LTC3890
applicaTions inForMaTion
Efficiency Considerations
2
3. I 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
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:
R
, but is chopped between the topside MOSFET
SENSE
andthesynchronousMOSFET.IfthetwoMOSFETshave
approximately the same R
, then the resistance
DS(ON)
of one MOSFET can simply be summed with the resis-
2
%Efficiency = 100% – (L1 + L2 + L3 + ...)
tances of L, R
and ESR to obtain I R losses. For
DS(ON)
SENSE
example, if each R
= 30mΩ, R = 50mΩ, R
L SENSE
where L1, L2, etc. are the individual losses as a percent-
age of input power.
= 10mΩ and R
= 40mΩ (sum of both input and
ESR
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.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of
the losses in LTC3890 circuits: 1) IC V current, 2) IN-
IN
2
TV regulator current, 3) I R losses, 4) topside MOSFET
CC
Efficiency varies as the inverse square of V
for the
OUT
transition losses.
sameexternalcomponentsandoutputpowerlevel. The
combined effects of increasingly lower output voltages
andhighercurrentsrequiredbyhighperformancedigital
systemsisnotdoublingbutquadruplingtheimportance
of loss terms in the switching regulator system!
1. The V current is the DC supply current given in the
IN
ElectricalCharacteristicstable,whichexcludesMOSFET
driverandcontrolcurrents. V currenttypicallyresults
IN
in a small (<0.1%) loss.
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high
2. INTV current is the sum of the MOSFET driver and
CC
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
input voltages (t
ypically 15V or greater). Transition
losses can be estimated from:
Transition Loss = (1.7) • V • 2 • I
• C
• f
IN
O(MAX)
RSS
from INTV to ground. The resulting dQ/dt is a current
CC
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
resistancelossescanbeminimizedbymakingsurethat
out of INTV that is typically much larger than the
CC
control circuit current. In continuous mode, I
GATECHG
= f(Q + Q ), where Q and Q are the gate charges of
T
B
T
B
the topside and bottom side MOSFETs.
SupplyingINTV fromanoutput-derivedsourcepower
CC
through EXTV will scale the V current required for
CC
IN
C has adequate charge storage and very low ESR at
IN
thedriverandcontrolcircuitsbyafactorof(DutyCycle)/
the switching frequency. A 25W supply will typically
require a minimum of 20µF to 40µF of capacitance
having a maximum of 20mΩ to 50mΩ of ESR. The
LTC38902-phasearchitecturetypicallyhalvesthisinput
capacitance requirement over competing solutions.
Other losses including body diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
(Efficiency). For example, in a 20V to 5V application,
10mAofINTV currentresultsinapproximately2.5mA
CC
of V current. This reduces the midcurrent loss from
IN
10% or more (if the driver was powered directly from
V ) to only a few percent.
IN
3890fb
25
Linear Systems [ Linear Systems ]
