Datasheet

LTC3631
16
3631fe
For more information www.linear.com/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
V
IN(RISING)
– 1.21V
• R1= 224k
R3 =
1.1V
V
IN(FALLING)
– 1.1V
• R1– R2 = 94.8k
V
IN
LTC3631
RUN
HYST
SS
3631 F09a
SW
V
IN
C
IN
C
SS
V
OUT
R
SET
C
OUT
V
FB
I
SET
GND
R1
L1
R2
VIAS TO GROUND PLANE
VIAS TO INPUT SUPPLY (V
IN
)
OUTLINE OF LOCAL GROUND PLANE
V
IN
C
IN
C
SS
R
SET
C
OUT
V
OUT
L1
GND
R1
R2
Example Layout
Figure 9. 24V to 3.3V, 100mA Regulator at 250kHz
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.
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.
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).
V
IN
LTC3631
RUN
2M
1µF
226k
95.3k
HYST
3631 F09
SW
V
IN
24V
V
OUT
3.3V
100mA
I
SET
SS
V
FB
GND
750k
10µF
47µH
240k