Datasheet

18
LTC1625
APPLICATIONS INFORMATION
WUU
U
+
C
SS
0.1µF
R
C
10k
C
C1
470pF
C
C2
220pF
M1
Si4412DY
C
IN

22µF
35V
×2
V
IN

12V TO 22V
V
OUT

3.3V
2A
M2
Si4412DY
D1
MBRS140T3
D
B

CMDSH-3
C
VCC
4.7µF
1625 F09
OPEN
INTV
CC
V
IN
TK
EXTV
CC
LTC1625
SYNC
V
PROG
SW
TG
BOOST
INTV
CC
BG
RUN/SS
FCB
I
TH
SGND
V
OSENSE
PGND
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
C
B
0.1µF
L1
15µH
+
C
OUT

100µF
10V
0.065Ω
×2
+
C
IN
: AVX TPSE226M035R0300
C
OUT
: AVX TPSD107M010R0065
L1: SUMIDA CDRH125-150MC
Figure 9. 3.3V/2A Fixed Output at 225kHz
C
IN
is chosen for an RMS current rating of at least 1A at
temperature. C
OUT
is chosen with an ESR of 0.033Ω for
low output ripple. The output ripple in continuous mode
will be highest at the maximum input voltage and is
approximately:
∆V
O
= (∆I
L(MAX)
)(ESR) = (0.83A)(0.033Ω) = 27mV
The complete circuit is shown in Figure 9.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1625. These items are also illustrated graphically in
the layout diagram of Figure 10. Check the following in
your layout:
1) Connect the TK lead directly to the drain of the topside
MOSFET. Then connect the drain to the (+) plate of C
IN
.
This capacitor provides the AC current to the top
MOSFET.
2) The power ground pin connects directly to the source of
the bottom N-channel MOSFET. Then connect the source
to the anode of the Schottky diode and (–) plate of C
IN
,
which should have as short lead lengths as possible.
3) The LTC1625 signal ground pin must return to the (–)
plate of C
OUT
. Connect the (–) plate of C
OUT
to power
ground at the source of the bottom MOSFET
4) Keep the switch node SW away from sensitive small-
signal nodes. Ideally the switch node should be placed
on the opposite side of the power MOSFETs from the
LTC1625.
5) Connect the INTV
CC
decoupling capacitor C
VCC
closely
to the INTV
CC
pin and the power ground pin. This
capacitor carries the MOSFET gate drive current.
6) Does the V
OSENSE
pin connect directly to the (+) plate of
C
OUT
? In adjustable applications, the resistive divider
(R1, R2) must be connected between the (+) plate of
C
OUT
and signal ground. Place the divider near the
LTC1625 in order to keep the high impedance V
OSENSE
node short.
7) For applications with multiple switching power con-
verters connected to the same V
IN
, ensure that the input
filter capacitance for the LTC1625 is not shared with the
other converters. AC input current from another con-
verter will cause substantial input voltage ripple that
may interfere with proper operation of the LTC1625. A
few inches of PC trace or wire (≈100nH) between C
IN
and V
IN
is sufficient to prevent sharing.