LTC3613
25
3613fa
APPLICATIONS INFORMATION
In some applications, a more severe transient can be caused
by switching in loads with large (>10F) input capacitors.
If the switch connecting the load has low resistance and
is driven quickly, then the discharged input capacitors are
effectively put in parallel with C
OUT
, causing a rapid drop in
V
OUT
. No regulator can deliver enough current to prevent
this problem. The solution is to limit the turn-on speed of
the load switch driver. A Hot Swap™ controller is designed
specifically for this purpose and usually incorporates cur-
rent limiting, short-circuit protection and soft starting.
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. 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, four main sources account for
most of the losses:
1. I
2
R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause
the efficiency to drop at high output currents. In
continuous mode the average output current flows
though the inductor L, but is chopped between the
top and bottom MOSFETs.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V.
3. INTV
CC
current. This is the sum of the MOSFET driver
and control currents. The MOSFET driver current re-
sults 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 INTV
CC
to ground. The resulting dQ/dt is a current
out of INTV
CC
that is typically much larger than the
controller I
Q
current.
Supplying INTV
CC
power through EXTV
CC
could save
several points of efficiency, especially for high V
IN
ap-
plications. 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/Effi-
ciency. For example, in a 20V to 5V application, 10mA
of INTV
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 has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R loss
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in cabling,
fuses or batteries.
Other losses, which include the C
OUT
ESR loss, bottom
MOSFET reverse-recovery loss and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current there is no change in efficiency.
Power losses in the switching regulator will reflect as a
longer than ideal on-time. This efficiency accounted on-
time in continuous mode can be calculated as:
t
ON(REAL)
≈
t
ON(IDEAL)
Efficiency
