LTC3547B_15 Datasheet, PDF (14/16 Pages) Linear Technology – Dual Monolithic 300mA Synchronous Step-Down Regulator

LTC3547B
APPLICATIO S I FOR ATIO
Design Example
As a design example, consider using the LTC3547B in
a portable application with a Li-Ion battery. The battery
provides a V_IN ranging from 2.8V to 4.2V. The load on each
channel requires a maximum of 300mA in active mode
and 2mA in standby mode. The output voltages are V_OUT1
= 2.5V and V_OUT2 = 1.8V.
Start with channel 1. First, calculate the inductor value
for about 40% ripple current (120mA in this example) at
maximum V_IN. Using a derivation of Equation 1:
L1 =
2.5V
2.25MHz • (120mA)
•


1

2.5V 
4.2V 
=
3.75μH
For the inductor, use the closest standard value of 4.7μH.
A 4.7μF capacitor should be more than sufficient for this
output capacitor. As for the input capacitor, a typical value
of C_IN = 4.7μF should suffice, as the source impedance of
a Li-Ion battery is very low.
The feedback resistors program the output voltage. To
maintain high efficiency at light loads, the current in these
resistors should be kept small. Choosing 2μA with the
0.6V feedback voltage makes R1~300k. A close standard
1% resistor is 280k, using Equation 4.
R2 =


V_OUT
0.6

1^
• R1=
887k
An optional 10pF feedback capacitor (C_F1) may be used
to improve transient response.
Using the same analysis for channel 2 (V_OUT2 = 1.8V),
the results are:
L2 = 3.81μH
R3 = 280k
R4 = 560k
Figure 4 shows the complete schematic for this example,
along with the efficiency curve and transient response.
V_IN
2.5V TO 5.5V
V_OUT2
1.8V AT 300mA
C1
4.7μF
L2
4.7μH
C_F2, 10pF
RUN2 V_IN RUN1
LTC3547B
SW2
SW1
L1
4.7μH
C_F1, 10pF
V_OUT1
2.5V AT 300mA
C_OUT2 R4
4.7μF 562k
V_FB2
V_FB1
R3
GND
280k
R1 R2
280k 887k
C_OUT1
4.7μF
3547b F04a
C1, C2, C3: TAIYO YUDEN JMK316BJ475ML
L1, L2: MURATA LQH32CN4R7M33
Figure 4a. Design Example Circuit
14
100
V_OUT = 1.8V
90
80
70
60
50
40
30
20
10
0
0.1
V_IN = 2.7V
V_IN = 3.6V
V_IN = 4.2V
1
10
100
OUTPUT CURRENT (mA)
1000
100
V_OUT = 2.5V
90
80
70
60
50
40
30
20
10
0
0.1
V_IN = 2.7V
V_IN = 3.6V
V_IN = 4.2V
1
10
100
1000
OUTPUT CURRENT (mA)
3547b F04b
Figure 4b. Efficiency vs Output Current
3547bfb