ADM1075 datasheet(21/52 Pages) AD

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Data Sheet

ADM1075

Rev. A | Page 21 of 52

RSENSE

SENSE–

VEE

GATE

VIN

SENSE+

ADM1075

RDROP = 15Ω

–48V RTN

–48V

11V

VEE

1µF

Rb1 = 100kΩ

Rb2 = 640Ω

Ib = 6µA TO 33µA

10.3V (5.5mA TO 10mA)

CIN

CLOAD

18V

75V

Figure 43. Wide Input Supply Range

CURRENT SENSE INPUTS

The load current is monitored by measuring the voltage drop

across an external sense resistor, RSENSE. An internal current

sense amplifier provides a gain of 25 or 50 (depending on the

model) to the voltage drop detected across RSENSE. The result is

compared to an internal reference and detects when an

overcurrent condition occurs.

RSENSE

SENSE–

VEE

GATE

VIN

SENSE+

ADM1075

OVER-

CURRENT

1V REF

×25/50

–

Figure 44. Hot-Swap Current Sense Amplifier

The SENSE± inputs can be connected to multiple parallel sense

resistors, which can affect the voltage drop detected by the

ADM1075. The current flowing through the sense resistors

creates an offset, resulting in reduced accuracy. To achieve

better accuracy, averaging resistors should be used to sum the

sense nodes of each sense resistor, as shown in Figure 45. The

typical value for the averaging resistors is 10 Ω. The value of the

averaging resistors is chosen to be much greater than the trace

resistance between the sense resistor terminals and the inputs to

the ADM1075. This greatly reduces the effects of differences in

the trace resistances.

SENSE–

VEE

GATE

VIN

SENSE+

ADM1075

BIAS

CURRENT

Figure 45. Connection of Multiple Sense Resistors to SENSE± Pins

CURRENT LIMIT REFERENCE

The current limit reference voltage determines the load current

level to which the ADM1075 limits the current during an

overcurrent event. This is the reference voltage to which the

gained up current sense voltage is compared to determine if the

limit is reached. This current limit voltage, shown in Figure 46,

is then converted to a gate current to regulate the GATE pin.

LIM

CURR

GATE

where gm, the gate transconductance, = 660 μS.

An internal current limit reference selector block continuously

compares the ISET, soft start, and foldback (derived from PLIM)

voltages, determines which is the lowest at any given time, and

uses it as the current limit reference. This ensures that the

programmed current limit, ISET, is used in normal operation

and the soft start and foldback features reduce the current limit

when required.

The foldback and soft start voltages change during different

stages of operation and are clamped to a lower level of 100 mV

(typical) to prevent zero current flow due to the current limit

being too low.

GATE

SENSE+

ADM1075

VEE

SENSE–

CURRENT

LIMIT

VOLTAGE

FLB

( = 0.1/PLIM)

ISET

TIMEOUT

CURRENT

LIMIT

CONTROL

REF

SELECT

1.0V

CURRENT

LIMIT

VCAP

10µA

GATE

DRIVE

LOGIC

PLIM

–

×25/50

–

Figure 46. Current Limit Reference Selection

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