LTC3619B_15 Datasheet, PDF (16/20 Pages) Linear Technology – 400mA/800mA Synchronous Step-Down DC/DC with Average Input Current Limit

LTC3619B
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
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
USB INPUT 5V
C_IN
10µF
RUN2 V_IN RUN1
V_OUT2
3.4V AT
800mA
+
L2
1.5µH
R4
276k
C_OUT2
R3
2.2mF
59k
×2
SuperCap
PGOOD2 PGOOD1
LTC3619B
SW2
SW1
L1
3.3µH
C_F1, 22pF
V_FB2
RLIM
V_FB1
GND
R1 R2
59k 118k
R_LIM
116k
C_LIM
2200pF
V_OUT1
1.8V AT
400mA
C_OUT1
10µF
3619B F04a
C_IN, C_OUT1: AVX 08056D106KAT2A
C_OUT2: VISHAY 592D228X96R3X2T20H
I_LIM = 475mA
L1: COILCRAFT LPS4012-332ML
L2: COILCRAFT LPS4012-152ML
Figure 4a. Design Example Circuit
3619bfb
16