Power Management Texas Instruments Incorporated IQ: What it is, what it isn’t, and how to use it By Chris Glaser Applications Engineer Introduction A device’s quiescent current, or IQ, is an important yet often misused parameter for low-power, energy-efficient designs. In many battery-powered applications, the current drawn from the battery in a standby condition with light or no load defines the total run time of the system. In integrated switch converters, the IQ is only one portion of this battery current. This article defines IQ and how it is measured, explains what IQ is not and how it should not be used, and gives design considerations on how to use IQ while avoiding common measurement errors. This article applies to any of the Texas Instruments (TI) TPS61xxx, TPS62xxx, TPS63xxx, or TPS650xx devices. What IQ is Unless otherwise noted in the datasheet for a part, IQ is defined as the current drawn by the IC in a no-load and nonswitching but enabled condition. “No load” means that no current leaves the IC to the output. Typically, this would be current leaving via the SW pin on buck converters or via the VOUT pin on boost converters. All of the IQ simply travels inside the IC to ground. “Nonswitching” means that no power switch in the IC is on (closed). This includes the main or control switch as well as the synchronous rectifier if both are integrated into the IC. In other words, the IC is in a high-impedance condition with a power stage that is completely disconnected from the output (except for integrated MOSFET body diodes on some devices that cannot be turned off). “Enabled” means that the IC is turned on via its EN pin and is not in a UVLO or other shutdown condition. IQ measures operating current, not shutdown current, so the device must be on. Lastly, IQ is meaningful only in power-save mode, so if this mode is an option for the particular device, it must be enabled. If the device runs in pulse-width-modulation (PWM) mode, then the input current to the power stage and switching losses more than dwarfs the miniscule amount of current, the IQ, required to run the device. IQ fundamentally comes from two inputs: VIN and VOUT. The datasheet lists whether the IQ comes from either or both pins. Figure 1 shows the IQ specification from the datasheet for the TI TPS61220/21/22,1 which are boost converters that draw their IQ from both VIN and VOUT. Typically, a buck converter draws IQ only from its input, while a boost converter or buck-boost converter draws IQ from both the input and the output. IQ measures the current required to operate the device’s basic functionality, which includes powering things like the internal precision reference voltage, an oscillator, a thermal shutdown or UVLO circuit, the device’s state machine or other logic gates, etc. IQ does not include any input current to the power stage or gate drivers, as it is measured in a nonswitching condition where these currents are zero. The reason for measuring IQ in this condition is that it is solely dependent on the IC, whereas the power-stage input current and gate-drive current are dependent on the selected external components, which in most cases dictate how often the IC switches in its power-save mode. Thus, IQ is an IC measurement, whereas including the other two currents is a system measurement. TI does not control and cannot guarantee such a system measurement but does control and can specify an IC measurement. In fact, TI guarantees the IQ specification and, for devices whose datasheets specify a maximum value for the IQ, tests it on each and every device that is produced. This is done by enabling the device, setting it to the test conditions specified in its datasheet, and then artificially raising (with externally applied voltages) the output voltage, FB pin, and any other pin voltages high enough to cause the IC not to switch. With no load and power-save mode enabled (if available), the input current to the IC becomes the IQ. What IQ isn’t IQ is not the no-load input current. As previously mentioned, the IQ is simply the “overhead” current required to operate the IC’s basic functionality. It does not include the Figure 1. IQ specification from TPS61220/21/22 datasheet DC/DC STAGE PARAMETER IQ Quiescent current TEST CONDITIONS VIN VOUT MIN IO = 0 mA, VEN = VIN = 1.2 V, VOUT = 3.3 V TYP MAX UNIT 0.5 0.9 µA 5 7.5 µA 18 High-Performance Analog Products www.ti.com/aaj 2Q 2011 Analog Applications Journal Power Management Texas Instruments Incorporated input current into the power stage (current Figure 2. No-load operation of TPS61220 that is actually transferred to the output) or current required to operate the gate drivers. Even at no load, the device still switches to VIN = 1.2 V keep the output regulated. Some losses VOUT = 3.3 V always exist at the output, such as loss from VOUT (AC-Coupled, 10 mV/div) the voltage divider used to set the output 2 voltage; leakage current into the load or through the output capacitor; pull-up resistors; etc. Because these losses cause voltage decay at the output capacitor, the IC must Phase #1 switch every so often to replenish the power lost. So, a no-load input-current measurePhase #2 ment violates the requirements that the IC must be in a nonswitching condition and that Switch Node (1 V/div) no current may leave the IC to recharge 14 VOUT. As an example, Figure 2 shows no-load IL (100 mA/div) operation for the TPS61220 boost converter, with an input voltage of 1.2 V and an output Time (500 µs/div) voltage of 3.3 V. The IC switches approximately every 1.75 ms to regulate the output voltage. This period depends on VIN, VOUT, and the external components and affects Figure 3. Switching pulse of TPS61220 during how much average input current is drawn. no-load operation During phase #1, the IC is switching—either the high-side MOSFET or the synchronous rectifying MOSFET is on. The input current is dominated by the current into the power VIN = 1.2 V stage, which averages about 70 mA (half of VOUT = 3.3 V VOUT (AC-Coupled, 10 mV/div) the peak current in the inductor). 