PM6680ATR datasheet(40/48 Pages) STMICROELECTRONICS

40 / 48 page

Design guidelines

PM6680A

40/48

Figure 40.

Bootstrap circuit

The bootstrap circuit capacitor value CBOOT must provide the total gate charge to the high

side MOSFET during turn on phase. A typical value is 100 nF.

The bootstrap diode D must charge the capacitor during the off time phases. The maximum

rated voltage must be higher than VINmax.

A resistor RBOOT on the BOOT pin could be added in order to reduce noise when the phase

node rises up, working like a gate resistor for the turn on phase of the high side MOSFET.

9.8

Design example

The following design example considers an input voltage from 16 V to 32 V(the typical value

is 24 V). The two switching outputs are OUT1 = 3.3 V and OUT2 = 1.8 V and must deliver a

maximum current of 2.5 A. The selected switching frequencies are about 290 kHz for OUT1

section and about 440 kHz for OUT2 section (see Table 6).

Inductor selection

OUT1: ILOAD = 2.5 A, 45 % ripple current.

Equation 36

We choose standard value L= 8.2

µH.

∆I

L(max) = 1.16 A @VIN = 24 V.

ILRMS = 2.523 A

Ipeak = 2.5 A + 0.58 A = 3.83 A

OUT2:ILOAD=2.5 A, 35 % ripple current.

CBBOOOOTT

RBBOOOOTT

PM6680ATR 数据表(PDF) 40 Page - STMicroelectronics