Datasheet

11
LTC3440
3440fb
SOFT-START
The soft-start function is combined with shutdown. When
the SHDN/SS pin is brought above typically 1V, the IC is
enabled but the EA duty cycle is clamped from the V
C
pin.
COMPONENT SELECTION
Figure 5. Soft-Start Circuitry
–
+
9
10
V
IN
ERROR AMP
1.22V
15μA
FB
R1
R2
C
P1
V
C
V
OUT
8
SHDN/SS
C
SS
1V
ENABLE SIGNAL
R
SS
SOFT-START
CLAMP
TO PWM
COMPARATORS
CHIP
ENABLE
3440 F05
–
+
V
CI
Inductor Selection
The high frequency operation of the LTC3440 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
L
VVV
f I Ripple V
H
L
VV V
f I Ripple V
H
IN MIN OUT IN MIN
OUT MAX OUT
OUT IN MAX OUT
OUT MAX IN MAX
>
()
μ
>
()
μ
() ()
()
()
() ()
•–
•••
,
•–
•••
where f = operating frequency, MHz
A detailed diagram of this function is shown in Figure 5.
The components R
SS
and C
SS
provide a slow ramping
voltage on the SHDN/SS pin to provide a soft-start
function.
OPERATIO
U
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Recommended Component Placement. Traces Carrying
High Current are Direct. Trace Area at FB and V
C
Pins are Kept
Low. Lead Length to Battery Should be Kept Short
3440 F06
GND
C2
D2
LTC3440
MULTIPLE
VIAS
L1
R
T
V
C
FB
SHDN/SS
V
IN
V
OUT
MODE/SYNC
SW1
GND
SW2
D1
V
IN
R1 R2
V
OUT
C1
1
2
3
4
5
10
9
8
7
6