利用PMBus數(shù)字電源系統(tǒng)管理器進(jìn)行電流檢測(cè)——第一部分

This is the first article in a 2-part series. Part 1 introduces the digital power system manager (DPSM) family and covers the primary methods of current sensing. LTpowerPlay? is also introduced, and energy metering described. Part 2 covers current sensing on high voltage or negative supplies, accuracy, and highlights the digital aspects of the DSPM family.

本系列文章分為兩部分，這是第一部分。第一部分介紹數(shù)字電源系統(tǒng)管理器(DPSM)系列，并說明電流檢測(cè)的主要方法。另外還會(huì)介紹LTpowerPlay?并討論電能計(jì)量。第二部分探討高壓或負(fù)電源上的電流檢測(cè)及精度，并重點(diǎn)介紹DSPM系列的數(shù)字方面。

Introduction
簡(jiǎn)介
Board level designers are tasked with giving life to a board, monitoring its health, adjusting settings, running diagnostics, bringing it offline for inspection, trouble-shooting when things are not quite right, and gracefully powering down a complex board without incident. In the world of designing and developing power supplies, power management may not only be desirable, but a hard requirement. The power system manager aggregates a variety of functions, such as power-on sequencing, detecting faults, margin testing, coordinating shutdown, measuring voltages, measuring currents, and collecting data for analysis. Measuring supply current with LTC297x devices is the focus of this article.1

電路板級(jí)設(shè)計(jì)人員的任務(wù)是賦予電路板以生命，監(jiān)視其健康狀況，調(diào)整設(shè)置，運(yùn)行診斷，脫機(jī)進(jìn)行檢查，在出現(xiàn)問題時(shí)排除故障，以及在無事故的情況下有序地關(guān)斷復(fù)雜的電路板。在電源設(shè)計(jì)和開發(fā)的世界里，電源管理可能不僅僅是一種需要，更是一項(xiàng)硬性要求。電源系統(tǒng)管理器聚合了多種功能，例如上電時(shí)序管理、故障檢測(cè)、裕量測(cè)試、協(xié)調(diào)關(guān)斷、測(cè)量電壓、測(cè)量電流以及收集數(shù)據(jù)進(jìn)行分析。使用LTC297x器件測(cè)量電源電流是本文的重點(diǎn)1。

For supplies that power high value components, such as FPGAs, CPUs, and optical transceivers, it may be important to measure the current drawn from the supply rail. For these critical supply rails, this data allows the board designer to gain insight into its performance. When current is measured and the current value is in a digital format, the device can compute power and energy, and the system host can perform unique calculations, look for trends in the data, schedule tasks, etc.

對(duì)于為FPGA、CPU、光收發(fā)器等高價(jià)值器件供電的電源，測(cè)量其從電源軌汲取的電流可能很重要。對(duì)于這些關(guān)鍵電源軌，電路板設(shè)計(jì)人員可以通過此數(shù)據(jù)深入了解其性能。當(dāng)電流信息被測(cè)量到，且電流值為數(shù)字格式，那么器件就可以計(jì)算功率和電能，系統(tǒng)主機(jī)也可以執(zhí)行獨(dú)特的計(jì)算、從數(shù)據(jù)中分析趨勢(shì)、安排任務(wù)等。

Many technical articles and application notes are written on the topic of sensing current, but none have covered the topic specifically for DPSMs. This article covers both the analog and digital aspects and describes various supporting circuits for measuring current of low voltage, high voltage, and negative rails.

圍繞電流檢測(cè)這一主題已有許多技術(shù)文章和應(yīng)用筆記問世，但沒有一篇是專門針對(duì)DPSM的。本文涵蓋了模擬和數(shù)字方面，并描述了用于測(cè)量低壓、高壓和負(fù)電源軌的各種支持電路。

The LTC297x DPSM Family
LTC297x DPSM系列
The focus of this article is power system managers that have built-in current measurement. Table 1 describes the differences between these devices.

本文的重點(diǎn)是內(nèi)置電流測(cè)量功能的電源系統(tǒng)管理器。表1說明了這些器件之間的差異。

The LTC2977/LTC2979/LTC2980/LTM2987 can be configured to monitor current, but there are some limitations. Only odd-numbered channels support current measurement, and measurements are returned in unscaled units of volts. This is covered in more detail in Part 2.

