LTC3417A_15 Datasheet, PDF (16/22 Pages) Linear Technology – Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator

LTC3417A
applications information
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 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 efficiency (SYNC/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)
1.5A
1A
∆I_L
525mA
350mA
Using the 1.5MHz frequency setting (FREQ = V_IN), we get
the following equations for L1 and L2:
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
•
1.5MHz
1A
(^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
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 com-
ponents 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_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.
L1
=
1.8V
1.5MHz • 525mA
⎛⎝⎜₁
–
1.8V
4.2V
⎞
⎠⎟
=
1.3µ
H
Use 1.5µH.
L2
=
2.5V
1.5MHz • 350mA
⎛⎝⎜_1–
2.5V
4.2V
⎞
⎠⎟
=
1.9µH
Use 2.2µH.
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