Datasheet

LTC3816
38
3816f
applicaTions inForMaTion
For the synchronous MOSFETs, assume that the two
bottom MOSFETs share the inductor current equally. The
power dissipation for one MOSFET is:
P
V V
V
I R
P
SYNC
IN OUT
IN
LOAD MAX DS ON
SYNC
=
( )
+
( )
–
( ) ( )
2
1 δ
==
( )
+
( )
=
12 0 75
12
13 5 1 0 25 2 8
0 598
2
V V
V
A m
W
– .
. . .
.
Ω
The total power dissipation of the bottom MOSFETs is
2 × 0.598W = 1.196W.
For this application, the maximum input RMS current
happens when V
IN
= V
IN(MIN)
and can be determined from
the formula:
I I
V V V
V
RMS MAX LOAD MAX
OUT IN MIN OUT
IN MIN
( ) ( )
( )
( )
–
≈
( )
II A
V V V
V
A
RMS MAX( )
. – .
.≈
( )
≈27
0 75 5 0 75
5
9 64
The minimum RMS current rating of the input capacitor
must exceed 9.64A. To meet this ripple current requirement
with V
IN(MAX)
= 24V, select two Sanyo OS-CON 25SVP56
capacitors or higher voltage rating capacitor as the input
supply bulk capacitance. In addition, place a couple of
high performance ceramic capacitors in parallel with the
bulk capacitors.
The output capacitor value is determined by:
C C
L I
V V
BULK CER
LOAD
OUT OUT LOAD
+ =
( )
( )
∆
∆
2
2
( )
Most regulator designs allow a slight transient overshoot
for a short duration. If this is limited to 40mV, we have:
∆V AVP I I
mV
A
A
OUT AVP LOAD MAX LOAD MIN( ) ( ) ( )
• –
•
=
( )
= 3 27 –– . .
( ) ( )
1 5 76 5A mV
V V V
OUT LOAD OUT AVP OVERSHO
( )
=
= +∆ ∆ ∆
OOT
BULK CER
mV mV mV
C C
µH A
= + =
+ =
76 5 40 116 5
0 33 27 1 5
. .
. – .
AA
V mV
µF
( )
( )( )
=
2
2 0 75 116 5
1228
. .
The ESR of the output capacitor is determined by the load
transient requirement. If the output voltage jump due to
the capacitor ESR is limited to ∆V
OUT(AVP)
:
R
V
I
mV
A A
m
ESR
OUT AVP
LOAD
< =
( )
=
∆
∆
Ω
( )
– .
.
75
27 1 5
2 94
The above requirements are easily satisfied by three
Sanyo POSCAP 2TPF330M6 330µF (ESR = 6mΩ) bulk
capacitors in parallel, twenty 10µF and some 1µF high
performance ceramic capacitors in the processor socket
cavity. With three bulk capacitors in parallel, the effective
ESR is 2mΩ, and the maximum steady-state output ripple
voltage is given by:
∆ ∆V I R
f C C
A
OUT L ESR
OSC BULK CER
≈ +
+
( )








=
1
8
5 5
• •
.
22
1
8 400 3 330 20 10
11 1 4
m
kHz µF µF
mV
Ω+
+
( )






= +
• • • •
. 44 12 44mV mV= .