Datasheet

LTC3727A-1
22
3727a1fa
Effi ciency Considerations
The percent effi ciency 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 effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
%Effi ciency = 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, four main sources usually account for most of
the losses in LTC3727A-1 circuits: 1) LTC3727A-1 V
IN
current (including loading on the 3.3V internal regulator),
2) INTV
CC
regulator current, 3) I
2
R losses, 4) Topside
MOSFET transition losses.
1. The V
IN
current has two components: the fi rst is the
DC supply current given in the Electrical Characteris-
tics table, which excludes MOSFET driver and control
currents; the second is the current drawn from the 3.3V
linear regulator output. V
IN
current typically 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
power through the EXTV
CC
switch input
from an output-derived source will scale the V
IN
current
required for the driver and control circuits by a factor of
(Duty Cycle)/(Effi ciency). For example, in a 20V to 5V
application, 10mA of INTV
CC
current results in approxi-
mately 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.
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 fl ows 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, R
SENSE
and ESR to obtain I
2
R losses. For example, if
each R
DS(ON)
= 30mΩ, R
L
= 50mΩ, R
SENSE
= 10mΩ and
R
ESR
= 40mΩ (sum of both input and output capacitance
losses), then the total resistance is 130mΩ. 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. Effi ciency 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!
Figure 8. Active Voltage Positioning Applied to the LTC3727A-1
I
TH
R
C
R
T1
INTV
CC
C
C
3727 F08
LTC3727A-1
R
T2
4. Transition losses apply only to the topside MOSFET(s),
and become signifi cant 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% effi ciency 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