LTC3417 Datasheet, PDF (16/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

LTC3417
APPLICATIO S I FOR ATIO
Design Example
As a design example, consider using the LTC3417 in a
portable application with a Li-Ion battery. The battery
provides a V_IN from 2.5V to 4.2V. One output requires 1.8V
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_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
SW1
SW2
I_LOAD(MAX)
1.4A
800mA
∆I_L
490mA
280mA
Using the 1.5MHz frequency setting (FREQ = V_IN), we get
the following equations for L₁ and L₂:
L1=
1.8V
1.5MHz • 490mA
⎛
⎝⎜
1–
1.8V ⎞
4.2V ⎠⎟
= 1.4µH
Use 1.5µH.
L2
=
2.5V
1.5MHz • 280mA
⎛
⎝⎜
1–
2.5V ⎞
4.2V ⎠⎟
=
2.4µH
Use 2.2µH.
C_OUT selection is based on load step droop instead of ESR
requirements. For a 5% output droop:
C_OUT1
=
2.5
•
1.
1.3A
5MHz (5 %
•
1.8V)
=
24µF
C_OUT2
=
2.5
•
1
0.7A
. 5MHz (5 %
•
2.5V)
=
9.
3µF
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_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.
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