LTC3417A LINER [Linear Technology], LTC3417A Datasheet - Page 16

• Manufacturer Part Number: LTC3417A
• Description: Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3417A
the R
0.09Ω and the R
0.163Ω. The power dissipated by the part is:
The DFN package junction-to-ambient thermal resistance,
θ
of the regulator operating in a 70°C ambient temperature
is approximately:
Remembering that the above junction temperature is
obtained from an R
the junction temperature based on a higher R
it increases with temperature. However, we can safely
assume 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
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
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.
16
JA
CONVERTER OUTPUT
PD = I
PD = 1.5
PD = 366mW
T
T
L
, is about 43°C/W. Therefore, the junction temperature
J
J
is estimated as 0.35I
= 0.366 • 43 + 70
= 85.7°C
DS(ON)
1
SW1
SW2
2
2
• R
IN
resistance of the 1.5A P-channel switch is
• 0.09 + 1
DS(ON)1
from 2.8V to 4.2V. One load requires 1.8V
DS(ON)
U
DS(ON)
+ I
2
• 0.163
of the 1A P-channel switch is
2
LOAD(MAX)
U
2
at 25°C, we might recalculate
• R
I
LOAD(MAX)
1.5A
DS(ON)2
1A
IN
).
W
). Reasonable values
DS(ON)
525mA
350mA
U
∆I
L
since
L
Using the 1.5MHz frequency setting (FREQ = V
the following equations for L
C
requirements. For a 2.5% output droop:
The closest standard values are 47µF and 22µF.
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 LTC3417A 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
L
Us
L
Use
C
C
1
2
OUT
OUT
selection is based on load step droop instead of ESR
=
e e
=
1 5
2 2
1 5
1
2
1 5
.
TH1
.
.
=
.
=
MHz
µ
µ
MHz
2 5
2 5
H
H
and 2.87k and 6800pF on I
. •
. • •
.
1 8
.
2 5
.
.
•
•
1 5
1 5
V
525
V
350
.
.
MHz
MHz
mA
mA
1 5
(
(
⎛
⎝ ⎜
⎛
⎝ ⎜
.
5
1
5
1
A
1
% • .
1
% • .
A
–
–
and L
1 8
4 2
2 5
4 2
1 8
.
2 5
.
.
.
V
V
V
V
2
V
V
:
⎞
⎠ ⎟
⎞
⎠ ⎟ ⎟
)
)
=
=
=
=
TH2
1 3
1 9
28
13
.
.
. The output
µ
µ
µ
µ
IN
F
H
F
H
), we get
3417afa