LTC2977/LTC2979/LTC2980/LTM2987可配置用來監(jiān)視電流，但存在一些限制。只有奇數(shù)通道支持電流測(cè)量，并且測(cè)量值以未縮放的單位(V)返回。這在第二部分中有更詳細(xì)的介紹。

This article focuses on the LTC2971/LTC2972/LTC2974/LTC2975 devices due to their ability to measure output current and allow the system/software to read back values in units of amps using the READ_IOUT command.

本文重點(diǎn)介紹LTC2971/LTC2972/LTC2974/LTC2975器件，它們能夠測(cè)量輸出電流，并允許系統(tǒng)/軟件利用READ_IOUT命令回讀以安培(A)為單位的值。

Note AN105: Current Sense Circuit Collection Making Sense of Current covers a wide variety of circuits and scenarios. A portion of this collection applies to the ADI digital power system managers.

應(yīng)用筆記AN105：幫助弄懂電流的電流檢測(cè)電路集合介紹了廣泛的電路和使用場(chǎng)景。該集合的一部分適用于ADI數(shù)字電源系統(tǒng)管理器。

Table 1. Family of LTC297x DPSM Devices

表1.LTC297x DPSM器件系列
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1?? ?Unless otherwise specified, the term LTC297x in this application note refers to the LTC2971, LTC2972, LTC2974, LTC2975, LTC2977, LTC2979, LTC2980, and LTM2987. It does not include the LTC2970.

除非另有說明，本應(yīng)用筆記中的LTC297x指LTC2971、LTC2972、LTC2974、LTC2975、LTC2977、LTC2979、LTC2980和LTM2987，而不包括LTC2970。

PSM Basics
PSM基礎(chǔ)
Power system managers provide a digital view of key voltage and current readings of a power supply. This is a powerful feature of the product family: a system host or LTpowerPlay can support initial board bring up, debugging, validating or collecting baseline data, or looking for trends. While some power supply channels do not require accurate current readings, many critical output channels need highly accurate current measurements.

電源系統(tǒng)管理器提供關(guān)于電源的關(guān)鍵電壓和電流讀數(shù)的數(shù)字視圖。這是該產(chǎn)品系列的一個(gè)強(qiáng)大特性：系統(tǒng)主機(jī)或LTpowerPlay可以支持電路板初始啟動(dòng)、調(diào)試、驗(yàn)證或收集基線數(shù)據(jù)，或者尋找趨勢(shì)。雖然一些電源通道不需要精確的電流讀數(shù)，但許多關(guān)鍵的輸出通道需要高度精確的電流測(cè)量。

Various current sense options will be covered in this article, including trade-offs between cost, complexity, and accuracy.

本文將介紹各種電流檢測(cè)選項(xiàng)，包括成本、復(fù)雜性和精度之間的權(quán)衡。

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Figure 1. READ_IOUT telemetry plot in LTpowerPlay.
圖1.LTpowerPlay中的READ_IOUT遙測(cè)曲線

Current Sensing Options
電流檢測(cè)選項(xiàng)
The LTC2971/LTC2972/LTC2974/LTC2975 managers accurately measure output current. Use these devices when possible because they have dedicated current sense pins and PMBus commands that provide telemetry values in amps.

LTC2971/LTC2972/LTC2974/LTC2975管理器能夠精確測(cè)量輸出電流。應(yīng)盡可能使用這些器件，因?yàn)樗鼈兙哂袑Ｓ玫碾娏鳈z測(cè)引腳和PMBus命令，可提供以安培為單位的遙測(cè)值。

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Figure 2. Current sensing with series shunt.
圖2.使用串聯(lián)分流器進(jìn)行電流檢測(cè)

For example, wire the ISENSE lines to a shunt, configure a few registers, and the chip does the rest. The chip converts a measured sense voltage to a current value. LTpowerPlay displays the current in real time as numerical values and in a telemetry plot.

例如，將ISENSE線連接到分流器，配置幾個(gè)寄存器，剩下的工作由芯片完成。芯片會(huì)將測(cè)得的電壓轉(zhuǎn)換為電流值。LTpowerPlay將電流實(shí)時(shí)顯示為數(shù)值和遙測(cè)曲線。

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Figure 3. PMBus register settings for output current measurement.
圖3.用于輸出電流測(cè)量的PMBus寄存器設(shè)置

It is also possible to use an LTC2977/LTC2979/LTC2980/LTM2987 to measure output current; however, the READ_IOUT command returns a voltage that must be converted to amps by the system host or LTpowerPlay. In practice, this means the firmware, rather than the chip, must store the value of the series shunt.

