LTC3417 Linear Technology, LTC3417 Datasheet - Page 16

• Manufacturer Part Number: LTC3417
• Description: Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3417
Design Example
As a design example, consider using the LTC3417 in a
portable application with a Li-Ion battery. The battery
provides a V
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 efficiency (MODE = V
First, determine what frequency should be used. Higher
frequency results in a lower inductor value for a given ∆I
(∆I
for wire wound surface mount inductors are usually in the
range of 1µH to 10µH.
Using the 1.5MHz frequency setting (FREQ = V
the following equations for L
16
CONVERTER OUTPUT
L
Us
L
Use
L
1
2
is estimated as 0.35I
=
e e
=
1 5
1 5
2 2
1 5
SW1
SW2
.
.
.
.
MHz
µ
µ
MHz
IN
H
H
from 2.5V to 4.2V. One output requires 1.8V
1 8
.
.
2 5
.
.
•
•
V
490
V
280
U
mA
mA
LOAD(MAX)
U
IN
⎛
⎝ ⎜
⎛
⎝ ⎜
1
I
).
1
LOAD(MAX)
1
800mA
–
–
1.4A
and L
1 8
4 2
2 5
4 2
.
.
.
.
V
V
V
V
W
). Reasonable values
2
⎞
⎠ ⎟
:
⎞
⎠ ⎟ ⎟
=
=
1 4
2 4
.
.
µ
µ
490mA
280mA
H
IN
H
U
∆I
), we get
L
L
C
requirements. For a 5% output droop:
The closest standard values are 22µF and 10µF.
The output voltages can now be programmed by choosing
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 compo-
nents by examining the load step response, but a good
place to start for the LTC3417 is with a 5.9kΩ and 2200pF
filter on I
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.
OUT
C
C
OUT
OUT
selection is based on load step droop instead of ESR
1
2
TH1
=
=
2 5
2 5
. •
and 2.87k and 6800pF on I
. • •
1 5
1 5
.
.
MHz
MHz
1 3
0 7
(
(
.
5
.
5
% • .
% • .
A
A
1 8
2 5
www.DataSheet4U.com
V
V
)
)
=
=
TH2
24
9 3
.
µ
. The output
µ
F
F
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