PM6681A STMicroelectronics, PM6681A Datasheet - Page 33

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

PM6681A
Maximum conduction losses are approximately:
Equation 20
where R
are approximately:
Equation 21
where ton and toff are the switching times of the turn off and turn off phases of the MOSFET.
As general rule, high side MOSFETs with low gate charge are recommended, in order to
minimize driver losses.
Below there is a list of possible choices for the high side MOSFET.
Table 14.
The power dissipation of the low side MOSFET is given by:
Equation 22
Maximum conduction losses occur at the maximum input voltage:
Equation 23
Choose a synchronous rectifier with low R
variation of the phase node voltage can bring up even the low side gate through its gate
drain capacitance CRSS, causing cross-conduction problems. Choose a low side MOSFET
that minimizes the ratio C
Below there is a list of some possible low side MOSFETs.
Manufacturer
P
switching
DS(on)
ST
ST
High side MOSFET manufacturer
is the drain-source on resistance of the high side MOSFET. Switching losses
=
V
IN
×
I (
P
LOAD
conduction
STS12NH3LL
STS17NH3LL
RSS
P
conduction
(max)
Type
/C
GS
2
=
−
(C
R
P
=
∆
DSon
DLowSide
2
GS
I
R
L
DSon
)
= C
×
×
DS(on)
t
⎛
⎜
⎜
⎝
on
1
×
Gate charge (nC)
ISS
−
=
×
V
V
P
f
V
IN
sw
. When high side MOSFET turns on, the fast
OUT
V
- C
conduction
IN
min
OUT
max
+
RSS
10
18
×
V
I
IN
⎞
⎟
⎟
⎠
LOAD
).
×
×
I
LOAD
I (
LOAD
(max)
(max)
(max)
2
Rated reverse voltage (V)
2
2
+
∆
2
Design guidelines
I
L
)
×
30
30
t
off
×
f
sw
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