LTC3417AIFE(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3417AIFE Datasheet PDF : 20 Pages
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LTC3417A
APPLICATIO S I FOR ATIO
the RDS(ON) resistance of the 1.5A P-channel switch is
0.09Ω and the RDS(ON) of the 1A P-channel switch is
0.163Ω. The power dissipated by the part is:
PD = I12 • RDS(ON)1 + I22 • RDS(ON)2
PD = 1.52 • 0.09 + 12 • 0.163
PD = 366mW
The DFN package junction-to-ambient thermal resistance,
θJA, is about 43°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
TJ = 0.366 • 43 + 70
TJ = 85.7°C
Remembering that the above junction temperature is
obtained from an RDS(ON) at 25°C, we might recalculate
the junction temperature based on a higher RDS(ON) since
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.
Using the 1.5MHz frequency setting (FREQ = VIN), 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.
COUT selection is based on load step droop instead of ESR
requirements. For a 2.5% output droop:
COUT1
=
2.5
•
1.5A
1 . 5MHz (5 %
•
1.8V
)
=
28µF
COUT2
=
2.5
•
1A
1 . 5MHz (5 %
•
2.5V)
=
13µF
Design Example
As a design example, consider using the LTC3417A in a
portable application with a Li-Ion battery. The battery
provides a VIN 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 = VIN).
First, determine what frequency should be used. Higher
frequency results in a lower inductor value for a given ∆IL
(∆IL is estimated as 0.35ILOAD(MAX)). Reasonable values
for wire wound surface mount inductors are usually in the
range of 1µH to 10µH.
CONVERTER OUTPUT
SW1
SW2
ILOAD(MAX)
1.5A
1A
∆IL
525mA
350mA
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 ITH1 and 2.87k and 6800pF on ITH2. 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.
3417afa
16
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