LTC3838-2_15 Datasheet, PDF (39/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
Note that it is expected that this DTR feature will cause
additional loss on the bottom MOSFET, due to its body
diode conduction. The bottom MOSFET temperature may
be higher with a load of frequent and large load steps. This
is an important design consideration. Experiments on the
demo board show a 20°C increase when a continuous
100% to 50% load step pulse train with 50% duty cycle
and 100kHz frequency is applied to the output.
If not needed, this DTR feature can be disabled by tying
the DTR pin to INTV_CC, or simply leave the DTR pin open
so that an internal 2.5µA current source will pull itself up
to INTV_CC.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percentage efficiency
can be expressed as:
%Efficiency = 100% – (L1% + L2% + L3% + ...)
where L1%, L2%, etc. are the individual losses as a per-
centage of input power. Although all dissipative elements
in the circuit produce power losses, several main sources
usually account for most of the losses:
1. I²R loss. These arise from the DC resistances of the
MOSFETs, inductor, current sense resistor and is the ma-
jority of power loss at high output currents. In continu-
ous mode the average output current flows though the
inductor L, but is chopped between the top and bottom
MOSFETs. If the two MOSFETs have approximately the
same R_DS(ON), then the resistance of one MOSFET can
simply be summed with the inductor’s DC resistances
(DCR) and the board traces to obtain the I²R loss. For
example, if each R_DS(ON) = 8mΩ, R_L = 5mΩ, and R_SENSE
= 2mΩ the loss will range from 15mW to 1.5W as the
output current varies from 1A to 10A. This results in loss
from 0.3% to 3% a 5V output, or 1% to 10% for a 1.5V
output. Efficiency varies as the inverse square of V_OUT
for the same external components and output power
level. The combined effects of lower output voltages
and higher currents load demands greater importance
of this loss term in the switching regulator system.
2. Transition loss. This loss mostly arises from the brief
amount of time the top MOSFET spends in the satura-
tion (Miller) region during switch node transitions. It
depends upon the input voltage, load current, driver
strength and MOSFET capacitance, among other fac-
tors, and can be significant at higher input voltages or
higher switching frequencies.
3. DRV_CC current. This is the sum of the MOSFET driver
and INTV_CC control currents. The MOSFET driver cur-
rents result 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 DRV_CC to ground. The resulting dQ/dt is a
current out of DRV_CC that is typically much larger than
the controller I_Q current. In continuous mode,
I_GATECHG = f • (Qg_(TOP) + Qg_(BOT)),
where Qg_(TOP) and Qg_(BOT) are the gate charges of the
top and bottom MOSFETs, respectively.
Supplying DRV_CC power through EXTV_CC could increase
efficiency by several percent, especially for high V_IN
applications. Connecting EXTV_CC to an output-derived
source will scale the V_IN current required for the driver
and controller circuits by a factor of (Duty Cycle)/(Ef-
ficiency). For example, in a 20V to 5V application, 10mA
of DRV_CC current results in approximately 2.5mA of V_IN
current. This reduces the mid-current loss from 10%
or more (if the driver was powered directly from V_IN)
to only a few percent.
4. C_IN loss. The input capacitor filters large square-wave
input current drawn by the regulator into an averaged
DC current from the supply. The capacitor itself has
a zero average DC current, but square-wave-like AC
current flows through it. Therefore the input capacitor
must have a very low ESR to minimize the RMS current
loss on ESR. It must also have sufficient capacitance
to filter out the AC component of the input current to
prevent additional RMS losses in upstream cabling,
fuses or batteries. The LTC3838-2’s 2-phase architecture
improves the ESR loss.
38382f
For more information www.linear.com/3838-2
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