MIC5191_06 Datasheet, PDF (8/15 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter / LDO Controller

Micrel, Inc.
Application Information
Designing with the MIC5191
Anatomy of a transient response
A voltage regulator can maintain a set output voltage
while its exterior world is pushing and pulling in its
demand for power. The measure of a regulator is
generally how accurately and effectively it can maintain
that voltage, regardless of how the load demands power.
One measure of regulator response is the load step.
This is an intuitive look at how the regulator responds to
a change in load current. Figure 2 is a look at the
transient response to a load step.
V ¹ it
C
Output voltage vs. Time
during recovery is
1 directly proportional to
BW
gain vs. frequency
Time
Figure 2. Typical Transient Response
At the start of a circuit's power demand, the output
voltage is regulated to its set point, while the load current
runs at a constant rate. For many different reasons, a
load may ask for more current without warning. When
this happens, the regulator needs some time to
determine the output voltage drop. This is determined by
the speed of the control loop. So, until enough time has
elapsed, the control loop is oblivious to the voltage
change. The output capacitor must bear the burden of
maintaining the output voltage.
∆V = L ^di
dt
Since this is a sudden change in voltage, the capacitor
will try to maintain voltage by discharging current to the
output. The first voltage drop is due to the output
capacitor's ESL (equivalent series inductance). The ESL
will resist a sudden change in current from the capacitor
and drop the voltage quickly. The amount of voltage
drop during this time will be proportional to the output
capacitor's ESL and the speed at which the load steps.
Slower load current transients will reduce this effect.
MIC5191
∆V ↓= L ^di
dt ↑
Placing multiple small capacitors with low ESL in parallel
can help reduce the total ESL and reduce voltage droop
during high speed transients. For high speed transients,
the greatest voltage deviation will generally be caused
by output capacitor ESL and parasitic inductance.
∆V ↓= L ↓ ^di
dt
After the current has overcome the effects of the ESL,
the output voltage will begin to drop proportionally to
time and inversely proportional to output capacitance.
∫ ∆V = 1 idt
C
The relationship to output voltage variation will depend
on two aspects, loop bandwidth and output capacitance.
The output capacitance will determine how far the
voltage will fall over a given time. With more capacity-
ance, the drop in voltage will fall at a decreased rate.
This is the reason that for the same bandwidth, more
capacitance provides a better transient response.
∫ ∆V ↓= 1 idt
↑C
Also, the time it takes for the regulator to respond is
directly proportional to its gain bandwidth. Higher
bandwidth control loops respond quicker causing a
reduced droop on the supply for the same amount of
capacitance.
∫ ∆V ↓= 1 idt ↓
C
Final recovery back to the regulated voltage is the final
phase of transient response and the most important
factors are gain and time. Higher gain at higher
frequency will get the output voltage closer to its
regulation point quicker. The final settling point will be
determined by the load regulation, which in proportional
to DC (0Hz) gain and the associated loss terms.
There are other factors that contribute to large signal
transient response, such as source impedance, phase
margin and PSRR. For example, if the input voltage
drops due to source impedance during a load transient,
this will contribute to the output voltage deviation by
filtering through to the output reduced by the loops
PSRR at the frequency of the voltage transient. It is
straightforward: good input capacitance reduces the
source impedance at high frequencies. Having between
35° and 45° of phase margin will help speed up the
recovery time. This is caused by the initial overshoot in
response to the loop sensing a low voltage.
December 2006
8
M9999-122206