LTC3606B
16
3606bfb
Design Example
As a design example, consider using the LTC3606B in a
USB-GSM application, with V
IN
= 5V, I
INMAX
= 500mA,
with the output charging a SuperCap of 4.4mF. The load
requires 800mA in active mode and 1mA in standby mode.
The output voltage V
OUT
= 3.4V.
First, calculate the inductor value for about 40% ripple
current (320mA in this example) at maximum V
IN
. Using
a derivation of Equation (1):
L1=
3.4V
2.25MHz •(320mA)
•1
3.4V
5V
=1.51μH
For the inductor, use the closest standard value of 1.5µH.
The 4.4mF supercaps are used to deliver the required
2A pulses to power the RF power amplifi ers, while the
LTC3606B recharges the supercap after the pulse ends,
see Figure 4c. As for the input capacitor, a typical value
Figure 4a. Design Example Circuit
V
IN
USB INPUT 5V
L1
1.5µH
R2
280k
R1
59k
C
IN
10µF
C
IN
: AVX 08056D106KAT2A
C
OUT
: VISHAY 592D228X96R3X2T20H
+
V
OUT
3.4V AT
800mA
I
LIM
= 475mA
R
LIM
116k
C
LIM
2200pF
R
PGD
499k
C
OUT
2.2mF
s2
SuperCap
PGOOD
L1: COILCRAFT LPS4012-152ML
V
IN
RUN
LTC3606B
V
FB
SW
PGOOD
GND
RLIM
3606B F04
APPLICATIONS INFORMATION
of C
IN
= 10µF should suffi ce, if the source impedance 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 10µA with the
0.6V feedback voltage makes R1~60k. A close standard
1% resistor is 59k. Using Equation (2).
R2=
V
OUT
0.6
1 •R1= 276k, 280k for 1%
A feedforward capacitor is not used since the 4.4mF
SuperCap will inhibit any fast output voltage transients.
Figure 4 shows the complete schematic for this example,
along with the effi ciency curve and transient response.
Input current limit is set at 475mA average current, 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
.
