Current-Limit Circuit (LIM)
The current-limit circuit employs a valley and sink cur-
rent-sensing algorithm that uses the on-resistance of
the low-side MOSFET as a current-sensing element, to
eliminate costly sense resistors. The current-limit circuit
is also temperature compensated to track the on-resis-
tance variation of the MOSFET over temperature. The
current limit is adjustable with an external resistor at
LIM, and accommodates MOSFETs with a wide range
of on-resistance characteristics (see the
Setting the
Valley Current Limit
section). The adjustment range is
from 30mV to 300mV for the valley current limit, corre-
sponding to resistor values of 6kΩ to 60kΩ. The valley
current-limit threshold across the low-side MOSFET is
precisely 1/10th of the voltage at LIM, while the sink
current-limit threshold is 1/20th of the voltage at LIM.
Valley current limit acts when the inductor current flows
towards the load, and LX is more negative than GND
during the low-side MOSFET on-time. If the magnitude
of current-sense signal exceeds the valley current-limit
threshold at the end of the low-side MOSFET on-time,
the MAX15026 does not initiate a new PWM cycle and
lets the inductor current decay in the next cycle. The
controller also rolls back the internal reference voltage
so that the controller finds a regulation point deter-
mined by the current-limit value and the resistance of
the short. In this manner, the controller acts as a con-
stant current source. This method greatly reduces
inductor ripple current during the short event, which
reduces inductor sizing restrictions, and reduces the
possibility for audible noise. After a timeout, the device
goes into hiccup mode. Once the short is removed, the
internal reference voltage soft-starts back up to the nor-
mal reference voltage and regulation continues.
MAX15026
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
______________________________________________________________________________________ 11
Internal 5.25V linear regulator output.
Active-high enable input.
Regulator output voltage.
Regulator internal soft-start and soft-stop signal.
Regulator high-side gate-driver output.
Regulator low-side gate-driver output.
rising while below the UVLO threshold.
EN is low.
is higher than the UVLO threshold. EN is low.
EN is pulled high. DH and DL start switching.
Normal operation.
drops below UVLO.
goes above the UVLO threshold. DH and DL
start switching. Normal operation.
EN is pulled low. V
enters soft-stop.
EN is pulled high. DH and DL start switching.
Normal operation.
drops below UVLO.
(21 Seiten)
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