2 Figure 3 shows an enlarged view of phase #1. Once the output voltage drops below the threshold, the TPS61220 begins a switching pulse by turning the control Phase #2 MOSFET on. The SW pin goes low, causing Switch Node the inductor current to ramp up. It then (1 V/div) turns off the control MOSFET and turns on the rectifying MOSFET, allowing current to Phase #1 flow to the output. The output voltage 4 1 increases as this energy is transferred into IL (100 mA/div) the output capacitor. When the inductor current reaches zero, all the energy has been Time (500 ns/div) delivered to the output; so the rectifying MOSFET turns off, and the IC goes into a sleep mode (phase #2). At this point, both MOSFETs are off (open), so the SW pin is in a state of high impedance. The inductor and parasitic switching time (phase #1), the average input current over capacitances on that pin ring until it reaches its DC value, this time must be higher than the IC’s IQ. However, because which equals the input voltage. the duration of phase #1 is very short, the average input During phase #2, the IC is high impedance, and the outcurrent is usually only slightly greater than the input curput voltage drops due to leakage at the output. Because rent that is due to the IQ. the IC is not switching, the current consumed by the IC To address this difference between the IQ and the noduring this time is the IQ. Phases #1 and #2 define a load input current, the datasheets of some ICs have typical switching period over which the average input current is specifications for the no-load input current in the electricalculated. Due to the high input current during the cal characteristics table. Others have graphs that show the 19 Analog Applications Journal 2Q 2011 www.ti.com/aaj High-Performance Analog Products Power Management Texas Instruments Incorporated Figure 4. Graph of no-load input current from TPS61220/21/22 datasheet 80 Device Enabled 70 Input Current, IIN (µA) 60 50 TPS61222, VOUT = 5 V TPS61221, VOUT = 3.3 V 40 TPS61220, VOUT = 1.8 V 30 20 10 0 0.7 1.7 2.7 3.7 4.7 Input Voltage, VIN (V) no-load input current for a particular circuit. Figure 4 shows such a graph from the TPS61220/21/22 datasheet.1 Alternatively, Figure 5 shows the IQ specification in an electrical characteristics table. This table is taken from the datasheet for the TI TPS62120/22,2 which are highefficiency buck converters. The typical specification of 13 µA is valid only for the specific test conditions stated. For both the TPS61220 and TPS62120, note that the noload input current is higher than the IC’s IQ. Figure 4 shows that the no-load input current to the TPS61221 boost converter is 20 µA with a VIN of 1.2 V and a VOUT of 3.3 V. This is much higher than the IQ in Figure 1 of 5 µA at VOUT and 0.5 µA at VIN with the same test conditions. This difference is explained later in this article under item #3 of “Design considerations.” How to use IQ Knowing the IQ assists the designer in comparing the lowpower performance of different ICs. However, an IC’s IQ is only part of the system’s input current, which is affected by three things: each IC’s internal design (its IQ), the external components around each IC, and the overall system configuration. Because the input current is a combination of these three items, IQ losses may or may not be the dominant loss for a particular system and may or may not be the determining factor in the battery’s run time. If the end application truly operates the IC at no output load, then an IC with lower IQ typically has lower no-load input current, which results in longer battery run time. This assumes that both ICs have a power-save mode and that it is enabled. However, power-save modes can behave differently among different ICs, resulting in vastly different no-load input currents. If the application does not run at no load but instead runs in a “standby” or “hibernate” mode in which the proc essor or another load still draws some current, then the usefulness of IQ quickly decreases. To demonstrate, consider the TPS62120 powering TI’s MSP430™ and other circuitry that altogether consume 100 µA at 2 V. With an 8-V input, the TPS62120 is running at 60% efficiency (see Figure 5. No-load input-current specification from TPS62120/22 datasheet PARAMETER TEST CONDITIONS MIN TYP MAX UNIT SUPPLY IQ Quiescent current IOUT = 0mA, Device not switching, EN = VIN, regulator sleeps 11 IOUT = 0mA, Device switching, VIN = 8 V, VOUT = 1.8V 13 18 µA µA 20 High-Performance Analog Products www.ti.com/aaj 2Q 2011 Analog Applications Journal Power Management Texas Instruments Incorporated Figure 6. Efficiency graph for TPS62120 100 VIN = 4 V 90 VIN = 2.5 V VIN = 6 V Efficiency (%) 80 VIN = 10 V 70 60 VIN = 12 V VIN = 8 V 50 VOUT = 2 V L = 18 µH LPS3015 COUT = 4.7 µF 40 30 0.1 VIN = 15 V 1 10 100 Output Current, IOUT (mA) Figure 6 2 ), resulting in an input current of 2 V × 100 µA = 42 µA. 0.6 × 8 V This input current includes the IQ (11 µA), which is a very significant portion of the total input current (about 26%). If, however, the standby load increases to 1 mA, the input current at 8 V is 2 V × 1 mA = 313 µA. 0.8 × 8 V Now the 11 µA of IQ is not significant at all (about 3.5%). To accurately estimate the input current in a system’s standby mode, the load current drawn must be known. Simply using the IQ in place of this light-load input current does not accurately estimate the battery current drawn. Any efficiency graph in a datasheet indicates the total circuit efficiency and includes the IQ losses. Therefore, the IQ losses should not be added to the losses given in the graphs. Design considerations Numerous errors can be made when IQ values are measured or taken from a datasheet. The following five considerations will help the designer avoid these errors. 