ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 24

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

LTC1876
APPLICATIO S I FOR ATIO
Voltage Positioning
Voltage positioning can be used to minimize peak-to-peak
output voltage excursions under worst-case transient
loading conditions. The open loop DC gain of the control
loop is reduced depending upon the maximum load step
specifications. Voltage positioning can be easily added to
the LTC1876 by loading the I
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage of the error amplifier, or 1.2V
(see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifier. The
maximum output voltage deviation can theoretically be
reduced to half or alternatively the amount of output
capacitance can be reduced for a particular application. A
complete explanation is included in Design Solutions 10.
(See: www.linear-tech.com)
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 LTC1876 circuits: 1) LTC1876 V
cluding loading on the 3.3V internal regulator), 2) INTV
regulator current, 3) I
transition losses.
24
Figure 8. Active Voltage Positioning Applied to the LTC1876
%Efficiency = 100% – (L1 + L2 + L3 + ...)
INTV
CC
R
R
T2
T1
U
R
C
C
C
2
R losses, 4) topside MOSFET
U
I
TH
TH
pin with a resistive divider
LTC1876
W
1876 F08
IN
current (in-
U
CC
1. The V
supply current given in the Electrical Characteristics table,
which excludes MOSFET driver and control currents; the
second is the current drawn from the 3.3V linear regulator
output. V
loss.
2. INTV
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
ground. The resulting dQ/dt is a current out of INTV
is typically much larger than the control circuit current. In
continuous mode, I
are the gate charges of the topside and bottom side
MOSFETs.
Supplying INTV
from an output-derived source will scale the V
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
mately 3mA of V
loss from 10% or more (if the driver was powered directly
from V
3. I
fuse (if used), MOSFET, inductor, current sense resistor,
and input and output capacitor ESR. In continuous mode
the average output current flows through L and R
but is “chopped” between the topside MOSFET and the
synchronous MOSFET. If the two MOSFETs have approxi-
mately the same R
MOSFET can simply be summed with the resistances of L,
R
R
= 40m
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
output power level. The combined effects of increasingly
lower output voltages and higher currents required by
high performance digital systems is not doubling but
SENSE
DS(ON)
2
R losses are predicted from the DC resistances of the
IN
CC
and ESR to obtain I
IN
= 30m , R
) to only a few percent.
IN
(sum of both input and output capacitance
current has two components: the first is the DC
current is the sum of the MOSFET driver and
OUT
current typically results in a small (<0.1%)
CC
IN
for the same external components and
power through the EXTV
current. This reduces the mid-current
GATECHG
L
DS(ON)
= 50m , R
2
, then the resistance of one
CC
R losses. For example, if each
=f(Q
current results in approxi-
SENSE
T
+Q
B
), where Q
= 10m and R
CC
switch input
IN
T
current
and Q
CC
SENSE
CC
1876fa
that
ESR
to
B
,