LTC3547 Datasheet, PDF (14/16 Pages) Linear Technology – Dual Monolithic 300mA Synchronous Step-Down Regulator

LTC3547
APPLICATIO S I FOR ATIO
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:
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 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 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
LTC3547
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
3547 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
OUTPUT CURRENT (mA)
1000
3547 F04b
Figure 4b. Efficiency vs Output Current
3547fa