LTC3858-2
26
38582f
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 LTC3858-2 circuits: 1) IC V
IN
current, 2) IN-
TV
CC
regulator current, 3) I
2
R losses, 4) topside MOSFET
transition losses.
1. The V
IN
current is the DC input 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 current 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
from an output-derived power source
through EXTV
CC
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 application,
10mA of INTV
CC
current results in approximately 2.5mA
of V
IN
current. This reduces the midcurrent loss from
10% or more (if the driver was powered directly from
V
IN
) to only a few percent.
APPLICATIONS INFORMATION
3. I
2
R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. 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 resis-
tances of L, R
SENSE
and ESR to obtain I
2
R losses. For
example, if each R
DS(ON)
= 30m, R
L
= 50m, R
SENSE
= 10m and R
ESR
= 40m (sum of both input and
output capacitance losses), then the total resistance
is 130m. This results in losses ranging from 3% to
13% as the output current increases from 1A to 5A for
a 5V output, or a 4% to 20% 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 (t
y
pically 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 minimum of 20µF to 40µF of capacitance
having a maximum of 20m to 50m of ESR. The
LTC3858-2 2-phase architecture typically halves this
input capacitance requirement over competing solu-
tions. Other losses including Schottky conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
