Precision dual voltage regulator controller ADM1051/AD51A

    Abstract: ADM1051/1051A is a precision bi-stabilized voltage controller produced by Analog Devices. It has two output voltages of 1.515V and 1.818V. Mainly used for power supply to bus terminals and interfaces on computer motherboards. This article introduces the working principle and pin functions of the chip, and finally gives its simple application circuit and connection precautions.

    Keywords: off current quiescent current adjustment voltage ADM1051

1 Overview

The ADM1051/ADM1051A is a precision bi-regulated voltage controller with two output voltages of 1.515V and 1.818V. It integrates two separate controllers on one chip, resulting in a series of advantages such as low quiescent current and low shutdown current. Its unique "HICCUP mode" enables it to protect external power supply components without the need for external components. At present, it has been widely used in personal computers, servers and workstations.

2 Chip packaging and pin functions

ADM1051/ADM1051A is packaged in SOIC and has 8 pins. Its pin configuration is shown in Figure 1. The function description of each pin is listed in Table 1.

Table 1 ADM1051 pin functions

3 Working principle

The ADM1051/ADM1051A contain an accurate 1.2V energy bandgap reference and two essentially identical channels consisting of control amplifiers. The main difference between these two channels is their respective output adjustment voltages, which are determined by the resistance ratio of their voltage detection inputs. Channel 1 normally outputs 1.515V, but can be adjusted to 3.3V. In addition to this, each channel also has a shutdown input that can be used to turn off the output of the amplifier. In addition, the device has a hold-up circuit to protect external common equipment.

The output of the ADM1051/1051A can be used to drive external N-channel MOSFETs that act as source followers. The advantage of using N-channel MOS transistors is that they are cheaper than P-channel MOS transistors under the same conditions and are more stable in a common-source setting. The functional block diagram of AD1051/1051A is shown in Figure 2.

3.1 Control amplifier unit

The reference voltage in the ADM1051/1051A can be amplified and buffered by controlling the amplifier and external MOS tubes. The output voltage of each channel can be determined by a feedback resistor network between the inverting input and the sense input of the control amplifier. Each amplifier drives the gate of a MOS tube. The drain of the MOS tube is connected to the unadjusted input voltage, and the source is connected to the regulated output voltage. The control amplifier has high current drive capability, which can quickly charge and discharge the gate capacitance of the external MOS tube. Therefore, they have good transient response to changes in load or input voltage.

3.2 Close input compatibility with TYPEDET signals

Each channel of the ADM1051 has a separate shutdown input that is controlled by a logic signal. This logic signal can be used to control the output on or off. When the shutdown input is high or not connected, the regulator operates normally; when the shutdown input is low, the enable input of the control amplifier is turned off, the output of the control amplifier becomes low, and the regulator is closed.

The AGP interface on the PC motherboard has two different working modes, both of which require a stable 3.3V or 1.5V voltage. Both modes can be controlled and switched by the TYPEDET signal on the PC motherboard. Since the output of the ADM1051/ADM1051A is compatible with the TYPEDET signal, the regulated output voltage of channel 1 can be selected through the Shutdown pin. This is a multi-level multi-function input through which the output voltage and off state of the regulator can be selected. The voltage regulator can be set to four different operating modes listed in Table 2 by setting the SHDM to different voltages.

Table 2 Four different working modes of voltage regulator

3.3 HICCUP error protection mode

HICCUP error protection mode is a simple protection method that can be used to protect external power supply equipment. It does not require additional external resistor or inductor pins to implement the protection function.

When the output is shorted, the output voltage will drop, and when the output voltage of a channel is less than 20% of the rated value, this situation is detected by the HICCUP comparator and the channel enters HICCUP mode. This mode has no effect on channel 2 of the ADM1051A. In order to prevent the device from accidentally entering HICCUP mode when it is powered on or when the channel is activated, the HICCUP mode will be out of synchronization with the two channels in the above two states for about 60ms. After this 60ms, the output voltage should have reached the normal value. During power-on, when VCC reaches the power-on swing of 6V to 9V, the isolation cycle is initiated. In the channel active state, when the SHDN pin goes high, the isolation cycle starts. At this time, as long as one channel is activated or closed, the isolation timeout affects both channels.

4 application circuit

4.1 PCB layout

In order to obtain the most suitable power supply regulation, the load should be as close as possible to the source of the output MOS tube, and the feedback to the SENSE input should be on a contact that is as close as possible to the load. The PCB trace from the load back to the SENSE input should be separate from the output rail and should not have any load current flowing through it. Similarly, the ground point of the ADM1051/1051A should be as close to the load ground as possible. Figure 3 shows examples of good and bad PCB layouts.

    4.2 Power supply decoupling

The power supply to the drain of the external MOS tube should be decoupled from the drain pin of the device as much as possible, and the capacitance between it and ground should be at least 100μF. The output of the source of the MOS tube should be decoupled from the drain pin of the component as much as possible. Decoupling capacitors should have a low equivalent series resistance (ESR), typically 50mΩ or less. The decoupling capacitor between the VCC pin of the ADM1051/1051A and ground should have a capacity of at least 1μF and should be as close as possible to the GND pin and VCC. In fact, the required decoupling device depends on the actual application. Since PC motherboards are often used in noisy environments, it may require the use of distributed decoupling to reduce the noise on the voltage rails to an acceptable level.

5 Summary

The ADM1051/1051A produced by Analog Devices is characterized by its precision dual voltage stabilization and can be widely used in personal computers, servers and workstations. As a controller, it will be more widely used in the power supply of bus terminals and interfaces on personal computer motherboards.