TI white paper "IQ: What is IQ and how to use it"

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TI white paper "IQ: What is IQ and how to use it" [Copy link]

Introduction
A device's quiescent current, or IQ, is an important but often misused parameter for low-power, high-performance designs. In many battery-powered applications, the current drawn by the battery during light-load or no-load standby conditions determines the total system run time. In integrated switching converters, IQ is only a fraction of this battery current. This article defines IQ, explains how to measure it, explains what IQ is and when it should not be used, and explains some design considerations for using IQ while avoiding common measurement errors. This article applies to all Texas Instruments (TI) TPS61xxx, TPS62xxx, or TPS650xx devices.
What is IQ?
Unless otherwise specified in the data sheet, all IQ definitions are: the current drawn by the IC in its no-load, non-switching, but active state. "No load" means no current is being drawn by the IC. Typically, this is the current drawn through the SW pin on a buck converter, or through the VOUT pin on a boost converter. All IQ flows only through the IC's internal circuit to ground. "Non-switching" means no power switch in the IC is on (off). It includes the main switch or control switch, and if both are integrated into the IC, the synchronous rectifier. In other words, the IC is in a high impedance state with a power stage that is completely disconnected from the output (except for the integrated MOSFET body diode on some devices that cannot be turned off). "Active" means that the IC is turned on via its EN pin and is not in UVLO or other shutdown state. IQ measures the operating current, not the shutdown current, so the device must be turned on. Finally, IQ is only meaningful in power-saving mode, so if this mode is an option for a particular device, it must be active. If the device is running in pulse-width modulation (PWM) mode, the power stage input current and switching losses significantly reduce the amount of current, which is the IQ required to run the device.

IQ is fundamentally derived from two inputs: VIN and VOUT. The datasheet specifies whether IQ is derived from either or both pins. Figure 1 shows the IQ specification from the TI TPS61220/21/22 datasheet. The TI TPS61220/21/22 are all boost converters that pull their IQ from both VIN and VOUT. In general, a buck converter pulls IQ only from its input, while a boost or buck converter pulls IQ from both the input and output. IQ measures the current required to operate the basic functions of the device, which includes powering things like the internal precision reference, oscillator, thermal shutdown or UVLO circuits, the device state machine, or other logic gates. IQ does not include any input current to the power stage or gate driver because it is measured in a non-switching state where the current is zero. The reason IQ is measured in this state is that it depends only on the IC, while the power stage input current and gate drive current depend on the external components selected, which in most cases dictate how often the IC switches in its power saving mode. Therefore, IQ is an IC measurement, while the measurement that includes the other two currents is a system measurement. TI cannot control or guarantee this system measurement, but it can control and specify the IC measurement. In fact, TI guarantees the IQ specification, and for those devices that specify a maximum IQ in the datasheet, TI tests each device individually and tests every device produced. The testing process involves activating the device, setting it to the test conditions specified in the datasheet, and then artificially raising (applying voltage externally) the output voltage, the FB pin voltage, and all other pin voltages to a level high enough that the IC does not switch. With no load and the power-saving mode activated (if active), the input current to the IC becomes IQ.
Misunderstood IQ
IQ is not the no-load input current. As mentioned earlier, IQ is only the "overhead" current required to operate the basic functions of the IC. It does not include the input current of the power stage (the current that actually goes to the output), or the current required to operate the gate driver. Even in the no-load condition, the device is still switching to keep the output in regulation. Some losses are always present at the output, such as losses in the voltage divider used to set the output voltage; leakage current into the load or through the output capacitor; and pull-up resistors. Because these losses cause voltage decay at the output capacitor, the IC must switch frequently to compensate for the power loss. Thus, the no-load input current measurement violates the
requirement that the IC must be in a non-switching state and that there is no current for the IC to recharge VOUT. For example, Figure 2 shows the no-load operation of the TPS61220 boost converter with an input voltage of 1.2 V and an output voltage of 3.3 V. The IC switches approximately every 1.75 ms to regulate the output voltage. This interval depends on VIN, VOUT, and external components, and affects the amount of average current drawn. During Phase #1, the IC is switching—either the high-side MOSFET or the synchronous rectifier MOSFET is on. The input current is dominated by the current into the power stage, which averages about 7 mA (half the inductor peak current).

