Datasheet

LTC3728
25
3728fg
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
specifi cations. 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 amplifi er, or
1.2V (see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifi er. 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).
Effi ciency Considerations
The percent effi ciency 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 effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
I
TH
R
C
R
T1
INTV
CC
C
C
3728 F08
LTC3728
R
T2
Figure 8. Active Voltage Positioning
Applied to the LTC3728
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) I
2
R losses, 4) Topside
MOSFET transition losses.
1. The V
IN
current has two components: the fi rst 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)/(Effi ciency). 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. I
2
R 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 fl ows 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 I
2
R losses.
For example, if each R
DS(ON)
= 30m, R
L
= 50m,