Datasheet
LTC3547
14
3547fa
APPLICATIO S I FOR ATIO
WUU
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Design Example
As a design example, consider using the LTC3547 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:
L
V
MHz mA
V
V
1
25
2 25 120
1
25
42
37=−
⎛
⎝
⎜
⎞
⎠
⎟
=
.
.•()
•
.
.
.55µH
For the inductor, use the closest standard value of 4.7µH.
A 4.7µF capacitor should be more than suffi cient for this
output capacitor. As for the input capacitor, a typical value
of C
IN
= 4.7µF should suffi ce, as the source impedance of
a Li-Ion battery is very low.
The feedback resistors program the output voltage. To
maintain high effi ciency 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:
R
V
Rk
OUT
2
06
1 1 887=−
⎛
⎝
⎜
⎞
⎠
⎟
=
.
•
An optional 10pF feedback capacity (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 effi ciency curve and transient response.
Figure 4a. Design Example Circuit
Figure 4b. Effi ciency vs Output Current
V
IN
RUN2 RUN1
LTC3547
V
FB2
SW2
SW1
V
FB1
C
F2
, 10pF C
F1
, 10pF
GND
V
IN
2.5V TO 5.5V
V
OUT2
1.8V AT 300mA
V
OUT1
2.5V AT 300mA
3547 F04a
R3
280k
R1
280k
R4
562k
L2
4.7
µH
L1
4.7µH
R2
887k
C
OUT2
4.7µF
C1
4.7
µF
C
OUT1
4.7µF
C1, C2, C3: TAIYO YUDEN JMK316BJ475ML L1, L2: MURATA LQH32CN4R7M33
OUTPUT CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.1 10 100 1000
3547 F04b
0
1
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 4.2V
V
OUT
= 2.5V
OUTPUT CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.1 10 100 1000
0
1
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 4.2V
V
OUT
= 1.8V