/51 Pages) STMICROELECTRONICS

11 / 51 page

AN2785

Bootstrap driver

Doc ID 14785 Rev 1

11/51

For example: given the typical value of 120

Ω for R

DS(on) and using a power MOS with a

total gate charge of 30 nC, the drop on the bootstrap DMOS is about 1 V if the TCHARGE is

6 ms. In fact:

Equation 4

VDROP has to be considered when the voltage drop on CBOOT is calculated: if this drop is too

high, or the circuit topology does not allow a sufficient charging time, an external diode can

be used.

When operating at very low frequencies, the high side driver ON time of the high-side driver

can be very long. Therefore the CBOOT voltage can drop because of HVG steady-state

consumption. To avoid extremely large capacitors (> 1-2 µF), an external charge pump can

be added (see Figure 7 as example). It is mandatory for the diodes to have a low parasitic

capacitance since C1 and C2 should be greater than the capacitance of the diodes. The

oscillator has to work in order to balance the high-voltage side consumption, and the

minimum frequency is fixed by the values of C1 and C2 (with C1 = C2 = 33 pF then f > 250

– 300 kHz).

Figure 6.

Bootstrap driver

120

VDROP

≈

⋅

TO LOAD

BOOT

H.V.

HVG

LVG

HVG

LVG

CBOOT

TO LOAD

H.V.

CBOOT

DBOOT

VCC

OUT

VCC

OUT

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