Datasheet

LTC3880/LTC3880-1
53
3880fc
For more information www.linear.com/LTC3880
APPLICATIONS INFORMATION
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 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 and R
SENSE
to ob-
tain 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.
Efficiency 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 significant 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% 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
adequate charge storage and very low ESR at the switch-
ing 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 LTC3880
2-phase architecture typically halves this input capacitance
requirement over competing solutions. Other losses
including Schottky conduction 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 ∆I
LOAD
(ESR), where ESR is the effective
series resistance of C
OUT
. ∆I
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 recov-
ery 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-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 fac-
tor 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.
The only two programmable parameters that affect loop
gain are the voltage range, bits 5 and 6 of the MFR_PWM_
CONFIG_LTC3880 command and the current range, bit 7
of the MFR_P
WM_MODE_L
TC3880 command. Be sure to
establish these settings prior to compensation calculation.
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%