ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 27

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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
optimization of control loop behavior, but it 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
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
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.
TH
external components shown in Figure 13 circuit will
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
C
OUT
filter sets the dominant pole-zero
(ESR), where ESR is the effective
. ΔI
LOAD
TH
pin waveforms that will
also begins to charge or
TH
pin not only allows
OUT
shifts by an
Placing a resistive load and 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
the feedback loop and is the filtered and compensated
control loop response.
The gain of the loop will be increased by increasing R
and the bandwidth of the loop will be increased by de-
creasing C
is decreased, the zero frequency 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
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
C
OUT
. If R
is greater than 1:50, the switch rise time
C
is increased by the same factor that C
LOAD
. Thus a 10μF capacitor would
TH
pin signal which is in
LTC3858-2
OUT
. No regulator can
27
38582f
C
C