也可以使用LTC2977/LTC2979/LTC2980/LTM2987來測(cè)量輸出電流，但是，READ_IOUT命令返回的是電壓，必須由系統(tǒng)主機(jī)或LTpowerPlay將其轉(zhuǎn)換為安培。實(shí)踐中，這意味著固件（而不是芯片）必須存儲(chǔ)串聯(lián)分流器的值。

A series shunt resistor is not the only way to sense current. Table 2 summarizes the current sensing options available for the DPSM family and their trade-offs. Accuracy, cost, board space, and other factors also need to be considered.

串聯(lián)分流電阻并非檢測(cè)電流的唯一方法。表2總結(jié)了DPSM系列可用的電流檢測(cè)選項(xiàng)及其權(quán)衡。精度、成本、電路板空間和其他因素也需要考慮。

Table 2. Summary of Current Sense Options

表2.電流檢測(cè)選項(xiàng)總結(jié)
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Shunt Resistor Sensing
分流電阻檢測(cè)
The most common sensing method uses a shunt resistor, sometimes called a current shunt. Whether the DC-to-DC converter is a switching regulator or linear regulator, the shunt resistor is placed in series with the output. The feedback resistor divider is connected to the output node such that the shunt is inside the feedback loop, which allows the regulator to compensate for the shunt resistor’s IR drop when load current is applied, significantly improving the load regulation.

最常見的檢測(cè)方法是使用分流電阻，有時(shí)也稱為檢流電阻。無論DC-DC轉(zhuǎn)換器是開關(guān)穩(wěn)壓器還是線性穩(wěn)壓器，分流電阻都與輸出串聯(lián)。反饋電阻分壓器連接到輸出節(jié)點(diǎn)，使得分流器位于反饋環(huán)路內(nèi)，這樣當(dāng)施加負(fù)載電流時(shí)，穩(wěn)壓器就能補(bǔ)償分流電阻的IR壓降，從而顯著提高負(fù)載調(diào)整率。

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Figure 4. Sense resistor inside the feedback loop.
圖4.反饋回路內(nèi)的檢測(cè)電阻

The PMBus command used to convert voltage to current is called IOUT_CAL_GAIN. This is the nominal resistance of the shunt resistor. The chip measures the small voltage drop across the shunt resistor via the ISENSE pins, performs the conversion internally, and returns the output current with the READ_IOUT command. The actual voltage that is sensed by the chip is available using the MFR_IOUT_SENSE_VOLTAGE command. The chip computes the output current with this equation:

用于將電壓轉(zhuǎn)換為電流的PMBus命令稱為IOUT_CAL_GAIN。這是分流電阻的標(biāo)稱電阻。芯片通過ISENSE引腳測(cè)量分流電阻上的小電壓降，在內(nèi)部執(zhí)行轉(zhuǎn)換，并使用READ_IOUT命令返回輸出電流。芯片檢測(cè)到的實(shí)際電壓可通過MFR_IOUT_SENSE_VOLTAGE命令獲得。芯片使用以下公式計(jì)算輸出電流：

When using a resistive shunt, set the MFR_IOUT_CAL_GAIN_TC value to the manufacturer’s specifications to compensate for temperature changes. Generally, shunts greater than 10 m? have lower temperature coefficients: <100 ppm/°C.

使用阻性分流器時(shí)，將MFR_IOUT_CAL_GAIN_TC值設(shè)置為制造商的規(guī)格以補(bǔ)償溫度變化。通常，大于10 mΩ的分流器具有較低的溫度系數(shù)：<100 ppm/°C。

The maximum differential sense voltage developed across the ISENSE pins is listed in the data sheet specifications. Most of the LTC297x devices are limited to ±170 mV of differential voltage. This provides more than enough range for the majority of applications. The max sense voltage is calculated as follows: VSENSE = RSNS × IOUT(MAX). Generally, the max sense voltage is determined first and the RSNS current sense resistor is calculated as follows: RSNS = VSENSE / IOUT(MAX). The max sense voltage is chosen to be a large enough signal, yet not create a power dissipation problem or IR drop in the output path. 50 mV to 80 mV is a good max sense voltage. Select a current sense resistor’s physical size so that it provides a power dissipation rating greater than the calculated power dissipation in the sense resistor: PD = RSNS × (IOUT(MAX))2.

