14
DS8241-03 January 2014
www.richtek.com
RT8241
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
ON
IN
OUT
LOAD(MAX)
T
(V
V
)
L
LIR I
×
−
=
×
where LIR is the ratio of peak-to-peak ripple current to the
maximum average inductor current. Select a low pass
inductor having the lowest possible DC resistance that
fits in the allowed dimensions. Ferrite cores are often the
best choice, although powdered iron is inexpensive and
can work well at 200kHz. The core must be large enough
not to saturate at the peak inductor current (I
PEAK
) :
PEAK
LOAD(MAX)
LOAD(MAX)
LIR
I
I
I
2
=
+
×
Output Capacitor Selection
The output filter capacitor must have ESR low enough to
meet output ripple and load transient requirement, 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 transient, 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 :
P P
LOAD(MAX)
V
ESR
I
−
≤
In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain at an
acceptable level of output voltage ripple :
P P
LOAD(MAX)
V
ESR
LIR I
−
≤
×
Organic semiconductor capacitor(s) or special polymer
capacitor(s) are recommended.
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
ESR C
4
π
=
≤
×
×
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 and unstable operation.
However, it is easy to add sufficient series resistance by
placing the capacitors a couple of inches downstream from
the inductor and connecting FB divider close to the
inductor. There are two related but distinct ways including
double pulsing and feedback loop instability to identify
the unstable operation. Double pulsing occurs due to noise
on the output or because the ESR is too low such 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 oscillation at the output after line
or load perturbations that can trip the over voltage
protection latch or cause the output voltage to fall below
the tolerance limit. The easiest method for stability
checking is to apply a very 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 probe. Do not allow more
than one ringing cycle after the initial step-response under-
or overshoot.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P
D(MAX)
= (T
J(MAX)
−
T
A
) /
θ
JA
where T
J(MAX)
is the maximum junction temperature, T
A
is
the ambient temperature, and
θ
JA
is the junction to ambient
thermal resistance.
For recommended operating condition specifications of
the RT8241, the maximum junction temperature is 125
°
C
Output Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as follows :
