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ADM1270 Datasheet(PDF) 16 Page - Analog Devices

Part No. ADM1270
Description  High Voltage Input Protection Device
Download  21 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADM1270 Datasheet(HTML) 16 Page - Analog Devices

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ADM1270
Data Sheet
Rev. A | Page 16 of 21
POWERING THE ADM1270
A supply voltage from 4 V to 60 V is required to power the
ADM1270 via the VCC/SENSE+ pin. The VCC/SENSE+ pin
provides the majority of the bias current for the device; the
remainder of the current needed to control the gate drive and to
best regulate the gate to source voltage (VGS) is supplied by the
SENSE− pin.
CURRENT SENSE INPUTS
The load current is monitored by measuring the voltage drop
across an external current sense resistor, RSENSE (see Figure 35).
An internal current sense amplifier provides a gain of 40 to the
voltage drop detected across RSENSE. The result is compared to an
internal reference and is used by the hot swap control logic to
detect an overcurrent condition.
40×
+
CURRENT
LIMIT
VCC/SENSE+
SENSE–
GATE
GND
LDO
VCAP
REFERENCE
RSENSE
Q1
4V TO 60V
+
Figure 35. Hot Swap Current Sense Amplifier
CURRENT-LIMIT REFERENCE
The current-limit reference voltage determines the load current
at which the ADM1270 limits the current during an overcurrent
event. This reference voltage is compared to the amplified current
sense voltage to determine when the current-limit threshold is
reached.
An internal current-limit reference selector block continuously
compares the ISET and FLB voltages to determine which
voltage is the lowest at any given time; the lowest voltage is used
as the current-limit reference. This behavior ensures that the
programmed current limit, ISET, is used in normal operation,
and that the foldback feature reduces the current limit when
required during startup and/or fault conditions.
40×
+
CURRENT
LIMIT
VCC/SENSE+
SENSE–
GATE
GND
LDO
VCAP
RSENSE
Q1
4V TO 60V
+
CURRENT-
LIMIT
CONTROL
REF
SELECT
ISET
FLB
2V
Figure 36. Current-Limit Reference Selection
The FLB voltage varies during different modes of operation
and, therefore, is clamped to a minimum level of 200 mV. This
behavior prevents zero current flow due to the current limit
being set too low. Figure 37 provides an example of how the
FLB and ISET voltages interact during startup as the ADM1270
turns on the FET and charges the load capacitance. Depending
on how the foldback feature is configured, the transition point
varies to ensure that the FET operates within the correct limits.
2V
0.2V
ISET
VISET
FLB
TIME
CURRENT-LIMIT
REFERENCE
Figure 37. Interaction of Foldback (FLB) and ISET Current Limits
SETTING THE CURRENT LIMIT (ISET)
The maximum current limit is partially determined by selecting
a sense resistor to match the current sense voltage limit on the
controller for the desired load current. However, as currents
become larger, the sense resistor value decreases for a given
current sense voltage. Choosing an appropriate current sense
resistor can be difficult due to the limited selection of low value
resistors. The ADM1270 provides an adjustable current sense
voltage limit to handle this issue. The device allows the user to
program the required current sense voltage limit from 12.5 mV
to 62.5 mV.


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