Understanding Quiescent and Shutdown Currents With Boost Converters
Quiescent current is an important parameter to consider when optimizing battery-powered applications. This article examines the difference between a boost converter’s quiescent and shutdown current to provide a deeper understanding of how the currents can be utilized in battery-powered applications.
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 (IQ) is an important parameter to consider when optimizing these applications; an improved IQ increases battery life. This article describes the difference between a boost converter’s IQ and shutdown current (ISD) to understand better how IQ and ISD can be utilized in battery-powered applications.
In many battery-powered applications, the battery output is relatively low, such as a 1.5 V output voltage (VOUT) from a single AA battery. Meanwhile, the back-end IC or subsidiary circuit requires a higher input voltage (VIN). 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 VIN 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 VOUT
|Type/Spec||Cylindrical Batteries||Coin Batteries||Phone/Tablet Batteries|
|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 VOUT among these batteries is 3.7 V, which may be insufficient for several applications. This is when a boost converter becomes vital.
MPS provides a variety of synchronous step-up converters that can be used to step up a battery’s VOUT to provide a sufficient VIN for the receiving system. The remainder of this article will focus on the differences between a boost converter’s ISD and IQ while using the MP28600, an ultra-low IQ 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.
ISD 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 VIN equal to 3.3 V. ISD is the current measured from the input (see Figure 2).
Figure 2. Measuring ISD. 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. ISD in an IC. Image used courtesy of Bodo’s Power Systems [PDF]
IQ 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 VIN of 3.3 V. IQ is the measured current from the MP28600’s input voltage pin (see Figure 4).
Figure 4. Measuring IQ. Image used courtesy of Bodo’s Power Systems [PDF]
Similar to ISD, it may be difficult to understand why there is an IQ. 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). IQ 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 IQ. The smaller the IQ, the longer the battery life.
Figure 5. IQ 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.