Datasheet

LTC3853
24
3853fa
on the current sense signal. The minimum on-time can be
affected by PCB switching noise in the voltage and current
loop. However, as the peak sense voltage decreases the
minimum on-time gradually increases to 130ns. This is
of particular concern in forced continuous applications
with low ripple current at light loads. If the duty cycle
drops below the minimum on-time limit in this situation,
a significant amount of cycle skipping can occur with
correspondingly larger current and voltage ripple.
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 usually account for most of the
losses in LTC3853 circuits: 1) IC V
IN
current, 2) INTV
CC
regulator current, 3) I
2
R losses, 4) topside MOSFET
transition losses.
1. The V
IN
current is the DC supply current given in
the Electrical Characteristics table, which excludes
MOSFET driver and control currents. V
IN
current typi-
cally results in a small (<0.1%) loss.
2. INTV
CC
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 INTV
CC
to ground. The resulting dQ/dt is a cur-
rent out of INTV
CC
that is typically much larger than the
control circuit current. In continuous mode, I
GATECHG
= f(Q
T
+ Q
B
), where Q
T
and Q
B
are the gate charges of
the topside and bottom side MOSFETs.
Supplying INTV
CC
power through EXTV
CC
from an
output-derived source will scale the V
IN
current required
for the driver and control circuits by a factor of (Duty
Cycle)/(Efficiency). For example, in a 20V to 5V applica-
tion, 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.
3. I
2
R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor.
In continuous mode, the average output current flows
through L and R
SENSE
, but is “chopped” between the
topside MOSFET and the synchronous MOSFET. If the
two MOSFETs have approximately the same R
DS(ON)
,
then the resistance of one MOSFET can simply be
summed with the resistances of L and R
SENSE
to obtain
I
2
R losses. For example, if each R
DS(ON)
= 10mΩ, R
L
= 10mΩ, R
SENSE
= 5mΩ, then the total resistance is
25mΩ. This results in losses ranging from 2% to 8%
as the output current increases from 3A to 15A for
a 5V output, or a 3% to 12% loss for a 3.3V output.
Efficiency varies as the inverse square of V
OUT
for the
same external components and output power level. The
combined effects of increasingly lower output voltages
and higher currents required by high performance digital
systems is not doubling but quadrupling the importance
of loss terms in the switching regulator system!
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Transition Loss = (1.7) V
IN
2
I
O(MAX)
C
RSS
f
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% efficiency degradation in portable systems. It is
very important to include these “system” level losses
during the design phase. 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. A 25W supply will typically require a
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