LTC3417A-2 Datasheet, PDF (16/20 Pages) Linear Technology – Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator

LTC3417A-2
APPLICATIONS INFORMATION
The power dissipated by the part is:
PD = I₁₂ • R_DS(ON)1 + I₂₂ • R_DS(ON)2
PD = 1.5² • 0.09 + 1² • 0.163
PD = 366mW
The DFN package junction-to-ambient thermal resistance,
θ_JA, is about 43°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
T_J = 0.366 • 43 + 70
T_J = 85.7°C
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.
Using the 1.5MHz frequency setting (FREQ = V_IN), 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.
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_OUT
2
=
2.5
•
1.5MHz
1A
(5%
•
2.5V
)
=
13μF
Design Example
As a design example, consider using the LTC3417A-2 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 ΔI_L
(ΔI_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
ΔI_L
525mA
350mA
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 compo-
nents by examining the load step response, but a good
place to start for the LTC3417A-2 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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