L6910A L6910

6/21

Soft Start

value for this current is of 35

µA and speeds-up the charge of the capacitor up to 0.5V. After that it becames

10

µA until the final charge value of approximatively 4V.

MOS begins to switch and the output voltage starts to increase.

No switching activity is observable if SS is kept lower than 0.5V and both mosfets are off.

Start will not take place, and the relative pin is internally shorted to GND. During normal operation, if any under-

voltage is detected on one of the two supplies, the SS pin is internally shorted to GND and so the SS capacitor

is rapidly discharged.

Figure 2. Soft Start (with Reference Present)

Driver Section

The driver capability on the high and low side drivers allows using different types of power MOS (also multiple

The low-side mos driver is supplied directly by Vcc while the high-side driver is supplied by the BOOT pin.

Adaptative dead time control is implemented to prevent cross-conduction and allow to use several kinds of mos-

fets. The upper mos turn-on is avoided if the lower gate is over about 200mV while the lower mos turn-on is

avoided if the PHASE pin is over about 500mV. The lower mos is in any case turned-on after 200ns from the

high side turn-off.

The peak current is shown for both the upper (fig. 3) and the lower (fig. 4) driver at 5V and 12V. A 3.3nF capac-

itive load has been used in these measurements.

Similarly, for the upper driver, the source peak current is 1.3A @ Vboot-Vphase = 12V and 600mA @ Vboot-

Vphase = 5V, and the sink peak current is 1.3A @ Vboot-Vphase =12V and 550mA @ Vboot-Vphase = 5V.

Timing Diagram

Vcc Turn-on threshold

Vin Turn-on threshold

0.5V

1V

Vcc

Vin

Vss

LGATE

Vout

to GND