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	I_OUT(MAX)(V)	V_IN(MIN)(V)	V_IN(MAX)(V)	V_DROPOUT(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	I_OUT(MAX)(V)	V_IN(MIN)(V)	V_IN(MAX)(V)	V_DROPOUT(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	I_OUT(MAX)(V)	V_IN(MIN)(V)	V_IN(MAX)(V)	V_DROPOUT(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.