ADM1041A datasheet(21/56 Pages) AD

21 / 56 page

ADM1041A

Rev. 0 | Page 21 of 56

CURRENT SENSING

Current is typically sensed by a low value resistor in series with

the positive output of the power supply, positioned just before

the OrFET or diode. For high voltages (12 V and higher), this

resistor is usually placed in the negative load. A pair of closely

matched voltage dividers connected to Pins 2 and 3 divide the

common-mode voltage down to approximately 2.0 V. The

divider ratio must be the same as used in the local and remote

voltage-sense circuits. Alternatively, current may be sensed by a

current transformer (CT) connected to Pin 8. The ADM1041A

must be configured via the SMBus to select one or the other.

See Figure 13.

CURRENT-TRANSFORMER INPUT

The ADM1041A can also be configured to sense current by

using a CT connected to Pin 8. In this case, the resistive current

sense is disabled. A separate single-ended amplifier has two

possible sensitivities that are selected via the SMBus. If the CT

option is selected, the gain of the 1.0, 2.1 buffer that follows the

gain trim stage is no longer configurable and is fixed at 1.0.

The share driver amplifier has a total of 100 mV positive offset

built into it. To use the device in CT mode, it is necessary to

compensate for this additional 100 mV offset. This is achieved

by adding in a positive offset on the CT input. This also allows

any negative amplifier offsets in the CT chain to be nulled out.

This offset cancellation is achieved by sourcing a current

through a resistance on the ICT pin. The resistor value is 40 kΩ

and so for 100 mV of offset cancellation a current of 2.5 μA is

required. It is possible to fine trim this current via Register 15h,

Bits 4–0, step size 170 nA. For example, 2.5 μA ≈ 15 × 170 nA;

so the code for Register 15h is decimal 15 or 0Fh. Refer to the

Current Transformer parameter in the Specifications table for

more details. See Figure 13.

CURRENT-SENSE CALIBRATION

Regardless of which means is used to sense the current, the end

result of the calibration process should produce the standard

current share signal between Pins 20 and 23, that is, 2.0 V at

100% load, excluding any additional share signal offset that

might be configured.

CURRENT-LIMIT ERROR AMPLIFIER

This is a low gain transconductance amplifier that takes its

input from one of the calibrated current stages described

previously. The amplifier requires only the output pin for loop

compensation, which typically consists of a series RC network-

to-common. A trimmable reference provides a wide range of

adjustment for the current limit. When the current signal

reaches the reference voltage, the output of the error amplifier

comes out of saturation and begins to drive a controlled current

source. The control threshold is nominally 1.0 V. This current

flows through a resistor in series with the trimmed voltage loop

signal and thereby attempts to increase the voltage signal above

the 1.5 V reference for that loop. The closed voltage loop reacts

by reducing the power supply’s output voltage and this results in

constant current operation. See Figure 13.

VREF

VDD

SET CURRENT-

LIMIT LEVEL

TRANSFORMER

CURRENT-

SENSE

CONFIGURATION

CURRENT

TRANSFORMER

CCMP

ICT

CS+

CS–/FS

GAIN = 10

CURRENT

LIMIT TO

VOLTAGE

ERROR AMP

IRS/ICT

SET

CURRENT

CURRENT-

OFFSET

(VSHARE,

IOUT = 0)

CURRENT

ERROR AMP

TO CURRENT-

SHARE DRIVE

AMPLIFIER

1.25V

OCP

COMPARATOR

OCP

SET GAIN

40k

REG 17h b7

OrFET SOURCE

0.5V

OrFET GATE

Figure 13. Current Sense

ADM1041A Datasheet(PDF) 21 Page - Analog Devices

ADM1041A Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Datasheet Download

Link URL