ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 21

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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:
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 LTC3827-1 circuits: 1) IC V
regulator current, 3) I
transition losses.
1. The V
2. INTV
3. I
%Efficiency = 100% – (L1 + L2 + L3 + ...)
supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the 3.3V
linear regulator output. V
small (<0.1%) loss.
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
out of INTV
control circuit current. In continuous mode, I
=f(Q
topside and bottom side MOSFETs.
Supplying INTV
input from an output-derived source will scale the V
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
in approximately 2.5mA of V
mid-current loss from 10% or more (if the driver was
powered directly from V
fuse (if used), MOSFET, inductor, current sense resis-
2
R losses are predicted from the DC resistances of the
T
CC
+Q
IN
current has two components: the first is the DC
current is the sum of the MOSFET driver and
B
), where Q
CC
CC
to ground. The resulting dQ/dt is a current
that is typically much larger than the
CC
U
power through the EXTV
T
2
and Q
R losses, 4) Topside MOSFET
U
IN
IN
) to only a few percent.
B
current typically results in a
IN
are the gate charges of the
current. This reduces the
W
IN
current, 2) INTV
CC
current results
U
CC
GATECHG
switch
CC
IN
4. Transition losses apply only to the topside MOSFET(s),
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
adequate charge storage and very low ESR at the switch-
ing frequency. A 25W supply will typically require a mini-
mum of 20μF to 40μF of capacitance having a maximum
of 20mΩ to 50mΩ of ESR. The LTC3827-1 2-phase
architecture typically halves this input capacitance
requirement over competing solutions. Other losses
including Schottky conduction losses during dead-time
and inductor core losses generally account for less than
2% total additional loss.
tor, and input and output capacitor ESR. In continuous
mode the average output current flows through L and
R
and the synchronous MOSFET. If the two MOSFETs
have approximately the same R
tance of one MOSFET can simply be summed with the
resistances of L, R
For example, if each R
R
and output capacitance losses), then the total resis-
tance 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.
Efficiency varies as the inverse square of V
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!
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
SENSE
SENSE
, but is “chopped” between the topside MOSFET
= 10mΩ and R
SENSE
ESR
DS(ON)
and ESR to obtain I
IN
= 40mΩ (sum of both input
2 I
O(MAX)
= 30mΩ, R
DS(ON)
LTC3827-1
C
, then the resis-
RSS
f
L
OUT
2
= 50mΩ,
R losses.
21
for the
IN
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