Power Supply Tracking for Linear Regulators

Introduction

The LTC2923 provides simple and versatile control over the power-up and power-down behavior of switching power supplies. It allows several supplies to track the voltage of a master supply, so that their relative voltages meet the stringent specifications for the power up of modern digital semiconductors, such as DSPs, microprocessors, FPGAs and ASICs. The LTC2923 is specifically designed to work with switching power supplies (see “Versatile Power Supply Tracking without MOSFETs” from Linear Technology Magazine, February, 2004 ) but it is easily adapted to linear regulators, including popular low-dropout (LDO) types. Summarized here are several techniques for controlling linear regulators with the LTC2923.

Monolithic Regulators

Table 1 lists three popular monolithic linear regulators that have been tested with the LTC2923. Using these three Power Supply Tracking for Linear Regulators monolithic LDOs with the LTC2923 is generally very simple:

  • The LTC3020 is a 100mA low dropout regulator (LDO) that operates with input supply voltages between 1V and 10V. Since its ADJ pin behaves like the feedback pin on most witching regulators, tracking the LTC3020’s output using the LTC2923 is simple. The standard circuits and design procedures shown in the LTC2923 data sheet require no modification when used with the LTC3020 (Figures 1 and 2).
  • The LTC3025 is a 300mA monolithic CMOS LDO that regulates input supplies between 0.9V and 5.5V, while a bias supply between 2.5V and 5.5V powers the part. Similar to the LT3020, the LTC3025’s ADJ pin is operationally identical to common switchers. For that reason, the LTC3025 combined with an LTC2923 provides a simple supply tracking solution for loads less than 300mA (Figures 1 and 2).
  • The LTC1844 CMOS LDO drives loads up to 150mA with input supply voltages between 1.6V and 6.5V. When used in conjunction with the LTC2923, a feedforward capacitor should be included as described in the “Adjustable Operation” section of the LTC1844 data sheet. Otherwise, no special considerations are necessary.
Table 1. New monolithic linear regulators
Regulator IOUT(MAX)(V) VIN(MIN)(V) VIN(MAX)(V) VDROPOUT(V)
LT3020 100mA 0.9 10 0.15
LTC1844 150mA 1.6 6.5 0.11
LTC3025 300mA 0.9 5.5 0.045

Figure 1. An LTC2923 causes the outputs of the LT3020 and LTC3025 to track during power-up and power-down.

Figure 2. The outputs of the LT3020 and LTC3025 low-dropout linear regulators ramp-up and ramp-down together. (Output of circuit in Figure 1.).

The LTC1761 Family of Monolithic, Bipolar Regulators

Table 2 shows the LTC1761 family of monolithic, bipolar low dropout regulators. These regulators cover a wide range of load currents and offer outstanding transient response and low noise, making them a popular choice for applications with loads less than 3A.

Table 2. LT1761 family of low-dropout linear regulators
Regulator IOUT(MAX)(V) VIN(MIN)(V) VIN(MAX)(V) VDROPOUT(V)
LT1761 100mA 1.8 20 0.30
LT1762 150mA 1.8 20 0.30
LT1962 300mA 1.8 20 0.27
LT1763 500mA 1.8 20 0.30
LT1963A 1.5A 2.1 20 0.34
LT1764A 3A 2.7 20 0.34

In these regulators, the ADJ pin draws excess current when the OUT pin drops below about 1V, a region of operation that LDOs do not normally experience. Nevertheless, an LDO which tracks another supply, enters this region when the output tracks below 1V (Figure 3). If this excess current is not accounted for, the output of the LDO will be slightly higher than ideal when it tracks below 1V. Three techniques have been used to successfully track outputs of this LDO family below 1V.

Figure 3. LT1761/LT1962/LT1762/LT1763/LT1963A/LT1764A with adjustable outputs only track above 1V unless modified as discussed in this article. The SHDN pin of the LDO is active before the ramp-up and after ramp-down.

If low dropout voltages are not necessary, simply connect two diodes in series with the OUT pin (Figure 4). In this configuration, the OUT pin remains two diode drops above the circuit’s output. As a result, the LDO remains in its normal region of operation even when the output is driven near ground. Since the feedback resistors are connected to the output, the LDO regulates the voltage at the circuit output instead of the LDO’s OUT pin. Diode voltage varies with both load current and temperature, so verify that the output is low enough at the minimum diode voltage. Likewise, the input voltage must be high enough to regulate the output when the diode drops are at their maximum. This solution effectively increases the dropout voltage of the linear regulator by two diode drops. Therefore, applications that require a low dropout voltage are better served by the solutions that follow.

Figure 4. Diodes placed in series with the OUT pin allow the LT1761 to track down to 0V.

