LTC3419 Datasheet, PDF (13/16 Pages) Linear Technology – Dual Monolithic 600mA Synchronous Step-Down Regulator

APPLICATIONS INFORMATION
V_IN
2.5V TO 5.5V
V_OUT2
C1
L2
C_F2
RUN2 V_IN RUN1
MODE
LTC3419
SW2
SW1
L1
C_F1
V_OUT1
C_OUT2
V_FB2
V_FB1
R4
GND
R2
R3
R1
C_OUT1
BOLD LINES INDICATE HIGH CURRENT PATHS
3419 F02
Figure 2. LTC3419 Layout Diagram (See Board Layout Checklist)
LTC3419
C_F1
C_F2
V_OUT1
R2
C_OUT1
R1
R3
VIA TO V_IN
R4
C_OUT2
V_OUT2
V_FB1
L1
RUN1
MODE
SW1
VIA TO GND
V_FB2
RUN2
SW2
V_IN
GND
C_IN
L2
3419 F03
Figure 3. LTC3419 Suggested Layout
Design Example
As a design example, consider using the LTC3419 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 600mA 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 (240mA in this example) at
maximum V_IN. Using a derivation of Equation 1:
L1 =
2.5V
2.25MHz • (240mA)
•
⎛
⎝⎜
1−
2.5V ⎞
4.2V ⎠⎟
=
1.87μH
For the inductor, use the closest standard value of
2.2μH.
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, 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 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= 187k
An optional 22pF feedback capacitor (C_F1) may be used
to improve transient response.
3419f
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