LTC3440 Datasheet, PDF (11/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

U
OPERATIO
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.
LTC3440
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.
V_IN
15μA
SOFT-START
CLAMP
TO PWM
V_CI
COMPARATORS
CHIP
ENABLE
ERROR AMP
1.22V
FB
9
V_C ^CP1
10
V_OUT
R1
R2
SHDN/SS
8
R_SS
C_SS
ENABLE SIGNAL
3440 F05
1V
Figure 5. Soft-Start Circuitry
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
L1
D1
MULTIPLE
VIAS
1 R_T
LTC3440
2 MODE/SYNC
V_C 10
FB 9
3 SW1
SW2
4
5 GND
D2 ^{SHDN/SS 8}
V_IN 7
V_OUT 6
C1
C2
V_OUT
R1
R2
V_IN
GND
3440 F06
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
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 > VIN(MIN) • VOUT – VIN(MIN) μH,
f • I_OUT(MAX) • Ripple • V_OUT
( ) _{L >} ^VOUT ^{• V}IN(MAX) ^{– V}OUT
μH
f • I_OUT(MAX) • Ripple • V_IN(MAX)
where f = operating frequency, MHz
3440fb
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