RT7275/76
18
DS7275/76-04 April 2017www.richtek.com
©
Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
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 (70%), 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 115% and short delay of 5μs and can actually
be triggered by incorrect component choices, particularly
for the RT7276 which does not sink current.
Output Capacitors Stability Criteria
The RT7275/76's ACOT
TM
control architecture uses an
internal virtual inductor current ramp and other
compensation that ensures stability with any reasonable
output capacitor. The internal ramp allows the IC to operate
with very low ESR capacitors and the IC is stable with
very small capacitances. Therefore, output capacitor
selection is nearly always a matter of meeting output
voltage ripple and transient response requirements, as
discussed in the previous sections. For the sake of the
unusual application where ripple voltage is unimportant
and there are few transients (perhaps battery charging or
LED lighting) the stability criteria are discussed below.
The equations giving the minimum required capacitance
for stable operation include a term that depends on the
output capacitor's ESR. The higher the ESR, the lower
the capacitance can be and still ensure stability. The
equations can be greatly simplified if the ESR term is
removed by setting ESR to zero. The resulting equation
gives the worst-case minimum required capacitance and
it is usually sufficiently small that there is usually no need
for the more exact equation.
The required output capacitance (C
OUT
) is a function of
the inductor value (L) and the input voltage (V
IN
) :
11
OUT
IN
5.23 10
C
VL
The worst-case high capacitance requirement is for low
VIN and small inductance, so a 5V to 3.3V converter is
used for an example. Using the inductance equation in a
previous section to determine the required inductance :
3.3V 5V 3.3V
L = = 1.6μH
5V 700kHz 1A
11
OUT
5.23 10
C = 6.6μF
5V 1.6μH
Therefore, the required minimum capacitance for the 5V
to 3.3V converter is :
11
OUT
5.24 10
C = 3.1μF
12V 1.4μH
Using the 12V to 1.05V typical application as another
example :
OUT
OUT
SW IN ESR OUT
V
C
2 f V (R 13647 L V )
Any ESR in the output capacitor lowers the required
minimum output capacitance, sometimes considerably.
For the rare application where that is needed and useful,
the equation including ESR is given here :
As can be seen, setting R
ESR
to zero and simplifying the
equation yields the previous simpler equation. To allow
for the capacitor's temperature and bias voltage coefficients,
use at least double the calculated capacitance and use a
good quality dielectric such as X5R or X7R with an
adequate voltage rating since ceramic capacitors exhibit
considerable capacitance reduction as their bias voltage
increases.
Feed-Forward Capacitor (C3)
The RT7275/76 are optimized for ceramic output capacitors
and for low duty cycle applications. This optimization
makes circuit stability easy to achieve with reasonable
output capacitors. However, the optimization affects the
quality factor (Q) of the circuit and therefore its transient
response. To avoid an under-damped response (high Q)
and its potential ringing, the internal compensation was
chosen to achieve perfect damping for low output voltages,
where the FB divider has low attenuation (V
OUT
is close
to V
REF
). 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 (V
OUT
> 1.5V) transient response is
improved by adding a small “feed-forward” capacitor (C3)
across the upper FB divider resistor, to increase the
circuit's Q and reduce damping to speed up the transient
response without affecting the steady-state stability of
the circuit. Choose a capacitor value that gives, together
with the divider impedance at FB, a time-constant between
100ns and 0.5μs. The divider impedance at FB is R1 in
parallel with R2. C3 can be safely left out in low-output
voltage circuits and if fast transient response is not required.
