LTC3731H_15 Datasheet, PDF (21/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
Applications Information
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:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Checking Transient Response
The regulator loop response can be checked by look-
ing 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_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 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-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 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 0.2 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_TH pin waveforms
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_TH pin signal which is in the
feedback loop and is the filtered and compensated control
loop response. The gain of the loop will be increased by
increasing R_C and the bandwidth of the loop will be in-
creased by decreasing C_C. If R_C is increased by the same
factor that C_C. is decreased, the zero frequency will be
kept 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
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_OUT, causing a rapid drop in V_OUT. No regulator can
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_LOAD is greater
than 2% of C_OUT, the switch rise time should be controlled
so that the load rise time is limited to approximately
1000 • R_SENSE • C_LOAD. Thus a 250µF capacitor and a 2mΩ
R_SENSE resistor would require a 500µs rise time, limiting
the charging current to about 1A.
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