LTC1625_15 Datasheet, PDF (18/24 Pages) Linear Technology – Synchronous Step-Down Switching Regulator

LTC1625
APPLICATIONS INFORMATION
C_SS
0.1µF
INTV_CC
C_C1
470pF
R_C
10k
OPEN
C_C2
220pF
1
EXTV_CC
16
V_IN
2
SYNC
3
RUN/SS
15
TK
14
SW
4
LTC1625
13
FCB
TG
5
I_TH
12
BOOST
6
SGND
7
V_OSENSE
8
V_PROG
11
INTV_CC
10
BG
9
PGND
D_B
CMDSH-3
C_B
0.1µF
C_VCC
4.7µF
+
C_IN: AVX TPSE226M035R0300
C_OUT: AVX TPSD107M010R0065
L1: SUMIDA CDRH125-150MC
+
M1
Si4412DY
L1
15µH
C_IN
22µF
35V
×2
V_IN
12V TO 22V
V_OUT
3.3V
2A
M2
Si4412DY
+
C_OUT
100µF
10V
0.065Ω
×2
D1
MBRS140T3
1625 F09
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.
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