LTC3407A Datasheet, PDF (12/16 Pages) Linear Technology – Dual Synchronous 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407A
APPLICATIONS INFORMATION
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I²R losses:
I²R losses = I_OUT2(R_SW + R_L)
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 C_IN 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 LTC3407A does not
dissipate much heat due to its high efficiency. However,
in applications where the LTC3407A 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 be turned off and the SW node
will become high impedance.
To prevent the LTC3407A 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 LTC3407A is
in dropout on both channels at an input voltage of 2.7V
with a load current of 600mA and an ambient 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) = 153mW
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 = 2 • 0.153 • 45 + 70 = 84°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3407A 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 600mA 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
1.5MHz • 300mA
•



1–
2.5V
4.2V


=
2.25μH
Choosing the closest inductor from a vendor of 2.2μH
inductor, results in a maximum ripple current of:
I_L
=
2.5V
1.5MHz • 2.2μH
•


1

2.5V
4.2V


=
307mA
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
≈
3
600mA
1.5MHz • (5% •
2.5V)
=
9.6μF
The closest standard value is 10μF. Since the output imped-
ance of a Li-Ion battery is very low, C_IN is typically 10μF.
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