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ADM1041AARQZ-REEL7 Datasheet(PDF) 22 Page - Analog Devices |
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ADM1041AARQZ-REEL7 Datasheet(HTML) 22 Page - Analog Devices |
22 / 56 page 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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Similar Description - ADM1041AARQZ-REEL7 |
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