Consider using the LT1761, LT1962, LT1762, or LT1763 voltage regulators when the load is less than 500mA and a low dropout voltage is necessary. A fixed output part, (such as the LTC1763A-1.5) can be used as an adjustable LDO if the SENSE pin is treated like an ADJ pin with a feedback voltage of 1.5V (Figure 5). The SENSE pin on the fixed output parts draws about 10μA regardless of the OUT pin’s voltage, unlike the ADJ pin on the adjustable parts. When choosing feedback resistors, minimize the output error by compensating for the extra 10μA of current that appears across the upper resistor. Also, use small valued resistors to minimize the error due to the 0μA to 20μA data sheet limits while avoiding values that are so small that the LTC2923’s 1mA IFBwill be unable to drive the output to ground. To satisfy these constraints, ensure that the parallel combination of the two feedback resistors is slightly greater than 1.5kΩ. For most output voltages, this reduces the output error due to the SENSE pin current to about 1%.

Figure 5. The fixed-output LT1763-1.5 can track down to 0V, has low dropout, and a resistive divider can be used for outputs greater than 1.5V.

For applications that require higher load currents and a low dropout voltage, the LT1963A and LT1764A may be appropriate. These parts are specified for 1.5A and 3A load currents respectively. Unfortunately, the SENSE pins on these fixed output parts draw about 600μA.

To use these parts, configure an operational amplifier to buffer the voltage from the feedback resistors to the SENSE pin of the 1.5V fixed output versions (Figure 6). If the op amp is configured with a voltage gain of 2, the 1.5V regulator in combination with the op amp behaves as an adjustable output regulator with a 0.75V reference voltage. The input to the op amp now serves as the ADJ input of the new regulator. This technique allows the use of the high current LT1963A/LT1764A where the voltage loss of series diodes would be unacceptable. It also works for the LT1761, LT1962, LT1762, and LT1763 in cases where the 10μA ADJ pin current produces an unacceptable output voltage error.

Figure 6. Using an op amp with the LT1963-1.5 allows lower output voltages and removes error due to the SENSE pin current.

Drivers for External, High Current Pass Devices

Table 3 summarizes the characteristics of the LT1575 and LT3150 low dropout regulators. These devices drive external N-channel MOSFET pass devices for high current/high power applications. The LTC3150 additionally includes a boost regulator that generates gate drive for the external FET.

Table 3. Drivers for external, high current pass devices
Regulator IOUT(MAX)(V) VIN(MIN)(V) VIN(MAX)(V) VDROPOUT(V)
LT3150 10A* 1.4 10 0.13
LT1575 * N/A 22 *

The LTC2923 tracks the outputs of the LT1575 and LT3150 without any special modifications. Because these linear regulators only pull the FET’s gate down to about 2.6V, low-threshold FETs may not allow the output to fall below a few hundred millivolts. This is acceptable for most applications.

作者

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Dan Eddleman

Dan Eddleman是一名模拟工程师,在凌力尔特工作超过15年,曾担任过IC设计人员、新加坡IC设计中心经理和应用工程师。

他的职业生涯始于凌力尔特,设计了LTC2923和LTC2925电源跟踪控制器、LTC4355高压双通道理想二极管OR和LTC1546多协议收发器。他还是世界首款以太网供电控制器LTC4255的设计团队成员。他拥有两项与这些产品相关的专利。

随后,他搬到新加坡管理凌力尔特的新加坡IC设计中心,负责设计产品的工程师团队,产品包括热插拔控制器、过压保护控制器、DC/DC开关模式电源控制器、电源监视器和超级电容器充电器。

回到米尔皮塔斯总部后,Dan作为应用工程师创建了Linduino,这是一个兼容Arduino的硬件平台,用于演示凌力尔特基于I2C和SPI的产品。Linduino可以方便地向客户分配C固件,同时也为凌力尔特的客户提供了简单的快速原型制作平台。

此外,在其担任应用工程师期间,他构思出了LTC2644/LTC2645 PWM至VOUT DAC,并开发了基于XOR的地址转换器电路,用于LTC4316/LTC4317/LTC4318 I2C/SMBUS地址转换器。他申请了与这两种产品相关的专利。Dan还开发了个多个参考设计,可满足较高的MIL-STD-1275 28V军用车辆规格要求。

Dan继续研究MOSFET的安全工作区,创建软件工具并在凌力尔特内部举办与SOA相关的培训课程。借助使用LTspice分配的SOAtherm模型,客户可以使用具有Spirito失控特性的热模型在其热交换电路仿真中仿真MOSFET SOA。

他拥有斯坦福大学电气工程硕士学位以及加州大学戴维斯分校电气工程与计算机工程学士学位。