LTC3548-2 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3548-2
Linear Technology
'LTC3548-2' PDF : 16 Pages View PDF
LTC3548-2
APPLICATIONS INFORMATION
The RDS(ON) for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. Thus, to obtain I2R losses:
I2R losses = IOUT2(RSW + RL)
4. Other hidden losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important
to include these system level losses in the design of a
system. The internal battery and fuse resistance losses
can be minimized by making sure that CIN has adequate
charge storage and very low ESR at the switching fre-
quency. Other losses including diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3548-2 does not
dissipate much heat due to its high efficiency. However,
in applications where the LTC3548-2 is running at high
ambient temperature with low supply voltage and high
duty cycles, such as in dropout, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 150°C,
both power switches will turn off and the SW node will
become high impedance.
To prevent the LTC3548-2 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:
TRISE = PD • θJA
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, TJ, is given by:
TJ = TRISE + TAMBIENT
As an example, consider the case when the LTC3548-2 is
at an input voltage of 2.7V with a load current of 400mA
and 800mA and an ambient temperature of 70°C. From
the Typical Performance Characteristics graph of Switch
12
Resistance, the RDS(ON) resistance of the main switch is
0.425Ω. Therefore, power dissipated by each channel is:
PD = I2 • RDS(ON) = 272mW and 68mW
The DFN package junction-to-ambient thermal resistance,
θJA, is 40°C/W. Therefore, the junction temperature of
the regulator operating in a 70°C ambient temperature is
approximately:
TJ = (0.272 + 0.068) • 40 + 70 = 83.6°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3548-2 in
an portable application with a Li-Ion battery. The battery
provides a VIN = 2.8V to 4.2V. The load requires a maximum
of 800mA in active mode and 2mA in standby mode. The
output voltage is VOUT = 1.8V. 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 VIN:
L
≥
1.8V
2.25MHz • 240mA
•
⎛
⎝⎜
1–
1.8V ⎞
4.2V ⎠⎟
=
1.9μH
Choosing a vendor’s closest inductor value of 2.2μH,
results in a maximum ripple current of:
ΔIL
=
1.8V
2.25MHz •
2.2μ
•
⎛
⎝⎜
1−
1.8V
4.2V
⎞
⎠⎟
=
208mA
For cost reasons, a ceramic capacitor will be used. COUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
COUT
≈
2.5
800mA
2.25MHz •(5%
• 1.8V)
=
9.9μF
A good standard value is 10μF. Since the output impedance
of a Li-Ion battery is very low, CIN is typically 10μF. Fol-
lowing the same procedure for VOUT2 = 2.5V, the inductor
value is derived as 4.7μH and the output capacitor value
is 4.7μF.
35482fb
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