ADM1275-3ACPZ-R7 datasheet(22/48 Pages) AD

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ADM1275

Data Sheet

Rev. D | Page 22 of 48

SENSE–

GND

GATE

VCC

SENSE+

BIAS

CURRENT

2.95V

TO 20V

Figure 49. Connection of Multiple Sense Resistors to the SENSE Pins

CURRENT LIMIT REFERENCE

The current limit reference voltage determines the load current

level to which the ADM1275 limits the current during an

overcurrent event. This reference voltage is compared to the

gained-up current sense voltage to determine whether the limit

is reached.

An internal current limit reference selector block continuously

compares the ISET, soft start, and foldback voltages to determine

which voltage is the lowest at any given time; the lowest voltage

is used as the current limit reference. This ensures that the

programmed current limit, ISET, is used in normal operation,

and that the soft start and foldback features reduce the current

limit when required during startup and/or fault conditions.

RSENSE

SENSE–

GND

GATE

VCC

ISET

FLB

SENSE+

ADM1275

OVER-

CURRENT

×50

Figure 50. Current Limit Reference Selection

The foldback and soft start voltages vary during different modes

of operation and are, therefore, clamped to minimum levels of

200 mV and 100 mV, respectively, to prevent zero current flow

due to the current limit being too low. Figure 51 provides an

example of how the soft start, foldback, and ISET voltages

interact during startup as the ADM1275 is enhancing the FET

and charging the load capacitances. Depending on how the soft

start and foldback features are configured, the hand-off point

can vary to ensure that the FET is operated correctly.

FLB

ISET

0.2V

0.1V

CURRENT LIMIT

REFERENCE

Figure 51. Interaction of Soft Start, Foldback, 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 requirements become smaller,

and resolution can be difficult to achieve when selecting the

appropriate sense resistor. The ADM1275 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 5 mV to 25 mV.

The default value of 20 mV is achieved by connecting the ISET

pin directly to the VCAP pin. This configures the device to use

an internal 1 V reference, which equates to 20 mV at the sense

inputs (see Figure 52).

ADM1275

VCAP

ISET

GND

Figure 52. Fixed 20 mV Current Sense Limit

To program the sense voltage from 5 mV to 25 mV, a resistor

divider is used to set a reference voltage on the ISET pin (see

Figure 53).

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