LTC3548 Datasheet, PDF (11/16 Pages) Linear Technology – Dual Synchronous, 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3548
APPLICATIO S I FOR ATIO
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 percentage
of input power.
Although all dissipative elements in the circuit produce
losses, 4 main sources usually account for most of the
losses in LTC3548 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
continuous mode, the average output current flows 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 (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 LTC3548 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3548 is running at high ambi-
ent 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 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 LTC3548 is in
dropout on both channels 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 Char-
acteristics graph of Switch Resistance, the R_DS(ON) resis-
tance 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
3548f
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