LTC3548A Datasheet, PDF (14/20 Pages) Linear Technology – Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3548A
APPLICATIONS INFORMATION
To prevent the LTC3548A from exceeding the maximum
junction temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum
junction temperature of the part. The temperature rise is
given by:
T_RISE = P_D • θ_JA
where P_D is the power dissipated by the regulator and θ_JA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T_J, is given by:
T_J = T_RISE + T_AMBIENT
As an example, consider the case when the LTC3548A is
in dropout on both channels at an input voltage of 2.7V
with a load current of 400mA and 800mA and an ambi-
ent temperature of 70°C. From the Typical Performance
Characteristics graph of Switch Resistance, the R_DS(ON)
resistance of the main switch is 0.425Ω. Therefore, power
dissipated by each channel is:
P_D = I² • R_DS(ON) = 272mW and 68mW
The MS package junction-to-ambient thermal resistance,
θ_JA, is 45°C/W. Therefore, the junction temperature of
the regulator operating in a 70°C ambient temperature is
approximately:
T_J = (0.272 + 0.068) • 45 + 70 = 85.3°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3548A in a
portable application with a Li-Ion battery. The battery pro-
vides a V_IN = 2.8V to 4.2V. The load requires a maximum
of 800mA in active mode and 2mA in standby mode. The
output voltage is V_OUT = 2.5V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the inductor value for about 30% ripple
current at maximum V_IN:
L
≥
2.5V
2.25MHz • 360mA
•
⎛
⎝⎜
1–
2.5V
4.2V
⎞
⎠⎟
= 1.25µH
Choosing the next highest standardized inductor value of
2.2μH, results in a maximum ripple current of:
ΔI_L
=
2.5V
2.25MHz • 2.2µH
•
⎛
⎝⎜
1−
2.5V
4.2V
⎞
⎠⎟
=
204mA
For cost reasons, a ceramic capacitor will be used. C_OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C_OUT
≈
2.5
800mA
2.25MHz • (5%
•
2.5V)
=
7.1µF
The closest standard value is 10μF. Since the output imped-
ance of a Li-Ion battery is very low, C_IN is typically 10μF.
The output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
current in these resistors should be kept small. Choosing
2μA with the 0.6V feedback voltage makes R1~300k. A close
standard 1% resistor is 280k, and R2 is then 887k.
The POR pin is a common drain output and requires a pull-
up resistor. A 100k resistor is used for adequate speed.
Figure 3 shows the complete schematic for this design
example. The specific passive components chosen allow
for a 1mm height power supply that maintains a high
efficiency across load.
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