ACT81460
Rev 1.0, 18-Dec-2018
input trigger to another regulator may or may not disable
the other regulator following it, depending on the spe-
cific CMI settings. The buck-boost has a load discharge
function designed to quickly pull the output voltage to
ground when it is disabled. The circuit connects an in-
ternal resistor (41ohm) from the output to PGND34
when the buck-boost is disabled.
boost converter. VINBB must have a dedicated input ca-
pacitor that is optimally placed to minimize the power
routing loop. Note that even though the buck-boost con-
verter has a separate input, the input must be con-
nected to the same voltage potential as the buck regu-
lators.
The VINBB pin requires a high quality, low-ESR, ce-
ramic input capacitor. 1uF capacitors are typically suit-
able, but this value can be increased without limit.
Smaller capacitor values can be used with lighter output
loads. Choose the input capacitor value to keep the in-
put voltage ripple less than 50mV.
POK and Output Fault Interrupt
The buck-boost features a Power-OK (POK) status bit
that can be read by the system microprocessor via the
I2C interface. If the output voltage is lower than the
power-OK threshold, typically 90% of the programmed
regulation voltage, the POK bit will be 0.
A capacitor’s actual capacitance is strongly affected by
its DC bias characteristics. The input capacitor is typi-
cally an X5R, X7R, or similar dielectric. Use of Y5U, Z5U,
or similar dielectrics is not recommended. Input capaci-
tor placement is critical for proper operation. Each
buck’s input capacitor must be placed as close to the IC
as possible. The traces from VINBx to the capacitor and
from the capacitor to PGNDx should as short and wide
as possible.
If the nFLTMSK bit is set to 1, the ACT81460 interrupts
the processor if the output voltage falls below the
Power-OK (POK) threshold. In this case, nIRQ asserts
low and remains asserted until either the regulator is
turned off or goes back into regulation, and the POK bit
has been read via I2C. The POK interrupt is cleared
when the register is read and the fault is no longer pre-
sent.
Inductor Selection
Overcurrent and Short Circuit Protection
The ACT81460 is designed to be used with a 1µH to
2.2µH. Use 1µH for a 3.3V output and use 2.2µH for a
5V output. Choose an inductor with a low DC-resistance,
and avoid inductor saturation by choosing inductors with
DC ratings that exceed the maximum output current by
at least 30%.
The buck-boost converter provides overcurrent and
short circuit protection. Overcurrent protection is
achieved with cycle-by-cycle current limiting. The peak
current threshold is fixed at 0.85A.
If the peak current reaches the programmed threshold,
the IC turns off the power FET for that switching cycle.
If the load current continues to increase, this condition
results in shutdown due to an UV condition from the
shortened switching cycle.
Output Capacitor Selection
The buck-boost regulator is designed to take advantage
of the benefits of ceramic capacitors, namely small size
and very-low ESR capacitors. VBBST must have a ded-
icated low-ESR capacitor bypass directly to PGND34
that is optimally placed to minimize the power routing
loop. It is designed to operate with 22µF to 44µF output
capacitors over most of its output voltage range. In or-
der to ensure stability, the Buck-Boost effective capaci-
tance must be greater than 8uF. The output capacitance
can be increased to reduce output voltage ripple and
improve load transients if needed.
If the short circuit or overload condition occurs quickly,
the cycle-by-cycle current can exceed the programmed
threshold. When it reaches 125% of the programmed
current for two consecutive switching cycles, the IC is-
sues an overcurrent warning and asserts nIRQ low.
When it reaches 125% of the programmed current for
eight consecutive switching cycles, the buck-boost con-
verter shuts down.
Compensation
Be sure to consider the capacitor’s DC bias effects and
maximum ripple current rating when using capacitors
smaller than 0805.
The buck-boost utilizes a proprietary internal compen-
sation scheme to simultaneously simplify external com-
ponent selection and optimize transient performance
over their full operating range. No compensation design
is required; simply follow a few simple guide lines de-
scribed below when choosing external components.
A capacitor’s actual capacitance is strongly affected by
its DC bias characteristics. The output capacitor is typi-
cally an X5R, X7R, or similar dielectric. Use of Y5U, Z5U,
or similar dielectrics are not recommended due to their
wide variation in capacitance over temperature and volt-
age ranges.
Input Capacitor Selection
The buck-boost converter has a dedicated input pin,
VINBB. Its ground pin, PGND34 is common with the
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