LTC3407AIDD Datasheet, PDF (11/16 Pages) Linear Integrated Systems – Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407
APPLICATIONS INFORMATION
produce the most improvement. Percent efficiency can
be expressed as:
%Efficiency = 100% - (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, 4 main sources usually account for most of the
losses in LTC3407 circuits: 1)V_IN quiescent current, 2)
switching losses, 3) I²R losses, 4) other losses.
1) The V_IN current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET driver
and control currents. V_IN current results in a small (<0.1%)
loss that increases with V_IN, even at no load.
2) The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current results
from switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high
to low again, a packet of charge dQ moves from V_IN to
ground. The resulting dQ/dt is a current out of V_IN that is
typically much larger than the DC bias current. In continu-
ous mode, I_GATECHG = f_O(Q_T + Q_B), where Q_T and Q_B are
the gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to V_IN
and thus their effects will be more pronounced at higher
supply voltages.
3) I²R losses are calculated from the DC resistances of the
internal switches, R_SW, and external inductor, R_L. In con-
tinuous mode, the average output current flowing through
inductor L, but is “chopped” between the internal top and
bottom switches. Thus, the series resistance looking into
the SW pin is a function of both top and bottom MOSFET
R_DS(ON) and the duty cycle (DC) as follows:
R_SW = (R_DS(ON)TOP)(DC) + (R_DS(ON)BOT)(1 – DC)
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_OUT)² (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 frequency.
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 LTC3407 does not dis-
sipate much heat due to its high efficiency. However, in
applications where the LTC3407 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 LTC3407 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 LTC3407 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 = I_OUT2 • R_DS(ON) = 153mW
The MS package junction-to-ambient thermal resistance,
θ_JA, is 45°C/W. Therefore, the junction temperature of
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