Ultra low power switching regulators

For micropower applications there are some interesting new devices around for helping power the circuitry. I am not thinking here about energy harvesting applications but low power battery operated devices where you don’t want the battery to go flat quickly when in standby but still need some regulated power to wake up a microcontroller, for example. If you just want something to be shut down and only woken by a button push then that isn’t much of a problem because the regulator can also be shut down – the button can restart the regulator. However, in some applications you need to keep regulated power alive and some circuitry working that will be periodically checking some parameter such as an accelerometer or RF receiver. In such cases you need a regulator which has a very low operating power even when lightly loaded and reasonable efficiency.

With lot of switching regulators the internal circuitry can be consuming several milli-amps even when doing almost nothing. Some of this will be powering the bandgap reference for regulating the voltage and the rest for other internal circuitry such as the oscillator. Switching regulators with standby currents of several micro-amps were not common but now there are a few with sub micro-amp standby currents such as the TPS62743 from Texas Instruments, the ADP5301 from Analog Devices and the LTC3388 from Linear Technology, which are all buck (step down) regulators. The lowest current of these is the ADP5301 at 180nA. Texas Instruments also make a module with a built in inductor and input/output capacitors as well in a 2.9mm x 2.3mm package. They are all synchronous regulators meaning the diode in a normal switching regulator is replaced with transistor to reduce the voltage drop associated with a diode to a few milli-volts with a transistor.

Ultra-low-power-switching-regulators

All three of the devices have programmable output voltages. The LTC3388 has two digital pins to program the voltage giving 4 voltages and 2 different versions to increase the choice of voltages to 8 in total. The ADP5301 has a single analog pin to program the output voltage with a single resistor. The TPS62743 has 3 digital voltage select pins to give 8 voltages.

Synchronous regulators usually use transistor (MOSFET) switches built in to the IC rather than external transistors. This has the advantage that the transistor switching speeds are part of the design of the IC so measures can be taken to minimize any clashes between the switches. One of the key pieces of circuitry in these ICs is the bandgap voltage reference which must have a very low current consumption to achieve the advertised quiescent current. One advantage of the voltage reference being on-chip is that it does not have to drive external circuitry – just the on-chip circuitry. Standard bandgap reference components need to have the ability to drive some current and so take more current.

While these are micropower regulators, some can delivery 0.5A. The LTC3388 can only supply 50mA compared to the 500mA of the ADP5301 and 300mA for the TPS62743 but the LTC3388 can handle input voltages up to 20V. The TPS62743 and ADP5301 are strictly low input voltage devices but still enough for a lithium rechargeable cell.

When the load current reduces the TPS62743 eventually reaches a power save mode where it uses single pulses to recharge the output capacitor through the inductor and then enters an active, but low power, quiescent state. When in that state it still needs to keep the voltage reference running so it can detect when to switch the inductor again. Also key is the very low current drain of the output voltage sense pin which is typically 40nA.

The ADP5301 is a little different in that it enters a hysteresis mode when load current is low so works in bursts. That keeps the current very low but also make the ripple at low currents a lot higher than at higher currents (i.e. when not in hysteresis mode). Like the TPS62743 it has a very low bias current feedback pin – typically 66nA. The LTC3388 also works in a hysteresis mode at low currents. The ADP5301 efficiency for 3.3V output voltage is shown below for hysteresis mode i.e. low currents.

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All three devices are capable of efficiencies of around 50% at 1µA output current and under certain circumstances more than that, so which one is best for your application depends on the detail such as ripple, output current, or input voltage requirements or price. They are all interesting devices which facilitate micropower design.

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