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DS8207L/M-08 September 2016 www.richtek.com
RT8207L/M
©
Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
This inductor ripple current also impacts transient-response
performance, especially at low V
IN
− V
VDDQ
differences.
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output filter
capacitors by a sudden load step. The peak amplitude of
the output transient (V
SAG
) is also a function of the output
transient. V
SAG
also features a function of the maximum
duty factor, which can be calculated from the on-time and
minimum off-time :
()
SAG
2
LOAD ON OFF(MIN)
OUT VDDQ IN ON VDDQ ON OFF(MIN)
V
I x L x (tt )
2 x C x V x V x t V x (t t )
where minimum off-time, t
OFF(MIN)
, is 400ns typically.
Output Capacitor Selection
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
Also, the capacitance must be high enough to absorb the
inductor energy going from a full-load to no-load condition
without tripping the OVP circuit.
For CPU core voltage converters and other applications
where the output is subject to violent load transients, the
output capacitor's size depends on how much ESR is
needed to prevent the output from dipping too low under a
load transient. Ignoring the sag due to finite capacitance :
PP
LOAD(MAX)
V
ESR
I
In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple :
PP
IR LOAD(MAX)
V
ESR
L x I
where V
P−P
is the peak-to-peak output voltage ripple.
Organic semiconductor capacitor(s) or specialty polymer
capacitor(s) are recommended.
For low input-to-output voltage differentials (V
IN
/V
VDDQ
<
2), additional output capacitance is required to maintain
stability and good efficiency in ultrasonic mode.
2
PEAK
SOAR
OUT VDDQ
(I ) x L
V
2 x C x V
where I
PEAK
is the peak inductor current.
Output Capacitor Stability
Stability is determined by the value of the ESR zero relative
to the switching frequency. The point of instability is given
by the following equation :
SW
ESR
OUT
f
1
f
2 x x ESR x C 4
The amount of overshoot due to stored inductor energy
can be calculated as :
Do not put high value ceramic capacitors directly across
the outputs without taking precautions to ensure stability.
Large ceramic capacitors can have a high ESR zero
frequency and cause erratic, unstable operation. However,
it is easy to add enough series resistance by placing the
capacitors a couple of inches downstream from the
inductor and connecting VDDQ or the FB voltage-divider
close to the inductor.
Unstable operation manifests itself in two related and
distinctly different ways: double-pulsing and feedback loop
instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there is not enough voltage
ramp in the output voltage signal. This
“fools” the error
comparator into triggering a new cycle immediately after
the 400ns minimum off-time period has expired. Double
pulsing is more annoying than harmful, resulting in nothing
worse than increased output ripple. However, it may
indicate the possible presence of loop instability, which
is caused by insufficient ESR.
Loop instability can result in oscillations at the output in
the form of line or load perturbations, which can trip the
over voltage protection latch or cause the output voltage
to fall below the tolerance limit.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully observe
the output-voltage-ripple envelope for overshoot and ringing.
It helps to simultaneously monitor the inductor current
with an AC current probe. Do not allow more than one
cycle of ringing after the initial step-response under- or
over-shoot.
