Datasheet
15
LTC3717
sn3717 3717fs
(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 6.
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.
APPLICATIO S I FOR ATIO
WUUU
• The ground plane layer should not have any traces and
it should be as close as possible to the layer with power
MOSFETs.
• Place C
IN
, C
OUT
, MOSFETs, D1 and inductor all in one
compact area. It may help to have some components on
the bottom side of the board.
• Place LTC3717 chip with pins 9 to 16 facing the power
components. Keep the components connected to pins
1 to 8 close to LTC3717 (noise sensitive components).
•
Use an immediate via to connect the components to
ground plane including SGND and PGND of LTC3717.
Use several bigger vias for power components.
• Use compact plane for switch node (SW) to improve
cooling of the MOSFETs and to keep EMI down.
• Use planes for V
IN
and V
OUT
to maintain good voltage
filtering and to keep power losses low.
Figure 6. Design Example: 1.25V/±10A at 250kHz
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
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
+
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
3717 F06a
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