Datasheet

LTC3865/LTC3865-1
26
3865fb
APPLICATIONS INFORMATION
Supplying INTV
CC
power through EXTV
CC
from an
output-derived source will scale the V
IN
current re-
quired 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 resistor.
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 resistance of one MOSFET can simply be
summed with the resistances of L and R
SENSE
to obtain
I
2
R losses. For example, if each R
DS(ON)
= 10mΩ, R
L
=
10mΩ, 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 for
a 5V output, or a 3% to 12% loss for a 3.3V output.
Effi ciency varies 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 signifi cant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Transition Loss = (1.7) V
IN
2
I
O(MAX)
C
RSS
f
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% effi ciency 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 adequate 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 max-
imum of 20mΩ to 50mΩ of ESR. The LTC3865 2-phase
architecture typically halves this input capacitance require-
ment over competing solutions. Other losses including
Schottky conduction losses during dead time and induc-
tor 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. The
availability of the I
TH
pin not only allows optimization of
control loop behavior but also provides a DC coupled and
AC fi ltered closed loop response test point. The DC step,
rise time and settling at this test point truly refl ects 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 Typical Application circuit will provide an adequate
starting point for most applications.