LTC3417A
16
3417afc
applicaTions inForMaTion
Remembering that the above junction temperature is
obtained from an R
DS(ON)
at 25°C, we might recalculate
the junction temperature based on a higher R
DS(ON)
since
it increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Design Example
As a design example, consider using the LTC3417A in
a portable application with a Li-Ion battery. The battery
provides a V
IN
from 2.8V to 4.2V. One load requires 1.8V
at 1.5A in active mode, and 1mA in standby mode. The
other load requires 2.5V at 1A 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 efficiency (SYNC/MODE = V
IN
).
First, determine what frequency should be used. Higher
frequency results in a lower inductor value for a given ∆I
L
(∆I
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)
∆I
L
SW1 1.5A 525mA
SW2 1A 350mA
Using the 1.5MHz frequency setting (FREQ = V
IN
), we get
the following equations for L1 and L2:
L1=
1.8V
1.5MHz • 525mA
1–
1.8V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 1.3µH
Use 1.5µH.
L2 =
2.5V
1.5MHz • 350mA
1–
2.5V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 1.9µH
Use 2.2µH.
C
OUT
selection is based on load step droop instead of ESR
requirements. For a 2.5% output droop:
C
OUT1
= 2.5 •
1.5A
1.5MHz 5% • 1.8V
( )
= 28µF
C
OUT2
= 2.5 •
1A
1.5MHz 5% • 2.5V
( )
= 13µF
The closest standard values are 47µF and 22µF.
The output voltages can now be programmed by choos-
ing the values of R1, R2, R3, and R4. To maintain high
efficiency, 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 com-
ponents by examining the load step response, but a
good place to start for the LTC3417A is with a 5.9kΩ and
2200pF filter 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.
