Datasheet

LTC3619B
16
3619bfb
applicaTions inForMaTion
Design Example
As a design example, consider using the LTC3619B in a
USB-GSM application, with V
IN
= 5V, I
INMAX
= 500mA, with
the output of channel 2 charging a SuperCap of 4.4mF.
The load on each channel requires a maximum of 400mA
and 800mA in active mode and 2mA in standby mode.
The output voltages are V
OUT1
= 1.8V and V
OUT2
= 3.4V.
Start with channel 1. First, calculate the inductor value
for about 40% ripple current (160mA in this example) at
maximum V
IN
. Using a derivation of Equation 1:
L1=
1.8V
2.25MHz • (160mA)
• 1−
1.8V
5V






= 3.2µH
For the inductor, use the closest standard value of 3.3µH.
A 10µF ceramic capacitor should be more than sufficient
for this output capacitor. As for the input capacitor, a
typical value of C
IN
= 10µF should suffice, if the source
impedance 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 10µA with the
Figure 4a. Design Example Circuit
0.6V feedback voltage makes R1~60k. A close standard
1% resistor is 59k. Using Equation 2.
R2=
V
OUT
0.6
− 1






• R1= 118k
An optional 22pF feedback capacitor (C
F1
) may be used
to improve transient response.
Using the same analysis for channel 2 (V
OUT2
= 3.4V),
the results are:
L2 = 1.5µH
R3 = 59k
R4 = 276k
A feed forward capacitor is not used on channel 2 since
the 4.4mF SuperCap will inhibit any fast output voltage
transients. Figure 4 shows the complete schematic for
this example, along with the efficiency curve and transient
response. Input current limit is set at 475mA average cur-
rent, R
LIM
= 116k, C
LIM
= 2200pF. See Programming Input
Current Limit section for selecting R
LIM
and Selection of
C
LIM
Capacitance section for C
LIM
.
V
IN
RUN2 RUN1
LTC3619B
V
FB2
SW2
SW1
PGOOD1PGOOD2
V
FB1
C
F1
, 22pF
GNDRLIM
V
IN
USB INPUT 5V
V
OUT2
3.4V AT
800mA
V
OUT1
1.8V AT
400mA
3619B F04a
R4
276k
R2
118k
R3
59k
R1
59k
L2
1.5µH
L1
3.3µH
C
OUT2
2.2mF
×2
SuperCap
C
OUT1
10µF
C
IN
10µF
R
LIM
116k
C
LIM
2200pF
+
I
LIM
= 475mA
C
IN
, C
OUT1
: AVX 08056D106KAT2A
C
OUT2
: VISHAY 592D228X96R3X2T20H
L1: COILCRAFT LPS4012-332ML
L2: COILCRAFT LPS4012-152ML