LTC3729_15 Datasheet, PDF (20/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
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_TH pin with a resistive divider
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage range of the error amplifier, or
1.2V (see Figure 8).
INTV_CC
R_T2
R_T1
I_TH
R_C
LTC3729
C_C
3729 F08
Figure 8. Active Voltage Positioning Applied to the LTC3729
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:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent‑
age 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_IN current
(including loading on the differential amplifier output),
2) INTV_CC regulator current, 3) I²R losses and 4) Topside
MOSFET transition losses.
1) The V_IN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control currents;
the second is the current drawn from the differential
amplifier 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 cur‑
rent. In continuous mode, I_GATECHG = (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 the ratio (Duty
Factor)/(Efficiency). For example, in a 20V to 5V application,
10mA of INTV_CC current results in approximately 3mA 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.
3) I²R losses are predicted from the DC resistances of the
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_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 I²R losses. For example,
if each R_DS(ON)=10mΩ, R_L=10mΩ, and R_SENSE=5mΩ,
then the total resistance is 25mΩ. 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_OUT for the same external
components and output power level. The combined effects
3729fb
20