RT6207A/B
Any sag is always short-lived, since the circuit quickly
sources current to regain regulation in only a few switching
cycles. With the RT6207B, any overshoot transient is
typically also short-lived since the converter will sink
current, reversing the inductor current sharply until the
output reaches regulation again. The RT6207A
discontinuous operation at light loads prevents sinking
current so, for that IC, the output voltage will soar until
load current or leakage brings the voltage down to normal.
VOUT
IN fSW
tON
tON
=
and DMAX =
V
tON tOFF(MIN)
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
Most applications never experience instantaneous full load
steps and the RT6207A/B high switching frequency and
fast transient response can easily control voltage regulation
at all times. Also, since the sag and soar both are
proportional to the square of the load change, if load steps
were reduced to 1A(from the 5Aexamples preceding) the
voltage changes would be reduced by a factor of almost
ten. For these reasons sag and soar are seldom an issue
except in very low-voltage CPU core or DDR memory
supply applications, particularly for devices with high clock
frequencies and quick changes into and out of sleep
modes. In such applications, simply increasing the amount
of ceramic output capacitor (sag and soar are directly
proportional to capacitance) or adding extra bulk
capacitance can easily eliminate any excessive voltage
transients.
load step, the output capacitor value, the inductor value
and the output voltage :
2
L(I
)
OUT
V
SOAR
=
2C
V
OUT
OUT
For the Typical Operating Circuit for 1.2V output, the circuit
has an inductor 1.8μH and 3 x 22μF output capacitance
with 5mΩ ESR each. The ESR step is 5A x 1.67mΩ =
8.35mV which is small, as expected. The output voltage
sag and soar in response to full 0A-5A-0A instantaneous
transients are :
1.2V
tON
=
= 153ns
12V650kHz
153ns
153ns 230ns
and DMAX
=
= 0.399
where 230ns is the minimum off time
1.8μH(5A)2
V
=
= 95mV
SAG
In any application with large quick transients, always
calculate soar to make sure that over-voltage protection
will not be triggered. Under-voltage is not likely since the
threshold is very low (60%), that function has a long delay
(250μs), and the IC will quickly return the output to
regulation. Over-voltage protection has a minimum
threshold of 120% and short delay of 10μs and can actually
be triggered by incorrect component choices, particularly
for the RT6207A which does not sink current.
266μF 12V0.3991.2V
1.8μH(5A)2
266μF1.2V
V
SOAR
=
= 284mV
The sag is about 7.92% of the output voltage and the soar
is a full 23.7% of the output voltage. The ESR step is
negligible here but it does partially add to the soar, so
keep that in mind whenever using higher-ESR output
capacitors.
The soar is typically much worse than the sag in high
input, low-output step-down converters because the high
input voltage demands a large inductor value which stores
lots of energy that is all transferred into the output if the
load stops drawing current. Also, for a given inductor, the
soar for a low output voltage is a greater voltage change
and an even greater percentage of the output voltage.
Feed-forward Capacitor (Cff)
The RT6207A/B are optimized for ceramic output
capacitors and for low duty cycle applications. However
for high-output voltages, with high feedback attenuation,
the circuit's response becomes over-damped and transient
response can be slowed. In high-output voltage circuits
(VOUT > 3.3V) transient response is improved by adding a
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6207A/B-02 December 2015
www.richtek.com
17
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