Understanding Quiescent and Shutdown Currents With Boost Converters

This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.

For battery-powered applications, an excellent user experience depends on long battery life and high-performance terminal devices. Quiescent current (I_Q) is an important parameter to consider when optimizing these applications; an improved I_Q increases battery life. This article describes the difference between a boost converter’s I_Q and shutdown current (I_SD) to understand better how I_Q and I_SD can be utilized in battery-powered applications.

 

Battery Types

In many battery-powered applications, the battery output is relatively low, such as a 1.5 V output voltage (V_OUT) from a single AA battery. Meanwhile, the back-end IC or subsidiary circuit requires a higher input voltage (V_IN). In these systems, a step-up converter (or boost converter) is typically used to step up the voltage from a battery output. Otherwise, there is an insufficient V_IN for the IC or subsidiary circuit.

Figure 1 shows a few different battery types, such as cylindrical, coin, and phone/tablet batteries.

 

Figure 1. Battery Types. Image used courtesy of Bodo’s Power Systems [PDF]

 

Table 1 shows battery types and their typical output voltages.

Table 1. Battery Types and Their Typical V_OUT

Type/Spec	Cylindrical Batteries				Coin Batteries			Phone/Tablet Batteries
	NiMH	Lithium-Iron	Lithium-Ion	NiCd	CR	SR	LR	Lithium-Ion
VOUT	1.2 V	1.5 V	3.7 V	1.2 V	3 V	1.55 V	1.5 V	3.7 V
Sizes	A, AA, AAA, AAAA, C, D, N, and F				Varies

 

Batteries should be selected based on the application needs. However, the maximum V_OUT among these batteries is 3.7 V, which may be insufficient for several applications. This is when a boost converter becomes vital.

 

Boost Converters

MPS provides a variety of synchronous step-up converters that can be used to step up a battery’s V_OUT to provide a sufficient V_IN for the receiving system. The remainder of this article will focus on the differences between a boost converter’s I_SD and I_Q while using the MP28600, an ultra-low I_Q synchronous boost converter. The MP28600 is a new IC solution with an nA level quiescent current and a small SOT563 (1.6mmx1.6mm) package. It is well-suited for wearable devices, industrial sensors, and battery-powered, handheld medical instruments.

 

Shutdown Current 

I_SD is the current drawn by the IC from a battery when the IC is switched off, and the battery is still connected to the system. Consider the scenario in which the MP28600’s enable signal is set to 0 V, and it is connected to a battery with a V_IN equal to 3.3 V. I_SD is the current measured from the input (see Figure 2).

 

Figure 2. Measuring I_SD. Image used courtesy of Bodo’s Power Systems [PDF]

 

It may be difficult to understand why the IC consumes current even though it has shut down. This is because some of the internal circuitry, such as the start-up loop, still leak small amounts of power to the ground (GND); this leakage consumes battery current (see Figure 3).

 

Figure 3. I_SD in an IC. Image used courtesy of Bodo’s Power Systems [PDF]

 

Quiescent Current

I_Q is the current consumed by the IC when it is enabled (but not switching) or when there is no load applied. This current can also be called operating quiescent, standby, and sleep mode current.

For example, set the MP28600’s enable signal to 5 V. The IC is connected to the battery, with a V_IN of 3.3 V. I_Q is the measured current from the MP28600’s input voltage pin (see Figure 4).

 

Figure 4. Measuring I_Q. Image used courtesy of Bodo’s Power Systems [PDF]

 

Similar to I_SD, it may be difficult to understand why there is an I_Q. Even if there is no load and the IC is not switching most of the time, some internal circuitry continues operating to maintain the device’s basic functions, which require a small amount of current. Figure 5 shows a control loop that consumes current.

Consider the following real-world example using a security sensor (see Figure 6). I_Q is essential for applications that run in a sleep state. 99% of the time, a security sensor does not detect an object or hazardous condition that triggers the alarm; however, it cannot shut down. In this scenario, the sensor can operate in sleep mode and wake up only when necessary. While in sleep mode, the IC consumes a small amount of power from the battery — this current is the operating I_Q. The smaller the I_Q, the longer the battery life.

 

Figure 5. I_Q in an IC. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 6. Security sensor functionality. Image used courtesy of Bodo’s Power Systems [PDF]

 

Differences Between Quiescent and Shutdown Current Summary

This article used the MP28600, a step-up (boost) converter, to describe the differences between quiescent and shutdown currents, which can be optimized in battery-powered applications. As electrical devices become more intelligent and diverse with the evolution of the Internet of Things devices, portable applications, medical instruments, and industrial sensors, low power consumption and small size are paramount.

MPS provides a robust portfolio of converters to meet any application’s needs.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.