Boost Converters for Keep-Alive Circuits Draw Only 8.5μA of Quiescent Current

Introduction

Industrial remote monitoring systems and keep-alive circuits spend most of their time idle. Many of these systems use batteries, so to maximize run time power losses, even during low power idle modes, must be minimized. Even at no load, power supplies draw some current to produce a regulated voltage for keep-alive circuits.

The LT8410/-1 DC/DC boost converter features ultralow quiescent current and integrated high value feedback resistors to minimize the draw on the battery when electronics are idle.

An entire boost converter takes very little space, as shown in Figure 1.

Figure 1. The LT8410/-1 is designed to facilitate compact board layout.

Ultralow Quiescent Current Low Noise Boost Converter with Output Disconnect

When a micropower boost converter is in regulation with no load, the input current depends mainly on two things—the quiescent current (required to keep regulation) and the output feedback resistor value. When the output voltage is high, the output feedback resistor can easily dissipate more power than the quiescent current of the IC. The quiescent current of the LT8410/-1 is a low 8.5μA, while the integrated output feedback resistors have very high values (12.4M/0.4M). This enables the LT8410/-1 to dissipate very little power in regulation at no load. In fact, the LT8410/-1 can regulate a 16V output at no load from 3.6V input with about 30μA of average input current. Figures 2, 3 and 4 show the typical quiescent and input current in regulation with no load.

Figure 2. Quiescent current vs temperature—not switching.

Figure 3. Quiescent current vs VCC voltage—not switching.

Figure 4. Average input current in regulation with no load.

The LT8410/-1 controls power delivery by varying both the peak inductor current and switch off time. This control scheme results in low output voltage ripple as well as high efficiency over a wide load range. As shown in Figure 5, even with a small 0.1μF output capacitor, the output ripple is typically less than 10mV. The part also features output disconnect, which disconnects the output voltage from the input during shutdown. This output disconnect circuit also sets a maximum output current limit, allowing the chip survive output shorts.

Figure 5. General purpose bias with wide input voltage and low output voltage ripple.

An Excellent Choice for High Impedance Batteries

A power source with high internal impedance, such as a coin cell battery, may show normal output voltage on a voltmeter, but its voltage can collapse under heavy current demands. This makes it incompatible with high switch-current DC/DC converters. The LT8410/-1 has an integrated power switch and Schottky diode, and the switch current limits are very low (25mA for the LT8410 and 8mA for the LT8410-1). This low switch current limit enables the LT8410/-1 to operate very efficiently from high impedance sources, such as coin cell batteries, without causing inrush current problems. Figure 6 shows the LT8410-1 charging an electrolytic capacitor. Without any additional external circuitry, the input current for the entire charging cycle is less than 8mA.

Figure 6. Capacitor charger with the LT8410-1 and charging waveforms.

Tiny Footprint with Small Ceramic Capacitors

Available in a tiny 8-pin 2mm × 2mm DFN package, the LT8410/-1 is internally compensated and stable for a wide range of output capacitors. For most applications, using 0.1μF output capacitor and 1μF input capacitor is sufficient. An optional 0.1μF capacitor at the VREF pin implements a soft-start feature. The combination of small package size and the ability to use small ceramic capacitors enable the LT8410/-1 to fit almost anywhere. Figure 1 shows the size of a circuit similar to that shown in Figure 4, illustrating how little board space is required to build a full featured LT8410/-1 application.

SHDN Pin Comparator and Soft-Start Reset Feature

An internal comparator compares the SHDN pin voltage to an internal voltage reference of 1.3V, giving the part a precise turn-on voltage level. The SHDN pin has built-in programmable hysteresis to reject noise and tolerate slowly varying input voltages. Driving the SHDN pin below 0.3V shuts down the part and reduces input current to less than 1μA. When the part is on, and the SHDN pin voltage is close to 1.3V, 0.1μA current flows out of the SHDN pin. A programmable enable voltage can be set up by connecting external resistors as shown in Figure 7.

Figure 7. Programming the enable voltage by using external resistors.

The turn-on voltage for the configuration is:

Equation 1

and the turn-off voltage is:

Equation 2

where R1, R2 and R3 are resistance in Ω. Programming the turn-on/turn-off voltage is particularly useful for applications where high source impedance power sources are used, such as energy harvesting applications.

By connecting an external capacitor (typically 47nF to 220nF) to the VREF pin, a soft-start feature can be implemented. When the part is brought and out of shutdown, the VREF pin is first discharged for 70μs with a strong pull down current, and then charged with 10μA to 1.235V. This achieves soft start since the output is proportional to VREF. Full soft-start cycles occur even with short SHDN low pulses since VREF is discharged when the part is enabled.

In addition, the LT8410/-1 features a 2.5V to 16V input voltage range, up to 40V output voltage and overvoltage protection for CAP and VOUT.

Conclusion

The LT8410/-1 is a smart choice for applications which require low quiescent current and low input current. The ultralow quiescent current, combined with high value integrated feedback resistors, keeps the average input current very low, significantly extending battery operating time. Low current limit internal switches (8mA for the LT8410-1, 25mA for the LT8410) make the part ideal for high impedance sources such as coin cell batteries. The LT8410/-1 is packed with features without compromising performance or ease of use and is available in a tiny 8-pin 2mm × 2mm package.

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Xiaohua Su