LTC3728L/LTC3728LX
26
3728lxff
applicaTions inForMaTion
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. 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 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 I
2
R losses. For example,
if each R
DS(ON)
= 30mΩ, R
L
= 50mΩ, R
SENSE
= 10mΩ
and R
ESR
= 40mΩ (sum of both input and output ca-
pacitance losses), then the total resistance is 130mΩ.
This results in losses ranging from 3% to 13% as the
output current increases from 1A to 5A for a 5V output,
or a 4% to 20% loss for a 3.3V output. Efficiency var-
ies as the inverse square of V
OUT
for the same external
components and output power level. The combined
effects of increasingly lower output voltages and higher
currents required by high performance digital systems
is not doubling, but quadrupling, the importance of loss
terms in the switching regulator system!
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high input
voltages (typically 15V or greater). Transition losses can
be estimated from:
Transition Loss = V
IN
( )
2
•
I
MAX
2
R
DR
( )
•
C
MILLER
( )
f
( )
1
5V – V
TH
+
1
V
TH
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% efficiency degradation in portable systems. It is very
important to include these system level losses during the
design phase. The internal battery and fuse resistance
losses can be minimized by making sure that C
IN
has ad-
equate charge storage and very low ESR at the switching
frequency. A 25W supply will typically require a minimum
of 20µF to 40µF of capacitance having a maximum of 20mΩ
to 50mΩ of ESR. The LTC3728L 2-phase architecture
typically halves this input capacitance requirement over
competing solutions. Other losses, including Schottky con-
duction losses during dead time and inductor core losses,
generally account for less than 2% total additional loss.
Checking Transient Response
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, V
OUT
shifts by an
amount equal to ΔI
LOAD
(ESR), where ESR is the effective
series resistance of C
OUT
. ΔI
LOAD
also begins to charge or
discharge C
OUT
generating the feedback error signal that
forces the regulator to adapt to the current change and
return V
OUT
to its steady-state value. During this recovery
time, V
OUT
can be monitored for excessive overshoot or
ringing, which would indicate a stability problem. OPTI-
LOOP compensation allows the transient response to be
optimized over a wide range of output capacitance and
ESR values.
The availability of the I
TH
pin not only allows
optimization of control loop behavior but also provides
a DC coupled and AC filtered closed loop response test
point. The DC step, rise time and settling at this test
point truly reflects the closed loop response.
Assuming a
predominantly second order system, phase margin and/
or damping factor can be estimated using the percentage
of overshoot seen at this pin. The bandwidth can also be
estimated by examining the rise time at the pin. The I
TH
external components shown in the Figure 1 circuit will
provide an adequate starting point for most applications.
The I
TH
series R
C
-C
C
filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the final PC layout is done and
the particular output capacitor type and value have been
determined. The output capacitors need to be selected
because the various types and values determine the loop
gain and phase. An output current pulse of 20% to 80%
of full-load current having a rise time of 1µs to 10µs will
