Datasheet

LTC3708
24
3708fb
APPLICATIONS INFORMATION
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3708. These items are also illustrated graphically in
Figure 14. Figure 15 further shows the current waveforms
present in the various branches of the 2-phase synchronous
Buck regulators operating in the continuous mode.
• Place the loop of M1, M3 and C
IN1
in a compact area.
This loop conducts high pulsating current and its area
needs to be minimized. Place M2, M4 and C
IN2
in the
same way.
• Place C
IN1
and C
IN2
within the distance of 1cm. Longer
distance may cause a large resonant loop.
• Connect the negative plates of C
OUT1
and C
DR1
to PGND1
before it joins PGND2 at the ground plane. Connect
C
OUT2
and C
DR2
in the same way so that power grounds
are separated before they meet at a single point.
• Cover the board area under the LTC3708 with a SGND
plane. For the LTC3708EUH, solder the back of the IC
to this plane. Separate SGND from the power ground
and connect all signal components (I
TH
, V
FB
, I
ON
, V
CC
,
EXTLPF, INTLPF, V
RNG
, TRACK and RUN/SS) to the
SGND plane before it joins PGND. Connect SGND to
the gound plane at a single point.
• Run SENSE
+
and SENSE
–
across the bottom MOSFET
(or R
SENSE
when a separate current sensing resistor
is used) with Kelvin connection (Figure 16). Route
SENSE
+
and SENSE
–
together with minimum PC trace
separation. The fi lter capacitor (when used) between
SENSE
+
and SENSE
–
should be as close to the LTC3708
as possible.
• Keep the high dV/dt nodes SW, TG and BOOST away
from sensitive small-signal nodes.
1μF
0.22μF
BOOST2
BAT54A
10Ω 4.7μF
PGND1
1μF
5V
TG1
17 4 31 21
14
13
15
16
20
18
19
11
12
7
30
10
8
20k
0.01μF
680pF0.1μF
10k
1%
C
IN
: UNITED CHEMI-CON THCR60EIH106ZT
C
OUT1
, C
OUT2
: SANYO POSCAP 4TPD470M
L1, L2: SUMIDA CEP125-1R0M
M1, M2: VISHAY Si4884
M3, M4: VISHAY Si4874
56pF
5V
PGOOD
100k
20k
1%
27
28
26
25
22
24
23
32
29
3
6
9
2
BOOST1
SW1
SENSE1
+
BG1
SENSE1
–
PGND1
V
RNG1
V
RNG2
I
ON1
V
FB1
TRACK2
EXTLPF
I
TH1
V
CC
TRACK1 FCB DRV
CC
LTC3708EUH
TG2
BOOST2
BOOST2
SW2
SENSE2
+
0.22μF
M2
M1
V
OUT1
2.5V
10A
V
IN
56pF
20k
L1
1μH
PGND1
L2
1μH
B340A
M3 M4
SENSE2
–
PGND2
V
RNG2
V
FB2
I
ON2
PWRGD
INTLPF
I
TH2
RUN/SS SGND
BG2
680pF
100pF
15
0.01μF
715k
31.6k
1%
R2
10k
1%
20k
1%
10k
1%
10k
511k 39k
V
IN
V
CC
11k
100pF 0.022μF
1nF
3708 F13
1μF
V
OUT1
1.8V
10A
C
OUT2
470μF
4V
22μF
6.3V
X7R
V
CC
B340A
+
C
OUT1
470μF
4V
C
IN
10μF
35V
s4
V
IN
7V TO 28V
22μF
6.3V
X7R
+
+
Figure 13. Design Example: 2.5V/10A and 1.8V/10A at 500kHz with Output Tracking