LTC3631_15 Datasheet, PDF (16/22 Pages) Linear Technology – High Efficiency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
Applications Information
The undervoltage lockout requirement on V_IN can be
satisfied with a resistive divider from V_IN to the RUN
and HYST pins. Choose R1 = 2M and calculate R2 and
R3 as follows:
R2 =
1.21V
• R1= 224k
V_IN(RISING) – 1.21V
R3 =
1.1V
• R1– R2 = 94.8k
V_IN(FALLING) – 1.1V
Choose standard values for R2 = 226k and R3 = 95.3k.
The I_SET pin should be left open in this example to select
maximum peak current (225mA). Figure 9 shows a com-
plete schematic for this design example.
V_IN
24V
V_IN
SW
2M
LTC3631
47µH
V_OUT
3.3V
100mA
1µF
RUN
I_SET
10µF
226k
SS
750k
HYST
V_FB
95.3k
GND
240k
3631 F09
2. Connect the (+) terminal of the input capacitor, C_IN, as
close as possible to the V_IN pin. This capacitor provides
the AC current into the internal power MOSFETs.
3. Keep the switching node, SW, away from all sensitive
small signal nodes. The rapid transitions on the switching
node can couple to high impedance nodes, in particular
V_FB, and create increased output ripple.
4. Flood all unused area on all layers with copper. Flooding
with copper will reduce the temperature rise of power
components. You can connect the copper areas to any
DC net (V_IN, V_OUT, GND or any other DC rail in your
system).
Example Layout
L1
V_IN
V_IN
SW
V_OUT
LTC3631
RUN
R1
C_IN
HYST
V_FB
C_OUT
SS
I_SET
C_SS
GND
R2
R_SET
3631 F09a
Figure 9. 24V to 3.3V, 100mA Regulator at 250kHz
V_IN
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3631. Check the following in your layout:
1. Large switched currents flow in the power switches
and input capacitor. The loop formed by these compo-
nents should be as small as possible. A ground plane
is recommended to minimize ground impedance.
L1
C_IN
C_OUT
V_OUT
R_SET C_SS
GND
VIAS TO GROUND PLANE
VIAS TO INPUT SUPPLY (V_IN)
OUTLINE OF LOCAL GROUND PLANE
R1
R2
3631fe
16
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