ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 25

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

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
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:
where L1, L2, etc. are the individual losses as a percent-
age of input power.
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
Figure 8. Active Voltage Positioning
Applied to the LTC3728
INTV
CC
R
R
T2
T1
R
C
C
C
TH
I
TH
pin with a resistive divider
LTC3728
3728 F08
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
rent (including loading on the 3.3V internal regulator),
2) INTV
MOSFET transition losses.
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
to ground. The resulting dQ/dt is a current out of INTV
that is typically much larger than the control circuit cur-
rent. In continuous mode, I
and Q
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)/(Effi ciency). For example, in a 20V to 5V ap-
plication, 10mA of INTV
2.5mA of V
from 10% or more (if the driver was powered directly from
V
3. I
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
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R
one MOSFET can simply be summed with the resistances
of L, R
IN
SENSE
) to only a few percent.
2
R losses are predicted from the DC resistances of
B
SENSE
, but is “chopped” between the topside MOSFET
CC
CC
are the gate charges of the topside and bottom
IN
IN
current has two components: the fi rst is the DC
current is the sum of the MOSFET driver and
regulator current, 3) I
IN
current typically results in a small (<0.1%)
and ESR to obtain I
current. This reduces the mid-current loss
CC
power through the EXTV
CC
current results in approximately
DS(ON)
GATECHG
2
R losses. For example, if
, then the resistance of
2
R losses, 4) Topside
=f(Q
LTC3728
T
+Q
CC
B
switch input
), where Q
IN
25
current
IN
3728fd
cur-
CC
CC
T