RT5789A/B
Application Information
The output stage of a synchronous buck converter is
composed of an inductor and capacitor, which stores and
delivers energy to the load, and forms a second-order low-
pass filter to smooth out the switch node voltage to maintain
a regulated output voltage.
and the approximate inductance can be calculated by the
selected input voltage, output voltage, switching frequency
(fSW), and inductor current ripple (ΔIL), as below :
V
V V
IN OUT
OUT
L =
V f
IN SW
I
L
Once the inductance is chosen, the inductor ripple current
(ΔIL) and peak inductor current (IL_PEAK) can be calculated,
as below :
100% Duty-Cycle
When the input voltage drops, these Buck converters
gradually increase the duty-cycle and will continuously
switch-on the high side MOSFET when the input voltage
drops below the regulated output voltage. This function is
especially suitable in battery powered applications, and
can extend application operation time when the battery is
almost depleted.
VOUT VIN VOUT
IL=
V fSW L
IN
1
2
IL_PEAK = IOUT_MAX
IL
1
2
IL_VALLEY = IOUT_MAX
IL
where IOUT_MAX is the maximum rated output current or
the required peak current.
Inductor Selection
When designing the output stage of the synchronous buck
converter, it is recommended to start with the inductor.
However, it may require several iterations because the
exact inductor value is generally flexible and is optimized
for low cost, small form factor, and high overall performance
of the converter. Further, inductors vary with manufacturers
in both material and value, and typically have a tolerance
of 20%.
The inductor must be selected to have a saturation current
and thermal rating which exceed the required peak inductor
current IL_PEAK. For a robust design to maintain control of
inductor current in overload or short-circuit conditions,
some applications may desire inductor saturation current
rating up to the high-side switch current limit of the device.
However, the built-in output under-voltage protection (UVP)
feature makes this unnecessary for most applications.
Three key inductor parameters to be specified for operation
with the device are inductance (L), inductor saturation
current (ISAT), and DC resistance (DCR), which affects
performance of the output stage. An inductor with lower
DCR is recommended for applications of higher peak
current or load current, and it can improve system
performance. Lower inductor values are beneficial to the
system in physical size, cost, DCR, and transient
response, but they will cause higher inductor peak current
and output voltage ripple to decrease system efficiency.
Conversely, higher inductor values can increase system
efficiency at the expense of larger physical size, slower
transient response due to the longer response time of the
inductor. Agood compromise among size, efficiency, and
transient response can be achieved by setting an inductor
current ripple (ΔIL) of about 20% to 50% of the desired full
output load current. To meet the inductor current ripple
(ΔIL) requirements, a minimum inductance must be chosen
IL_PEAK should not exceed the minimum value of the
device's high-side switch current limit because the device
will not be able to supply the desired output current. By
reducing the inductor current ripple (ΔIL) to increase the
average inductor current (and the output current), IL_PEAK
can be lowered to meet the device current limit
requirement.
For best efficiency, a low-loss inductor having the lowest
possible DCR that still fits in the allotted dimensions will
be chosen. Ferrite cores are often the best choice.
However, a shielded inductor, possibly larger or more
expensive, will probably give fewer EMI and other noise
problems.
The following design example is illustrated to walk through
the steps to apply the equations defined above. The
RT5789A/B's TypicalApplication Circuit for output voltage
of 1.2V at maximum output current of 6A and an input
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12
DS5789A/B-02 March 2018
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