LTC3407A-2 Datasheet, PDF (12/16 Pages) Linear Technology – Dual Synchronous 800mA,2.25MHz Step-Down DC/DC Regulator

LTC3407A-2
APPLICATIO S I FOR ATIO
the SW pin is a function of both top and bottom MOSFET
R_DS(ON) and the duty cycle (D) as follows:
R_SW = (R_DS(ON)TOP)(D) + (R_DS(ON)BOT)(1 – D)
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 ac-
count for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3407A-2 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3407A-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 be turned off and the SW node
will become high impedance.
To prevent the LTC3407A-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 tempera-
ture 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-2
is in dropout on both channels at an input voltage of 2.7V
with a load current of 800mA 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) = 272mW
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 • 0.272 • 45 + 70 = 94.5°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3407A-2 in a
portable application with a Li-Ion battery. The battery
provides a V_IN = 2.8V to 4.2V. The load requires a maxi-
mum 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
3407a2f
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