PM6681A STMicroelectronics, PM6681A Datasheet - Page 39

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

PM6681A
9.8
9.8.1
Typical components values are: R = 47 Ω and C = 1 µF.
●
●
●
Figure 43. Bootstrap circuit
The bootstrap circuit capacitor value C
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 V
A resistor R
node rises up, working like a gate resistor for the turn on phase of the high side MOSFET.
Design example
The following design example considers an input voltage from 7 V to 16 V. The two switching
outputs are OUT1 = 1.5 V and OUT2 = 1.05 V and must deliver a maximum current of 5 A.
The selected switching frequencies are about 290 kHz for OUT1 section and about 425 kHz
for OUT2 section (see Table 1).
Inductor selection
OUT1: I
We choose standard value L = 8.2 µH.
∆I
I
LRMS
L(max)
VREF capacitor. A 10 nF to 100 nF ceramic capacitor on V
ensure noise rejection.
LDO5 output capacitors. Bypass the output of each linear regulator with 1 µF ceramic
capacitor closer to the LDO pin and a 4.7 µF tantalum capacitor (ESR = 2 Ω). In most
applicative conditions a 4.7 µF ceramic output capacitor can be enough to ensure
stability.
Bootstrap circuit. The external bootstrap circuit is represented in the next figure:
= 2.53 A
LOAD
= 1.16 A @ V
BOOT
= 2.5 A, 45 % ripple current.
on the BOOT pin could be added in order to reduce noise when the phase
IN
= 24 V.
L
L
L
=
290
3
KHz
3 .
INmax
V
BOOT
⋅
C
C
⋅
(
20
24
R
B
B
.
R
O
O
B
B
O
O
O
O
T
T
V
V
O OT
must provide the total gate charge to the high
T
−
⋅
. 0
D
D
3
45
3 .
) V
⋅
B
P
B
P
2
O
H
O
H
5 .
O
A
A
O
L
L
S
S
T
D
D
T
≈
E
E
O
O
8
5
5
2 .
µ
H
REF
pin must be added to
Design guidelines
39/47