LTC3417A-1
16
3417a1fa
The power dissipated by the part is:
PD = I
1
2
• R
DS(ON)1
+ I
2
2
• R
DS(ON)2
PD = 1.5
2
• 0.09 + 1
2
• 0.163
PD = 366mW
The DFN package junction-to-ambient thermal resistance,
θ
JA
, is about 43°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
T
J
= 0.366 • 43 + 70
T
J
= 85.7°C
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-1 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 effi ciency (SYNC/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.
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 • 350m
A
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
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 LTC3417A-1 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 POR pin is an open 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.
CONVERTER OUTPUT I
LOAD(MAX)
ΔI
L
SW1 1.5A 525mA
SW2 1A 350mA
APPLICATIONS INFORMATION
