Datasheet

LTC3864
20
3864f
APPLICATIONS INFORMATION
but also provides a test point for the step-down regulator ’s
DC-coupled and AC-filtered closed-loop response. 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 this pin.
The ITH series R
ITH
-C
ITH1
filter sets the dominant pole-zero
loop compensation. Additionally, a small capacitor placed
from the ITH pin to signal ground, C
ITH2
, may be required to
attenuate high frequency noise. The values can be modified
to optimize transient response once the final PCB layout
is done and the particular output capacitor type and value
have been determined. The output capacitors need to be
selected because their various types and values determine
the loop feedback factor gain and phase. An output current
pulse of 20% to 100% 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 breaking the feedback loop. The general
goal of OPTI-LOOP compensation is to realize a fast but
stable ITH response with minimal output droop due to
the load step. For a detailed explanation of OPTI-LOOP
compensation, refer to Application Note 76.
Switching regulators take several cycles to respond to a
step in load current. When a load step occurs, V
OUT
im-
mediately 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 a feedback
error signal used by the regulator to return V
OUT
to its
steady-state value. During this recovery time, V
OUT
can
be monitored for overshoot or ringing that would indicate
a stability problem.
Connecting a resistive load in series with a power MOSFET,
then placing the two directly across the output capacitor
and driving the gate with an appropriate signal generator
is a practical way to produce a realistic load-step condi-
tion. 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 feedback loop response.
The gain of the loop increases with R
ITH
and the bandwidth
of the loop increases with decreasing C
ITH1
. If R
ITH
is
increased by the same factor that C
ITH1
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. In addition, a feedforward capacitor, C
FF
, can
be added to improve the high frequency response, as shown
in Figure 1. Capacitor C
FF
provides phase lead by creating
a high frequency zero with R
FB2
which improves the phase
margin. The output voltage settling behavior is related to
the stability of the closed-loop system and will demonstrate
overall performance of the step-down regulator.
In some applications, a more severe transient can be caused
by switching in loads with large (>10μF) input capacitors.
If the switch connecting the load has low resistance and
is driven quickly, then the discharged input capacitors are
effectively put in parallel with C
OUT
, causing a rapid drop in
V
OUT
. No regulator can deliver enough current to prevent
this problem. The solution is to limit the turn-on speed of
the load switch driver. A Hot Swap™ controller is designed
specifically for this purpose and usually incorporates cur-
rent limiting, short-circuit protection and soft starting.
Design Example
Consider a step-down converter with the following
specifications: V
IN
= 5V to 55V, V
OUT
= 5V, I
OUT(MAX)
= 2A,
and f = 350kHz (Figure 8).
The output voltage is programmed according to:
V
OUT
= 0.8V • 1+
R
FB2
R
FB1






If R
FB1
is chosen to be 80.6k, then R
FB2
would have to
be 422k.