ADM1270ACPZ-R2 datasheet(16/21 Pages) AD

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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

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

4V TO 60V

–

CURRENT-

LIMIT

CONTROL

REF

SELECT

ISET

FLB

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.

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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