16
LTC3717-1
sn37171 37171fs
APPLICATIO S I FOR ATIO
WUUU
Tying V
RNG
to 1.1V will set the current sense voltage range
for a nominal value of 110mV with current limit occurring
at 143mV. To check if the current limit is acceptable,
assume a junction temperature of about 40°C above a
70°C ambient with ρ
110°C
= 1.4:
I
mV
AA
LIMIT
≥
Ω
+=
143
14 0010
1
2
3 7 12 1
(.)(. )
(. ) .
and double check the assumed T
J
in the MOSFET:
P
VV
V
AW
BOT
=Ω=
25 125
25
12 1 1 4 0 010 1 02
2
.–.
.
(.)(.)(. ) .
T
J
= 70°C + (1.02W)(40°C/W) = 111°C
Because the top MOSFET is on roughly the same amount
of time as the bottom MOSFET, the same Si4874 can be
used as the synchronous MOSFET.
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary in this circuit.
C
IN
is chosen for an RMS current rating of about 5A at
85°C. The output capacitors are chosen for a low ESR of
0.013Ω to minimize output voltage changes due to induc-
tor ripple current and load steps. For current sinking
applications where current flows back to the input through
the top transistor, output capacitors with a similar amount
of bulk C and ESR should be placed on the input as well.
(This is typically the case, since V
IN
is derived from
another DC/DC converter.) The ripple voltage will be only:
∆V
OUT(RIPPLE)
= ∆I
L(MAX)
(ESR)
= (4A) (0.013Ω) = 52mV
However, a 0A to 10A load step will cause an output
change of up to:
∆V
OUT(STEP)
= ∆I
LOAD
(ESR) = (10A) (0.013Ω) = 130mV
An optional 22µF ceramic output capacitor is included to
minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 7.
PC Board Layout Checklist
When laying out a PC board follow one of the two sug-
gested approaches. The simple PC board layout requires
a dedicated ground plane layer. Also, for higher currents,
it is recommended to use a multilayer board to help with
heat sinking power components.
Figure 7. Design Example: 1.25V/±10A at 250kHz
RUN/SS
PGOOD
V
RNG
I
TH
SGND
I
ON
V
FB
V
REF
BOOST
TG
SW
PGND
BG
INTV
CC
V
CC
EXTV
CC
LTC3717-1
SENSE
+
SENSE
–
V
ON
DRV
CC
+
M2
Si4874
M1
Si4874
L1
0.68µH
D1
B320A
D2
B320A
C
OUT1-2
270µF
2V
×2
C
OUT3
22µF
6.3V
X7R
C
IN
22µF
6.3V
X7R
V
EXT
5V
V
IN
= 2.5V
V
OUT
1.25V
±10A
C
SS
0.1µF
C
C1
470pF
C
ON
0.01µF
C
C2
100pF
C
VCC
4.7µF
C
F
0.1µF
(OPT)
0.1µF
C
B
0.22µF
R
C
20k
R
ON
511k
R
F
1Ω
D
B
CMDSH-3
37171 F07
C
IN
, C
OUT1-2
: CORNELL DUBILIER ESRE181E04B
L1: SUMIDA CEP125-0R68MC-H
R
PG
100k
R3
11k
R4
39k
+
+
C
IN
180µF
4V
×2
10Ω