數(shù)據(jù)手冊(cè)規(guī)格中列出了ISENSE引腳上產(chǎn)生的最大差分檢測(cè)電壓。大多數(shù)LTC297x器件的差分電壓以±170 mV為限，這為大多數(shù)應(yīng)用提供了足夠的范圍。最大檢測(cè)電壓計(jì)算如下：VSENSE = RSNS × IOUT(MAX)。通常，首先確定最大檢測(cè)電壓，RSNS電流檢測(cè)電阻計(jì)算如下：RSNS = VSENSE / IOUT(MAX)。選擇的最大檢測(cè)電壓應(yīng)是一個(gè)足夠大的信號(hào)，但又不會(huì)在輸出路徑中造成功耗問題或IR壓降。50 mV至80 mV是一個(gè)很好的最大檢測(cè)電壓。選擇電流檢測(cè)電阻的物理尺寸，其功耗額定值應(yīng)大于檢測(cè)電阻的計(jì)算功耗：PD = RSNS × (IOUT(MAX))2。

A related method adds a ground referenced current sense amplifier (CSA) to provide a single-ended output that is fed into the current sense pins of the manager. This approach is typically used for level translating a rail that is higher than the 6 V limit of most LTC297x managers. The CSA should have good high-side common-mode performance. It is typical to power such a device from the sensed rail and GND. Details on this method are covered in Part 2 of this article.

一種相關(guān)方法是增加一個(gè)以地為基準(zhǔn)的電流檢測(cè)放大器(CSA)，其提供單端輸出，該輸出被饋送到管理器的電流檢測(cè)引腳。這種方法通常用于對(duì)高于大多數(shù)LTC297x管理器的6 V限值的電壓軌進(jìn)行電平轉(zhuǎn)換。CSA應(yīng)具有良好的高端共模性能。通常從被檢測(cè)的電壓軌和GND為此類器件供電。本文第二部分會(huì)詳細(xì)介紹這種方法。

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Figure 5. Current sense amplifier used for higher voltage rails.
圖5.用于更高電壓軌的電流檢測(cè)放大器

ADI offers many easy to use non-PSM μModule? devices with a small footprint. A PSM manager is a good companion device to control sequencing and supervise them. Most μModule devices have internal inductors. However, some also integrate the topside feedback resistor, making it impossible to add an external shunt resistor inside the feedback loop. One should select a μModule device that allows the use of an external topside feedback resistor for the highest voltage accuracy.

ADI公司提供了許多易于使用且小尺寸的非PSM μModule?器件。PSM管理器是很好的配套器件，可控制上電時(shí)序并實(shí)施監(jiān)控。大多數(shù)μModule器件都有內(nèi)置電感，但有些還集成了上方反饋電阻，因此無法在反饋環(huán)路內(nèi)添加外部分流電阻。應(yīng)當(dāng)選擇允許使用外部上方反饋電阻以獲得最高電壓精度的μModule器件。

Inductor DCR Sensing
電感DCR檢測(cè)
DCR sensing is the method that senses current through a buck regulator’s output inductor. An inductor can be modeled as an ideal inductance and a series resistance called DCR (see Figure 6). This is typically the preferred method for high current (>20 A) rails. The addition of a resistive shunt is an extra component, which dissipates power and generates heat.

DCR檢測(cè)是一種通過降壓穩(wěn)壓器的輸出電感檢測(cè)電流的方法。電感可以用理想電感和一個(gè)稱為DCR的串聯(lián)電阻來建模（見圖6）。這通常是高電流(>20 A)電源軌的首選方法。增加的阻性分流器是一個(gè)額外的元件，會(huì)消耗功率并產(chǎn)生熱量。

One must have access to both ends of the inductor to sense across it, and a filter network must be inserted between the sense points and the LTC297x sense pins. The filter network is a 2-stage differential RC low-pass filter. For convenience and small footprint, a 4-element resistor array can be used. The resistor values should be chosen such that the IR drop is small enough to prevent errors from the LTC297x input current, yet large enough to keep the capacitor values less than 1 μF.

