LTC3854 LINER [Linear Technology], LTC3854 Datasheet - Page 17

• Manufacturer Part Number: LTC3854
• Description: Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
4. Transition losses apply only to the topside MOSFET(s),
Other “hidden” losses such as copper trace and the bat-
tery internal resistance 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
switching frequency. A 25W supply will typically require a
minimum of 20μF to 40μF of capacitance having a maxi-
mum of 20mΩ to 50mΩ of ESR. 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
amount equal to ∆I
series resistance of C
discharge C
forces the regulator to adapt to the current change and
return V
time V
ringing, which would indicate a stability problem. The
availability of the ITH pin not only allows optimization of
IN
I
= 10mΩ, R
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
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!
and become significant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Transition Loss = 1.7V
2
has adequate charge storage and very low ESR at the
R losses. For example, if each R
OUT
OUT
can be monitored for excessive overshoot or
OUT
to its steady-state value. During this recovery
SENSE
generating the feedback error signal that
LOAD
= 5mΩ then the total resistance is
OUT
• ESR, where ESR is the effective
. ∆I
IN
2
LOAD
• I
O(MAX)
also begins to charge or
DS(ON)
• C
RSS
OUT
= 10mΩ, DCR
• f
shifts by an
OUT
S
for the
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 ITH external components shown
in the Typical Application circuit will provide an adequate
starting point for most applications.
The ITH series R
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
produce output voltage and ITH pin waveforms that will
give a sense of the overall loop stability without break-
ing the feedback loop. Placing a power MOSFET directly
across the output capacitor and driving the gate with an
appropriate signal generator is a practical way to produce
a realistic load step condition. The initial output voltage
step resulting from the step change in output current may
not be within the bandwidth of the feedback loop, so this
signal cannot be used to determine phase margin. This
is why it is better to look at the ITH pin signal which is
in the feedback loop and is the filtered and compensated
control loop response. The gain of the loop will be in-
creased by increasing R
will be increased by decreasing C
the same factor that C
will be kept the same, thereby keeping the phase shift the
same in the most critical frequency range of the feedback
loop. The output voltage settling behavior is related to the
stability of the closed-loop system and will demonstrate
the actual overall supply performance.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
OUT
, causing a rapid drop in V
C
-C
C
C
filter sets the dominant pole-zero
is decreased, the zero frequency
C
and the bandwidth of the loop
C
. If R
OUT
. No regulator can
LTC3854
C
is increased by

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