Electronicdesign 20178 Powerthinklarge 0
Electronicdesign 20178 Powerthinklarge 0
Electronicdesign 20178 Powerthinklarge 0
Electronicdesign 20178 Powerthinklarge 0
Electronicdesign 20178 Powerthinklarge 0

Efficiently Reduce High Supply Voltages To Accurate Low Voltage

Nov. 15, 2004
When a relatively high input of 24 V or more must be very accurately regulated down to a much lower voltage, such as 3.3 V or less, with very low quiescent current, there's no off-the-shelf solution available. For example, if one needs to...

When a relatively high input of 24 V or more must be very accurately regulated down to a much lower voltage, such as 3.3 V or less, with very low quiescent current, there's no off-the-shelf solution available. For example, if one needs to efficiently run a low-power microcontroller from a 24- or 48-V battery supply, it typically takes two cascaded regulators. And, the quiescent current is likely to be more than the load current.

The simple, low-cost circuit shown in the figure will do the job on a mere 75 µA. Moreover, the input voltage is limited only by the rating of the series-pass MOSFET. For best accuracy (0.1%) and lowest current (65 µA), an LM4041A or LM4051A micropower precision shunt reference is used for U1. For less critical applications, the lower-cost LMV431B will regulate at 0.5% on 80 µA. Less accurate grades are also available at lower cost.

This circuit produces highly regulated low voltages from inputs of 24 V or more while drawing under 75 µ A of quiescent current.

Operation is straightforward. When VIN is applied, R1 pulls up Q1's gate, turning it on. As VOUT rises, a portion is fed back to U1, causing it to pull the gate down. U1 will then hold the gate of Q1 at whatever voltage is necessary to keep VOUT at the desired set point. Therefore, the junction of R2 and R3 will be held equal to U1's reference voltage. C1 slows down Q1's turn-on to prevent overshoot and should be at least 10 times the FET's drain-to-gate capacitance. It can be made much larger if a slower turn-on is desired, though.

R1 is sized so that the lowest input voltage will supply the minimum current required by U1 to regulate. If the input voltage will vary over a very wide range, R1 can be replaced by a simple two-transistor current source to keep the quiescent current low, regardless of the input.

The output voltage equals 1 + (R2/R3) times the reference voltage of 1.212 V. Because the reference current is in the nanoamps, we can set the R2-R3 divider current at 10 µA and ignore it. The values shown should give an output of 3.315 V.

Q1 is selected based on the maximum input voltage, output current, and power dissipation. For just a few milliamps of output current, an SOT-23 device will suffice, while a larger package can supply several amps with no other circuit change. If desired, one could add a resistor in the FET's drain circuit to reduce its dissipation. This would also limit current in the event of a shorted output.

If an LM4041- or LM4051-type device is used, its cathode should not exceed 10 V. Thus, a logic-level or low-threshold FET should be used to ensure that the gate will only be a few volts higher than the output. If the FET's gate must be higher than 10 V, one or more diodes or a zener can be placed between U1 and the gate without degrading the accuracy. A zener could also be added across U1 for protection in case the output gets shorted to ground. For higher output voltages, the LMV431 should be used, because its cathode can withstand 35 V. For lower voltages at even lower current operation, the LM185/285/385 only requires 10 µA, but its cathode can't exceed 5.3 V.

Sponsored Recommendations

Understanding Thermal Challenges in EV Charging Applications

March 28, 2024
As EVs emerge as the dominant mode of transportation, factors such as battery range and quicker charging rates will play pivotal roles in the global economy.

Board-Mount DC/DC Converters in Medical Applications

March 27, 2024
AC/DC or board-mount DC/DC converters provide power for medical devices. This article explains why isolation might be needed and which safety standards apply.

Use Rugged Multiband Antennas to Solve the Mobile Connectivity Challenge

March 27, 2024
Selecting and using antennas for mobile applications requires attention to electrical, mechanical, and environmental characteristics: TE modules can help.

Out-of-the-box Cellular and Wi-Fi connectivity with AWS IoT ExpressLink

March 27, 2024
This demo shows how to enroll LTE-M and Wi-Fi evaluation boards with AWS IoT Core, set up a Connected Health Solution as well as AWS AT commands and AWS IoT ExpressLink security...

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!