LTC3634
17
3634fc
For more information www.linear.com/LTC3034
Checking Transient Response
The regulator loop response can be checked by observing
the response of the system to a load step. The ITH pin not
only allows optimization of the control loop behavior but
also provides a DC-coupled and AC filtered closed loop
response test point. The DC step, rise time, and settling
behavior at this test point reflect 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.
After choosing compensation values as discussed in the
previous section, the design should be tested to verify
stability. The component values may be modified slightly
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 their various types and values determine
the loop gain and phase. An output current pulse of 20%
to 100% of full load current having a rise time of ~1μs will
produce output voltage and ITH pin waveforms that will
give a sense of the overall loop stability without breaking
the feedback loop.
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 ΔI
LOAD
• ESR, where
ESR is the effective series resistance of C
OUT
. ΔI
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.
When observing the response of V
OUT
to a load step, the
initial output voltage step may not be within the bandwidth of
the feedback loop, so the standard second order overshoot/
DC ratio cannot be used to determine phase margin. The
output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a
review of control loop theory, refer to Application Note 76.
APPLICATIONS INFORMATION
In some applications, a more severe transient can be caused
by switching in loads with large (>10μF) input capacitors.
The discharged input capacitors are effectively put in paral-
lel with C
OUT
, causing a rapid drop in V
OUT
. No regulator
can deliver enough current to prevent this problem, if the
switch connecting the load has low resistance and is driven
quickly. 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.
INTV
CC
Regulator Bypass Capacitor
An internal low dropout (LDO) regulator produces the 3.3V
supply that powers the internal bias circuitry and drives
the gate of the internal MOSFET switches. The INTV
CC
pin
connects to the output of this regulator and must have a
minimum of 1μF ceramic bypass capacitance to ground.
This capacitor should have low impedance electrical
connections to the INTV
CC
and PGND pins to provide the
transient currents required by the LTC3634. This supply
is intended only to supply additional DC load currents as
desired and not intended to regulate large transient or AC
behavior, as this may impact LTC3634 operation.
Boost Capacitor
The LTC3634 uses a bootstrap circuit to create a voltage
rail above the applied input voltage V
IN
. Specifically, a boost
capacitor, C
BOOST
, is charged to a voltage approximately
equal to INTV
CC
each time the bottom power MOSFET is
turned on. The charge on this capacitor is then used to
supply the required transient current during the remainder
of the switching cycle. When the top MOSFET is turned on,
the BOOST pin voltage will be equal to approximately V
IN
+ 3.3V. For most applications, a 0.1μF ceramic capacitor
closely connected between the BOOST and SW pins will
provide adequate performance.
