RT5789A/B
voltage of 5V with inductor current ripple of 1.2A (i.e. 20%,
in the recommended range of 20% to 50%, of the maximum
rated output current) is taken as the design example. The
approximate minimum inductor value can first be
calculated as below :
usually occurs at 50% duty cycle, that is, VIN = 2 x VOUT.
The maximum IRMS as IRMS (Max), can be approximated
,
as 0.5 x IOUT_MAX, where IOUT_MAX is the maximum rated
output current. Besides, the variation of the capacitance
value with temperature, DC bias voltage, switching
frequency, and allowable peal-to-peak ripple voltage that
reflects back to the input, also need to be taken into
consideration. For example, the capacitance value of a
capacitor decreases as the DC bias across the capacitor
increases; also, higher switching frequency allows the use
of input capacitors of smaller capacitance values.
1.2 5 1.2
51500kHz1.2A
L =
= 0.5μH
where fSW is 1500kHz. The inductor current ripple will be
set at 1.2A, as long as the calculated inductance of 0.5μH
is used. However, the inductor of the exact inductance
value may not be readily available, and therefore an inductor
of a nearby value will be chosen. In this case, 0.47μH
inductance is available and actually used in the Typical
Application Circuit. The actual inductor current ripple (ΔIL)
and required peak inductor current (IL_PEAK) can be
calculated as below :
Ceramic capacitors are most commonly used to be placed
right at the input of the converter to reduce ripple voltage
amplitude because only ceramic capacitors have
extremely low ESR which is required to reduce the ripple
voltage. Note that the capacitors need to be placed as
close as to the input pins as possible for highest
effectiveness. Ceramic capacitors are preferred also due
to their low cost, small size, high RMS current ratings,
robust inrush surge current capabilities, and low parasitic
inductance, which helps reduce the high-frequency ringing
on the input supply.
1.2 5 1.2
51500kHz0.47μH
IL=
= 1.294A
1
2
1.294
IL_PEAK = IOUT_MAX
IL = 6 +
= 6.647A
2
For the 0.47μH inductance value, the inductor saturation
current and thermal rating should exceed 6.647A.
However, care must be taken when ceramic capacitors
are used at the input, and the input power is supplied by
a wall adapter, connected through a long and thin wire.
When a load step occurs at the output, a sudden inrush
current will surge through the long inductive wire, which
can induce ringing at the device's power input and
potentially cause a very large voltage spike at the VIN pin
to damage the device. For applications where the input
power is located far from the device input, it may be required
that the low-ESR ceramic input capacitors be placed in
parallel with a bulk capacitor of other types, such as
tantalum, electrolytic, or polymer, to dampen the voltage
ringing and overshoot at the input, caused by the long
input power path and input ceramic capacitor.
Input Capacitor Selection
Input capacitors are needed to smooth out the RMS ripple
current (IRMS) imposed by the switching currents and
drawn from the input power source, by reducing the ripple
voltage amplitude seen at the input of the converters. The
voltage rating of the input filter capacitors must be greater
than the maximum input voltage. It's also important to
consider the ripple current capabilities of capacitors.
The RMS ripple current (IRMS) of the regulator can be
determined by the input voltage (VIN), output voltage
(VOUT), and rated output current (IOUT) as the following
equation :
V
V
V
IN
V
OUT
OUT
I
= I
1
RMS
OUT
It is suggested to choose capacitors with higher
temperature ratings than required. Several ceramic
capacitors may be parallel to meet application
requirements, such as the RMS current, size, and height.
The Typical Application Circuit can use one 22μF, or two
10μF and one high-frequency-noise-filtering 0.1μF low-ESR
ceramic capacitors at the input.
IN
From the above, the maximum RMS input ripple current
occurs at maximum output load, which will be used as
the requirements to consider the current capabilities of
the input capacitors. Furthermore, for a single phase buck
converter, the duty cycle is approximately the ratio of
output voltage to input voltage. The maximum ripple voltage
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS5789A/B-02 March 2018
www.richtek.com
13
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