要在電感上進(jìn)行檢測(cè)，必須能夠接觸到電感的兩端，并且必須在檢測(cè)點(diǎn)和LTC297x檢測(cè)引腳之間插入一個(gè)濾波器網(wǎng)絡(luò)。濾波器網(wǎng)絡(luò)是一個(gè)兩級(jí)差分RC低通濾波器。為了方便和減小尺寸，可以使用4元件電阻陣列。電阻值的選擇應(yīng)使IR壓降足夠小，以防止LTC297x輸入電流造成誤差，同時(shí)又足夠大，以使電容值小于1 μF。

The LTC2971/LTC2972/LTC2974/LTC2975 data sheets provide guidance for selecting RC values.
LTC2971/LTC2972/LTC2974/LTC2975數(shù)據(jù)手冊(cè)提供了關(guān)于選擇RC值的指南。

Example:

示例：

Assume L = 2.2 μH, DCR = 10 m?, fSW = 500 kHz

假設(shè)L = 2.2 μH，DCR = 10 mΩ，fSW = 500 kHz

Let Rcm1 = Rcm2 = 1 kΩ

令Rcm1 = Rcm2 = 1 kΩ

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Figure 6. DCR inductor sensing with a 2-pole low-pass filter.
圖6.使用2極點(diǎn)低通濾波器的DCR電感檢測(cè)

DCR sensing offers a lossless measurement of current; however, the accuracy suffers due to variability of inductor winding resistance or DCR. It is not uncommon to see inductor DCR specifications as much as ±10% or only a maximum value. The actual DCR value will vary from inductor to inductor, and from lot to lot.

DCR檢測(cè)可實(shí)現(xiàn)電流的無損測(cè)量，但由于電感繞組電阻或DCR的差異，精度會(huì)受到影響。電感DCR規(guī)格高達(dá)±10%或只有一個(gè)最大值的情況并不少見。實(shí)際的DCR值會(huì)因電感和批次而異。

An alternative filtering scheme uses only two resistors and two capacitors. This reduces the component count from eight down to four; however, the filter performance is not as good as in Figure 7.

另一種濾波方案僅使用兩個(gè)電阻和兩個(gè)電容，因而元件數(shù)量從八個(gè)減少到四個(gè)，但濾波器的性能不如圖7所示的好。

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Figure 7. DCR inductor sensing with a simplified low-pass filter.
圖7.使用簡(jiǎn)化低通濾波器的DCR電感檢測(cè)

PMBus Configuration
PMBus配置
To configure the LTC297x using PMBus commands, the nominal value for the shunt resistor or inductor DCR is set using the IOUT_CAL_GAIN command. For inductors wound with copper wire, the DCR increases as inductor temperature increases. This will introduce errors in the READ_IOUT reading. This can be compensated for by setting the copper’s temperature coefficient with the MFR_IOUT_CAL_GAIN_TC command. The data sheet default for this value is 3900 ppm/°C. You may need to adjust the value to match your inductor, because this parameter can vary widely when the wire is an alloy, not pure copper. MFR_IOUT_CAL_GAIN_THETA is the thermal time constant that may be set. The LTC297x data sheets cover these in more detail.

要利用PMBus命令配置LTC297x，可使用IOUT_CAL_GAIN命令設(shè)置分流電阻或電感DCR的標(biāo)稱值。對(duì)于銅線纏繞的電感，DCR會(huì)隨著電感溫度升高而增加，這會(huì)在READ_IOUT讀數(shù)中引入誤差。使用MFR_IOUT_CAL_GAIN_TC命令設(shè)置銅的溫度系數(shù)可補(bǔ)償此誤差。在數(shù)據(jù)手冊(cè)中，該值的默認(rèn)值為3900 ppm/°C。用戶可能需要調(diào)整該值以匹配電感，因?yàn)楫?dāng)導(dǎo)線是合金而非純銅時(shí)，此參數(shù)可能會(huì)大幅改變。MFR_IOUT_CAL_GAIN_THETA表示熱時(shí)間常數(shù)，可對(duì)其進(jìn)行設(shè)置。LTC297x數(shù)據(jù)手冊(cè)詳細(xì)介紹了這些內(nèi)容。

It is important to place a temperature sensor (diode-connected bipolar transistor) close to the inductor to achieve a more accurate current temperature compensation. The LTC2971/LTC2972/LTC2974/LTC2975 devices have TSENSE pins that are connected to the sensor.

