ADM1041AARQZ-REEL7 datasheet(22/56 Pages) AD

Part #	ADM1041AARQZ-REEL7

Description	Secondary-Side Controller with Current Share and Housekeeping

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ADM1041A

Rev. 0 | Page 22 of 56

OVERCURRENT PROTECTION

When the current limit threshold is reached, the OCP com-

parator detects when the current error amplifier comes out of

saturation. Its threshold is nominally 0.5 V. This starts a timer

that, when it times out, causes an OCP condition to occur and

the power supply to shut down. If the current limit disappears

before the time has expired, the timer is reset. The time period

is configurable via the SMBus. Undervoltage blanking is applied

during the timer operation. See Figure 15.

CURRENT SHARE

The current-share method is the master–slave type, which

means that the power supply with the highest output current

automatically becomes the master and controls the share bus

signal. All other power supplies become slaves, and the share

bus signal causes them to increase their output voltages slightly

until their output currents are almost equal to that of the

master. This scheme has two major advantages. A failed master

power supply simply allows one of the slaves to become the new

master. A short-circuited share signal disables current sharing,

but all power supplies default to their normal voltage setting,

allowing a certain degree of passive sharing. Because this chip

uses a low voltage process, an external bidirectional amplifier is

needed for most existing share bus signal levels. The voltage

between Pins 20 and 23 is always controlled to 2.0 V full scale,

ignoring any offset. By connecting Pins 20 and 23 together, the

chip can produce a 2.0 V share signal directly without any

external circuits. To improve accuracy, the share signal is

referenced to the remote voltage sense negative (VS-) pin.

3V

1V

020

0

40

60

80 100%

VSHARE

IOUT

OFFSET

2V

Figure 14. Load Share Characteristic

CURRENT-SHARE OFFSET

To satisfy some customer specifications, the current-share

signal can be offset by a fixed amount by using a trimmable

current generator and a series resistor. The offset is added

on top of the 2.0 V full-scale, current-share output signal.

See Figure 15.

ISHARE DRIVE AMPLIFIER

This amplifier is a buffer with enough current source capability

to drive the current-share circuits of several slave power

supplies. It has negligible current sink capability. Refer to the

Differential Sense Amplifier section.

DIFFERENTIAL SENSE AMPLIFIER

This amplifier has unity gain and senses the difference between

the share bus voltage and the remote voltage sense negative pin.

When the power supply is the master, it forms a closed loop

with the ISHARE drive amplifier described previously, and

therefore it causes the share bus voltage between Pins 20 and 23

to equal the current-share signal at the noninverting input of

the ISHARE drive amplifier. When the power supply is a slave, the

output of the differential-sense amplifier exceeds the internal

current share signal, which causes the ISHARE drive amplifier to

be driven into cutoff. Because it is not possible to trim out

negative offsets in the op amps in the current-share chain, a

50 mV voltage source is used to provide a known fixed positive

offset. The share bus offset controlled current source must be

trimmed via the SMBus to take out the resulting overall offset.

See Figure 15.

ISHARE ERROR AMPLIFIER

This is a low gain transconductance amplifier that measures the

difference between the internal current share voltage and the

signal voltage on the external share bus. If two power supplies

have almost identical current-share signals, a 50 mV voltage

source on the inverting input helps arbitrate which power

supply becomes the master and prevents hunting between

master and slave roles. The amplifier requires only the output

pin for loop compensation, which typically consists of a series

RC network to common. When the power supply is a slave, the

output of the error amplifier comes out of saturation and begins

to drive a controlled current sink. The control threshold is

nominally 1.0 V. This current flows from a resistor in series

with the trimmed voltage loop signal and thereby attempts to

decrease the voltage signal below the 1.5 V reference for that

loop. The closed voltage loop reacts by increasing the power

supply’s output voltage until current share is achieved. The

maximum current sink is limited so that the power supply

voltage can be increased only a small amount, which is usually

limited to be within the customer’s specified voltage regulation

limit. This small voltage increase also limits the control range of

the current-share circuit and is called the capture range. The

capture range may be set via the SMBus to one of four values,

from 1% to 4% nominal. See Figure 15.

ISHARE CLAMP

This clamp keeps the current share-loop compensation

capacitor discharged when the current share is not required to

operate. The clamp is released during power-up when the

voltage reference and therefore the output voltage of the power

supply has risen to either 75% or 88% of its final value. This is

configurable via the SMBus. When the clamp is released, the

current share loop slowly walks in the current share and helps

to avoid output voltage spikes during hot swapping. See

Figure 15.

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ADM1041AARQZ-REEL7 Datasheet(PDF) 22 Page - Analog Devices

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