LTC3547B
14
3547bfb
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
OUTPUT CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.1 10 100 1000
3547b F04b
0
1
V
IN
= 2.7V
V
IN
= 3.6V
V
IN
= 4.2V
V
OUT
= 2.5V
APPLICATIO S I FOR ATIO
WUU
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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 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.
Figure 4a. Design Example Circuit
Figure 4b. Effi ciency vs Output Current
V
IN
RUN2 RUN1
LTC3547B
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
3547b 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
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.
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 effi ciency curve and transient response.
