LTC3731HGTR Linear Technology, LTC3731HGTR Datasheet - Page 21

• Manufacturer Part Number: LTC3731HGTR
• Manufacturer: Linear Technology

Specifications of LTC3731HGTR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIO S I FOR ATIO
Efficiency Considerations
The percent efficiency 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 efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Checking Transient Response
The regulator loop response can be checked by looking at
the load 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 I
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
ponents shown in the Figure 1 circuit will provide an
adequate starting point for most applications.
The I
loop compensation. The values can be modified slightly
%Efficiency = 100% – (L1 + L2 + L3 + ...)
TH
OUT
OUT
series R
can be monitored for excessive overshoot or
OUT
to its steady-state value. During this recovery
, generating the feedback error signal that
C
-C
LOAD
TH
U
C
OUT
pin not only allows optimization of
filter sets the dominant pole-zero
• ESR, where ESR is the effective
. ∆I
U
LOAD
also begins to charge or
W
OUT
TH
external com-
shifts by an
U
(from 0.2 to 5 times their suggested values) to maximize
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 decided
upon because the various types and values determine the
loop feedback factor gain and phase. An output current
pulse of 20% to 80% of full load current having a rise time
of <2µs will produce output voltage and I
that will give a sense of the overall loop stability without
breaking the feedback loop. 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 I
feedback loop and is the filtered and compensated control
loop response. The gain of the loop will be increased by
increasing R
increased by decreasing C
factor that C
the same, thereby keeping the phase 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 over-
all 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
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If C
than 2% of C
so that the load rise time is limited to approximately
1000 • R
R
the charging current to about 1A.
SENSE
OUT
resistor would require a 500µs rise time, limiting
SENSE
, causing a rapid drop in V
C
OUT
C
is decreased, the zero frequency will be kept
• C
and the bandwidth of the loop will be
, the switch rise time should be controlled
LOAD
. Thus a 250µF capacitor and a 2mΩ
C
TH
. If R
pin signal which is in the
C
is increased by the same
OUT
LTC3731H
. No regulator can
TH
pin waveforms
LOAD
is greater
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