Datasheet

LTC3417
16
3417fd
APPLICATIONS INFORMATION
Design Example
As a design example, consider using the LTC3417 in a
portable application with a Li-Ion battery. The battery
provides a V
IN
from 2.5V to 4.2V. One output requires
1.8V at 1.3A in active mode, and 1mA in standby mode.
The other output requires 2.5V at 700mA in active mode,
and 500µA in standby mode. Since both loads still need
power in standby, Burst Mode operation is selected for
good low load effi ciency (MODE = V
IN
).
First, determine what frequency should be used. Higher
frequency results in a lower inductor value for a given ΔI
L
(ΔI
L
is estimated as 0.35I
LOAD(MAX)
). Reasonable values
for wire wound surface mount inductors are usually in the
range of 1µH to 10µH.
CONVERTER OUTPUT I
LOAD(MAX)
ΔI
L
SW1 1.4A 490mA
SW2 800mA 280mA
Using the 1.5MHz frequency setting (FREQ = V
IN
), we get
the following equations for L
1
and L
2
:
L1=
1.8V
1.5MHz • 490mA
1–
1.8V
4.2V






= 1.4μH
Use 1.5μH.
L2=
2.5V
1.5MHz • 280m
A
1–
2.5V
4.2V






= 2.4μH
Use 2.2μH.
C
OUT
selection is based on load step droop instead of ESR
requirements. For a 5% output droop:
C
OUT1
= 2.5 •
1.3A
1.5MHz 5% • 1.8V
( )
= 24μF
C
OUT2
= 2.5 •
0.7A
1.5MHz 5% • 2.5V
( )
= 9.3μF
The closest standard values are 22µF and 10µF.
The output voltages can now be programmed by choos-
ing the values of R1, R2, R3, and R4. To maintain high
effi ciency, the current in these resistors should be kept
small. Choosing 2µA with the 0.8V feedback voltages makes
R2 and R4 equal to 400k. A close standard 1% resistor is
412k. This then makes R1 = 515k. A close standard 1%
is 511k. Similarily, with R4 at 412k, R3 is equal to 875k.
A close 1% resistor is 866k.
The compensation should be optimized for these compo-
nents by examining the load step response, but a good
place to start for the LTC3417 is with a 5.9k and 2200pF
fi lter on I
TH1
and 2.87k and 6800pF on I
TH2
. The output
capacitor may need to be increased depending on the
actual undershoot during a load step.
The PGOOD pin is a common drain output and requires a
pull-up resistor. A 100k resistor is used for adequate speed.
Figure 4 shows a complete schematic for this design.