ltc3729euh-trpbf Linear Technology Corporation, ltc3729euh-trpbf Datasheet - Page 20

• Manufacturer Part Number: ltc3729euh-trpbf
• Description: 550khz, Polyphase, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3729
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 easily be added to
the LTC3729 by loading the I
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage range 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 LTC3729 circuits: 1) LTC3729 V
(including loading on the differential amplifier output),
20
Figure 8. Active Voltage Positioning Applied to the LTC3729
%Efficiency = 100% – (L1 + L2 + L3 + ...)
INTV
CC
R
R
T2
T1
U
R
C
C
C
U
I
TH
TH
pin with a resistive divider
LTC3729
W
3729 F08
U
IN
current
2) INTV
MOSFET transition losses.
1) The V
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control
currents; the second is the current drawn from the differ-
ential amplifier output. V
small (<0.1%) 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 the ratio
(Duty Factor)/(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
total resistance is 25m . This results in losses ranging
from 2% to 8% as the output current increases from 3A to
15A per output stage for a 5V output, or a 3% to 12% loss
per output stage for a 3.3V output. Efficiency varies as
the inverse square of V
nents and output power level. The combined effects of
SENSE
DS(ON)
2
R losses are predicted from the DC resistances of the
IN
CC
and ESR to obtain I
CC
=10m , R
IN
) to only a few percent.
regulator current, 3) I
current is the sum of the MOSFET driver and
current has two components: the first is the
CC
IN
power through the EXTV
L
current. This reduces the mid-current
GATECHG
=10m , and R
DS(ON)
OUT
IN
2
, then the resistance of one
CC
for the same external compo-
R losses. For example, if each
= (Q
current typically results in a
current results in approxi-
2
T
R losses and 4) Topside
+ Q
SENSE
B
), where Q
=5m , then the
CC
switch input
IN
sn3729 3729fas
T
current
and Q
CC
SENSE
CC
that
to
B
,