LTC3728_15 Datasheet, PDF (25/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
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 LTC3728 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).
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.com).
INTV_CC
R_T2
R_T1
I_TH
R_C
LTC3728
C_C
3728 F08
Figure 8. Active Voltage Positioning
Applied to the LTC3728
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 LTC3728 circuits: 1) LTC3728 V_IN cur-
rent (including loading on the 3.3V internal regulator),
2) INTV_CC regulator current, 3) I²R losses, 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 cur-
rents; the second is the current drawn from the 3.3V
linear regulator 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 current. In continuous mode,
I_GATECHG = f(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
a factor of (Duty Cycle)/(Efficiency). For example, in a
20V to 5V application, 10mA of INTV_CC current results
in approximately 2.5mA 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 resis-
tor, 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 resis-
tance 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) = 30mΩ, R_L = 50mΩ,
3728fg
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