Figure 3 shows a larger view of phase #1. Once the output voltage drops below the threshold, the TPS61220 begins a switching pulse by turning on the control MOSFET. The SW pin goes low, causing the inductor current to ramp up. It then turns off the control MOSFET and then turns on the rectifier MOSFET, allowing current to flow to the output. As this energy enters the output capacitor, the output voltage rises. When the inductor current reaches zero, all the energy is transferred to the output; thus, the rectifier MOSFET turns off and the IC goes into sleep mode (phase #2). At this point, both MOSFETs are off (on), so the SW pin is in a high impedance state. The inductor and the parasitic capacitance of this pin ring until it reaches its DC value, which is equal to the input voltage. During phase #2, the IC is high impedance and the output voltage drops due to leakage at the output. Since the IC is not switching, the current consumed by the IC during this period is IQ. By calculating the average input current, phases #1 and #2 define a switching period. Due to the high input current during the switching period (Phase #1), the average input current during this time must be higher than the IQ of the IC. However, since the duration of Phase #1 is very short, the average input current is generally slightly larger than the input current due to IQ.

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To account for this discrepancy between IQ and no-load input current, some IC datasheets have typical specifications for no-load input current in the electrical characteristics table. Other IC datasheets have graphs showing no-load input current for specific circuits. Figure 4 shows a graph from the TPS61220/21/22 datasheet.

1. In addition, Figure 5 shows the IQ specification in the electrical characteristics table. This table is selected from the TI TPS62120/22 datasheet.

2. The TPS62120/22 are high efficiency step-down converters. The 13 μA typical specification is valid only for the specific test conditions specified. When using the TPS61220 and TPS62120, be aware that the no-load input current is higher than the IQ of the IC. Figure 4 shows the TPS61221 boost converter with a no-load input current of 20 μA, and VIN of 1.2V and VOUT of 3.3V. This result is higher than the IQ of 5 μA shown in Figure 1 at VOUT and 0.5 μA shown in Figure 1 at VIN under the same test conditions. The reason for this difference will be explained in Item 3, "Design Considerations," of this article.

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How to Use IQ
Understanding IQ can help designers compare the low-power performance of different ICs. However, the IQ of an IC is only a component of the system input current, which is affected by three factors: the internal design of each IC (its IQ), the external components surrounding each IC, and the total system configuration. Since the input current is a combination of these three items, the IQ loss may or may not be the dominant loss in a particular system and may or may not be the determining factor in battery run time.
If the end application truly operates the ICs with no output load, the IC with the lower IQ will generally have a lower no-load input current, resulting in longer battery run time. This assumes that both ICs have a power-saving mode and that it is active. However, the power-saving modes operate differently from one IC to another, resulting in significantly different no-load input currents.

If the application is not running at no load, but is instead operating in a “standby” or “sleep” mode (in which the processor or another load still draws some current), the effectiveness of IQ quickly decreases. To illustrate this point, consider using the TPS62120 to power a TI MSP430 and other circuitry, which consumes a total of 100 μA at 2V. With an 8-V input, the TPS62120 operates at 60% efficiency (see Figure 62), resulting in an input current of:

