ADM1276-3ACPZ-RL(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1276-3ACPZ-RL
(Rev.:Rev0)

Hot Swap Controller and Digital Power and Energy Monitoring with PMBus Interface

Analog Devices 

2.95V

TO 20V

Q1

SENSE+

SENSE–

VCC

BIAS

CURRENT

GATE

GND

Figure 43. 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 ADM1276 limits the current during an over-

current 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 pro-

grammed 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

Q1

SENSE+

SENSE–

VCC

×50

ISET

SS

FLB

OVER-

GATE

CURRENT

ADM1276

GND

Figure 44. 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 45 provides an

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

during startup as the ADM1276 is enhancing the FET and charging

the load capacitances. Depending on how the soft start and fold-

back features are configured, the hand-off point can vary to

ensure that the FET is being operated within the correct limits.

V

1V

ISET

ADM1276

SS

FLB

CURRENT-LIMIT

REFERENCE

0.2V

0.1V

t

Figure 45. 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 ADM1276 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 46).

VCAP

C1

ISET ADM1276

GND

Figure 46. 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 47).

Rev. 0 | Page 19 of 48

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