LTC3606B_15 Datasheet, PDF (16/20 Pages) Linear Technology – 800mA Synchronous Step-Down DC/DC with Average Input Current Limit

LTC3606B
APPLICATIONS INFORMATION
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
• 1 ^3.4V =1.51μH
2.25MHz • (320mA)
5V
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 amplifiers, while the
LTC3606B recharges the supercap after the pulse ends,
see Figure 4c. 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 = ^VOUT 1 • R1= 276k, 280k for 1%
0.6
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 efficiency 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.
V_IN
USB INPUT 5V
PGOOD
C_IN
R_PGD
499k
10μF
V_IN
SW
LTC3606B
RUN
PGOOD
RLIM
V_FB
GND
C_LIM
R_LIM
2200pF _116k
I_LIM = 475mA
L1
1.5μH
R2
280k
R1
59k
V_OUT
3.4V AT
800mA
+ C_OUT
2.2mF
s2
SuperCap
C_IN: AVX 08056D106KAT2A
C_OUT: VISHAY 592D228X96R3X2T20H
L1: COILCRAFT LPS4012-152ML
3606B F04
Figure 4a. Design Example Circuit
3606bfb
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