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ADM1270 Datasheet(PDF) 15 Page - Analog Devices
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ADM1270 Datasheet(HTML) 15 Page - Analog Devices
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Rev. A | Page 15 of 21
THEORY OF OPERATION
When circuit boards are inserted into a live backplane, discharged
supply bypass capacitors draw large transient currents from the
backplane power bus as they charge. These transient currents
can cause permanent damage to connector pins, as well as voltage
dips on the backplane supply that can reset other boards in the
The ADM1270 is designed to control the inrush current when
powering on the system, allowing a board to be inserted safely
into a live backplane by protecting it from excess currents.
The ADM1270 is a current-limiting controller that provides
inrush current limiting and overcurrent protection for modular
or battery-powered systems. The voltage developed across a
sense resistor in the power path is measured with a current
sense amplifier via the VCC/SENSE+ and SENSE− pins. A
default limit of 50 mV is set, but this limit can be adjusted, if
required, using a resistor divider network from the VCAP pin
to the ISET pin.
The ADM1270 limits the current through the sense resistor by
controlling the gate voltage of an external P-channel FET in the
power path, via the GATE pin. The sense voltage and, therefore,
the load current is maintained below the preset maximum. The
ADM1270 protects the external FET by limiting the time that
the FET remains on while the current is at its maximum value.
This current-limit time is set by the choice of capacitors
connected to the TIMER pin and the TIMER_OFF pin. This
current-limit time helps to maintain the FET in its SOA.
In addition to the timer function, there is a foldback pin (FLB)
that is used to provide additional FET protection. The current
limit is linearly reduced by the voltage on the FLB pin, so that
for large drain to source voltage (V
) voltage drops, the actual
current limit used by the device is lower, again helping to ensure
the FET is kept within its SOA.
A minimum voltage clamp ensures that even if the FLB voltage
is 0 V, the current is never reduced to zero, which otherwise
prevents the device from powering up.
The ADM1270 features OV and UV protection, programmed
using external resistor dividers on the UV and OV pins.
A PWRGD signal can be used to indicate when the output
supply is greater than a voltage programmed using an external
resistor divider on the FB_PG pin.
To protect the system from a reverse polarity input supply, there
is a provision made to control an additional external P-channel
FET with the RPFG pin. This feature allows for a low on-resistance,
low voltage drop FET to be used in place of a diode to perform
the same function, thus saving power losses and improving overall
efficiency. The reverse voltage protection FET prevents negative
input voltages that can damage the load or the ADM1270.
Figure 34. Simplified Functional Block Diagram
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