RT6254A/B
Output Ripple Voltage
The output voltage transient undershoot and overshoot each
have two components : the voltage steps caused by the
output capacitor's ESR, and the voltage sag and soar due
to the finite output capacitance and the inductor current
slew rate. Use the following formulas to check if the ESR
is low enough (typically not a problem with ceramic
capacitors) and the output capacitance is large enough to
prevent excessive sag and soar on very fast load step
edges, with the chosen inductor value.
Output ripple voltage at the switching frequency is caused
by the inductor current ripple and its effect on the output
capacitor's ESR and stored charge. These two ripple
components are called ESR ripple and capacitive ripple.
Since ceramic capacitors have extremely low ESR and
relatively little capacitance, both components are similar
in amplitude and both should be considered if ripple is
critical.
The amplitude of the ESR step up or down is a function of
the load step and the ESR of the output capacitor :
VRIPPLE = VRIPPLE(ESR) VRIPPLE(C)
VRIPPLE(ESR) = IL RESR
IL
VESR_STEP = IOUT RESR
VRIPPLE(C)
=
8COUT fSW
The amplitude of the capacitive sag is a function of the
load step, the output capacitor value, the inductor value,
the input-to-output voltage differential, and the maximum
duty cycle. The maximum duty cycle during a fast transient
is a function of the on-time and the minimum off-time since
the ACOTTM control scheme will ramp the current using
on-times spaced apart with minimum off-times, which is
as fast as allowed. Calculate the approximate on-time
(neglecting parasitics) and maximum duty cycle for a given
input and output voltage as :
The typical operating circuit design for the RT6254A/B,
the output voltage is 1V, inductor ripple current is 1.23A,
and using 2 pieces of 22μF output capacitor with about
5mΩ ESR, the output voltage ripple components are :
VRIPPLE(ESR) = IL RESR = 1.23A5m = 6.15mV
IL
1.23A
VRIPPLE(C)
=
=
8COUT fSW
844μF500kHz
= 6.99mV
VRIPPLE = VRIPPLE(ESR) VRIPPLE(C) = 13.13mV
VOUT
IN fSW
tON
tON
=
and DMAX =
V
tON tOFF(MIN)
Output Transient Undershoot and Overshoot
In addition to output ripple voltage at the switching
frequency, the output capacitor and its ESR also affect
the voltage sag (undershoot) and soar (overshoot) when
the load steps up and down abruptly. TheACOTTM transient
response is very quick and output transients are usually
small. However, the combination of small ceramic output
capacitors (with little capacitance), low output voltages
(with little stored charge in the output capacitors), and
low duty cycle applications (which require high inductance
to get reasonable ripple currents with high input voltages)
increases the size of voltage variations in response to
very quick load changes. Typically, load changes occur
slowly with respect to the IC's 500kHz switching frequency.
The actual on-time will be slightly longer as the IC
compensates for voltage drops in the circuit, but we can
neglect both of these since the on-time increase
compensates for the voltage losses. Calculate the output
voltage sag as :
2
L(I
)
OUT
V
SAG
=
2C
V
D V
MAX OUT
OUT
IN(MIN)
The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor value
and the output voltage :
2
L(I
)
OUT
V
SOAR
=
2C
V
OUT
OUT
But some modern digital loads can exhibit nearly
instantaneous load changes and the following section
shows how to calculate the worst-case voltage swings in
response to very fast load steps.
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12
DS6254A/B-02 March 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
