ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 22

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3827-1
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 ΔI
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.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The availability of the I
allows optimization of control loop behavior but also
provides a DC coupled and AC filtered closed loop re-
sponse 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 band-
width can also be estimated by examining the rise time at
the pin. The I
circuit will provide an adequate starting point for most
applications.
The I
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
22
TH
OUT
OUT
series R
can be monitored for excessive overshoot or
OUT
to its steady-state value. During this recovery
TH
generating the feedback error signal that
external components shown in Figure 13
C
-C
LOAD
U
C
OUT
filter sets the dominant pole-zero
(ESR), where ESR is the effective
. ΔI
U
LOAD
also begins to charge or
W
OUT
TH
pin not only
shifts by an
U
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 I
give a sense of the overall loop stability without breaking
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 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 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
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited to
approximately 25 • C
require a 250μs rise time, limiting the charging current to
about 200mA.
LOAD
OUT
to C
, causing a rapid drop in V
OUT
C
C
is decreased, the zero frequency will be kept
is greater than 1:50, the switch rise time
and the bandwidth of the loop will be
LOAD
C
. Thus a 10μF capacitor would
. If R
TH
TH
C
pin signal which is in the
is increased by the same
pin waveforms that will
OUT
. No regulator can
38271fd