This input current includes IQ (11 μA), which is a very large portion of the total input current (about 26%). However, if the standby load is increased to 1 mA, the input current at 8V is:
Now, the 11 μA of IQ is insignificant (about 3.5%). To accurately estimate the input current of a system in standby mode, we must know the load current being drawn. Using IQ in place of this light load input current alone does not accurately estimate the battery current being drawn. The efficiency graphs in the datasheet indicate the total circuit efficiency and include IQ losses. Therefore, IQ losses should not be added to the losses in the graphs.
Design Considerations
Many mistakes can be made when measuring IQ values or obtaining them from datasheets. The following five considerations can help designers avoid these mistakes.
1. The IQ of an IC cannot be modified. IQ cannot be influenced from outside the IC. IQ varies with input voltage and temperature, but the behavior of the IC's internal circuitry determines this variation. If the IC is operated in forced PWM mode or a load is applied to the output, the IQ no longer applies to the circuit, but the input current does. In an application, there are many aspects that can affect the input current but cannot affect the IQ.
2. Specified operating conditions need to be considered. IQ is specified only for the recommended operating conditions and certain test conditions of the IC, especially input voltage and output voltage. For any IC, the input voltage is above the recommended maximum (but below the absolute maximum) or the input voltage is below the recommended minimum (but above the UVLO level). For a buck converter, IQ is valid only when the input voltage is greater than the output voltage and the device is not in dropout (100% mode). For a boost converter, the input voltage must be lower than the output voltage so that the IC is not in power-down mode.

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3. Input current is usually connected to the output. Most of the synchronous boost IQ is usually derived from the output voltage. Since this power must ultimately come from the input, the input current in the no-load state is much higher than the IQ because the input current of the boost converter must be greater than its output current. Consider the TPS61220, which can boost 1.2V to 3.3V. With an IQ of 5 μA at VOUT and an IQ of 0.5 μA at VIN, and assuming 100% conversion efficiency, the input current for IQ is:

Due to non-IQ losses, such as switching losses and gate drive losses, the circuit actually draws about 20 μA of input current under no-load conditions (as shown in Figure 4). Importantly, this 20 μA of input current is much greater than the IQ of the IC (5.5 μA) because a boost converter like the TPS61220 draws most of its IQ from the output voltage.
4. Find all possible input current paths. When measuring IQ on an evaluation module (EVM) or other board, the designer should ensure that the board input current is fully fed into the IC and not elsewhere on the board. With small IQ values, leakage from capacitors or other devices (even when the device is off) can be significant and affect the board input current. In addition, on some EVMs and most end-equipment boards, the input voltage or output voltage is routed to pull-up resistors, indicator LEDs, or other devices that sink current under certain conditions. Obviously, this current drawn is not part of the IC IQ. Finally, the IC's IQ is not as important as a system parameter because the total input
current is what is really needed; moreover, it can be easily measured under specified test conditions.
5. Measurement methods vary widely. To accurately measure low-power input current or efficiency in power-save mode, it is important to follow the test steps detailed in Reference 3.
Conclusion
IQ is an important IC design parameter for modern low-power DC/DC converters, partially defining the current drawn from the battery under light load conditions. IQ is not the no-load input current of the IC because the IC only consumes IQ current when it is no-load, active, and non-switching. Due to leakage at the output, the IC must switch to keep the output voltage regulated. Rather than using the IC's IQ as an estimate of the battery current draw, the designer should measure and use the system's no-load input current. A better way to estimate the battery current draw is to define the system load when the system is in a low-power mode and then measure the actual battery current drawn at that operating point. By doing this rather than using only IQ, we can accurately estimate the battery run time.
References For
more information about this article, visit www.ti.com/lit/litnumber and replace "litnumber" with the specific TI Lit. #
to download the Acrobat Reader file for the materials listed below.
Document Title
Literature TitleTI
Lit. #
1. "Low Input Voltage Boost Converter in 6-Pin SC-70 Package" from TPS61220/21/22 Datasheet
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2. "15V, 75mA High Efficiency Buck Converter" from TPS62120/22 Datasheet
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3. "Making Accurate PFM Mode Efficiency Measurements" from Application Report, Author: Jatan Naik
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Related
Websitespower.ti.com
www.ti.com/sc/device/TPS61220
www.ti.com/sc/device/TPS62120