LTC3440_15 Datasheet, PDF (11/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3440
Operation
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. 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
SOFT-START
CLAMP
TO PWM
V_CI
COMPARATORS
15µA
ERROR AMP
1.22V
FB
9
V_C ^CP1
10
V_OUT
R1
R2
SHDN/SS
8
CHIP
ENABLE
1V
Figure 5. Soft-Start Circuitry
R_SS
C_SS
ENABLE SIGNAL
3440 F05
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor 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
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
3440fc
For more information www.linear.com/LTC3440
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