1.The IQ of an IC cannot be changed. Nothing can be done from outside the IC that affects the IQ. The IQ does vary over input voltage and temperature, but the behavior of the IC’s internal circuitry sets this variation. If the IC is operated in forced PWM mode or a load is attached to the output, then the IQ is no longer applicable to the circuit, and the input current becomes applicable instead. Many things can be done in an application that affect the input current, but not the IQ. 2.Specified operating conditions need to be considered. IQ is specified only for an IC’s recommended operating conditions and for certain test conditions, specifically an input voltage and an output voltage. For any IC, the specified IQ is not guaranteed when the input voltage is above the recommended maximum (but less than the absolute maximum) or when 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 when the device is not in dropout (100% mode). For a boost converter, the input voltage must be less than the output voltage so that the IC is not in down mode. 3.Input current is often linked to the output. The majority of the IQ for a synchronous boost usually comes from the output voltage. Since this power must ultimately come from the input, the input current in a noload condition is substantially higher than the IQ because the input current for a boost converter must be greater than its output current. Consider the TPS61220 boosting from 1.2 V to 3.3 V. With an IQ of 5 µA at VOUT and 0.5 µA at VIN, and assuming 100% conversion efficiency, the input current from the IQ alone is 3.3 V × 5 µA + 0.5 µA = 14.25 µA. 1.2 V The circuit actually draws about 20 µA of input current at no load (as shown in Figure 4) simply because of non-IQ losses such as switching losses and gate-drive 21 Analog Applications Journal 2Q 2011 www.ti.com/aaj High-Performance Analog Products Power Management Texas Instruments Incorporated losses. The important point is that this 20 µA of input current is much greater than the IC’s IQ of 5.5 µA because the TPS61220 is a boost converter that draws most of its IQ from the output voltage. 4.Look for all possible input-current paths. When measuring the IQ on an evaluation module (EVM) or other board, the designer should ensure that the input current to the board is going entirely into the IC and not to other places on the board. Leakages from capacitors or other devices, even if the devices are disabled, may be significant due to the small IQ values and may affect the input current to the board. 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 may sink current under some conditions. Obviously, this current draw is not part of the IC’s IQ. Finally, the IC’s IQ is of no importance as a system parameter, since total input current is actually what is needed; and that is easily measured at the required test conditions. 5.Measurement techniques can make a big difference. To get accurate measurements of the lowpower input current or the efficiency in power-save mode, it is critical to follow the test setup detailed in Reference 3. not the IC’s no-load input current, as the IC consumes the IQ current only in a no-load, enabled, and nonswitching condition. Due to leakage at the output, the IC must switch to keep the output voltage regulated. Instead of using an IC’s IQ as an estimate of the battery’s current draw, the designer should measure and use the no-load input current to the system. An even better way to estimate the battery’s current draw is to define the system’s load when the system is in low-power mode and then measure the battery’s actual current draw at this operating point. Doing this instead of simply using IQ allows accurate prediction of battery run times. Conclusion Related Web sites IQ is an important IC design parameter in modern lowpower DC/DC converters and partially defines the current drawn from the battery in light-load conditions. The IQ is power.ti.com www.ti.com/sc/device/TPS61220 www.ti.com/sc/device/TPS62120 References For more information related to this article, you can down load an Acrobat® Reader® file at www.ti.com/lit/litnumber and replace “litnumber” with the TI Lit. # for the materials listed below. Document Title TI Lit. # 1. “Low input voltage step-up converter in 6 pin SC-70 package,” TPS61220/21/22 Datasheet . . . slvs776 2. “15V, 75mA high efficient buck converter,” TPS62120/22 Datasheet . . . . . . . . . . . . . . . . . . . . slvsad5 3. Jatan Naik, “Performing accurate PFM mode efficiency measurements,” Application Report . slva236 22 High-Performance Analog Products www.ti.com/aaj 2Q 2011 Analog Applications Journal TI Worldwide Technical Support Internet TI Semiconductor Product Information Center Home Page support.ti.com TI E2E™ Community Home Page e2e.ti.com Product Information Centers Americas Phone +1(972) 644-5580 Brazil Phone 0800-891-2616 Mexico Phone 0800-670-7544 Fax Internet/Email +1(972) 927-6377 support.ti.com/sc/pic/americas.htm Europe, Middle East, and Africa Phone European Free Call International Russian Support 00800-ASK-TEXAS (00800 275 83927) +49 (0) 8161 80 2121 +7 (4) 95 98 10 701 Note: The European Free Call (Toll Free) number is not active in all countries. If you have technical difficulty calling the free call number, please use the international number above. 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