AN994

Bootstrap driver

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6

Bootstrap driver

A bootstrap circuitry is required to supply the high voltage section. This function is normally

accomplished by a high-voltage fast recovery diode (see Figure 12). In the L638xE, a

patented integrated structure replaces the external diode. This structure is comprised of a

high-voltage DMOS—driven synchronously with the low-side driver (LVG)—with a diode in

series, as shown in Figure 13.

An internal charge pump (also shown in Figure 13) provides the DMOS driving voltage. The

diode connected in series to the DMOS has been added to avoid current flowing in the

opposite direction.

6.1

loss. It must be:

to take into account the leakage losses.

Another example: HVG’s steady state consumption is lower than 200 µA (which is the case

for L6385E, L6386E and L6388E, whereas for L6384E and L6387E it is lower than 100 µA).

capacitor means a voltage drop of 1 V.

The internal bootstrap driver provides great advantages; it avoids use of the external fast

recovery diode (which usually has a high leakage current). This type of structure can only

charge the capacitor.

is 125

Ω). At low frequencies this drop is negligible, but when the frequency is increased it

must be taken into account.

The following equation is useful to compute the drop on the bootstrap DMOS.

C

EXT

Q

gate

V

gate

---------------

=

V

drop

I

ch

e

arg

R

DS on

()

V

drop

Q

gate

T

ch

e

arg

-------------------R

DS on

()

=

→

⋅

=