必須將溫度傳感器（二極管連接的雙極性晶體管）靠近電感放置，以實(shí)現(xiàn)更準(zhǔn)確的電流溫度補(bǔ)償。LTC2971/LTC2972/LTC2974/LTC2975器件具有連接到傳感器的TSENSE引腳。

IMON
IMON
IMON pins are gaining popularity in many regulators, both switching and linear. These regulators have a current sense output pin that provides a means of monitoring the regulator’s load current. The advantages of the IMON method are that it is lossless and there is no common-mode voltage to worry about because the LTC297x ISENSE pins do not connect to VOUT. The IMON pin is a single-ended output signal that represents a fraction of the output current, and it can be either a voltage output or a current output, which requires a resistor connected to GND. Current output IMON pins allow the user to select a resistor value, and hence set the maximum full load voltage.

IMON引腳在許多穩(wěn)壓器（包括開關(guān)和線性）中越來越受歡迎。這些穩(wěn)壓器有一個(gè)電流檢測(cè)輸出引腳，藉此可監(jiān)視穩(wěn)壓器的負(fù)載電流。IMON方法的優(yōu)點(diǎn)在于它是無損的，并且無需擔(dān)心共模電壓，因?yàn)長TC297x ISENSE引腳不連接到VOUT。IMON引腳是單端輸出信號(hào)，代表輸出電流的一小部分，它可以是電壓輸出或電流輸出，需要一個(gè)電阻連接到GND。電流輸出IMON引腳允許用戶選擇電阻值，從而設(shè)置最大滿載電壓。

A single-ended voltage can be a much larger signal than a voltage developed across a current shunt or inductor DCR. The LTC2972 and LTC2971 devices even have a configuration bit to allow larger signal levels. It is called the imon_sense bit. The bit is located in the MFR_CONFIG command and is a paged command.

單端電壓可以是比電流分流器或電感DCR兩端產(chǎn)生的電壓大得多的信號(hào)。LTC2972和LTC2971器件甚至有一個(gè)配置位來支持更大的信號(hào)電平，它被稱為imon_sense位。該位位于MFR_CONFIG命令中，是一個(gè)分頁命令。

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Figure 8. IMON bit in the MFR_CONFIG register.
圖8.MFR_CONFIG寄存器中的IMON位

The IMON resistor value should be chosen to allow wide dynamic range under all load conditions. In general, IMON accuracy is good under medium and heavy load current conditions but loses accuracy under light loads. Check the regulator’s data sheet specifications for more details.

選擇的IMON電阻值應(yīng)使得在所有負(fù)載條件下都能提供寬動(dòng)態(tài)范圍。一般情況下，IMON精度在中負(fù)載和重負(fù)載電流條件下較好，但在輕負(fù)載下會(huì)下降。有關(guān)更多詳細(xì)信息，請(qǐng)查閱穩(wěn)壓器的數(shù)據(jù)手冊(cè)規(guī)格。

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Figure 9. PSM measures current with IMON.
圖9.PSM利用IMON測(cè)量電流

Some regulators combine a current limit function with the IMON pin. The pin may be called IMON/ILIM. Be careful not to select an IMON resistor value such that the IMON voltage activates the current limit circuit under full load. Examples include linear regulators, such as the LT3072 and LT3086. In other cases, such as the LT3094 and LT3045, there is an ILIM pin that functions as a current limit and may be used as an output current monitor. In the case of some switching regulators, the pin may be called IMON and a built-in current limit function may not be obvious. Examples include the LT8652S and LT8708. The current limit circuit has a foldback and does not shut down the output. To shut off the output, an LTC298x will detect an overcurrent condition and pull VOUT_EN low, disabling the regulator’s output.

一些穩(wěn)壓器將限流功能與IMON引腳結(jié)合在一起，該引腳可稱為IMON/ILIM。請(qǐng)注意，所選的IMON電阻值不應(yīng)使得IMON電壓在滿載時(shí)會(huì)激活限流電路。示例包括線性穩(wěn)壓器，例如LT3072和LT3086。其他例子有LT3094和LT3045等，一個(gè)ILIM引腳起到限流作用，可用作輸出電流監(jiān)視器。對(duì)于某些開關(guān)穩(wěn)壓器，該引腳可稱為IMON，內(nèi)置的限流功能可能并不明顯。示例包括LT8652S和LT8708。限流電路具有折返功能，不會(huì)關(guān)閉輸出。若要關(guān)閉輸出，LTC298x會(huì)檢測(cè)過流狀況并將VOUT_EN拉低，從而禁用穩(wěn)壓器輸出。

Input Current Sensing
輸入電流檢測(cè)
A power system may have a single input supply that powers a number of down-stream regulators. The input supply current may be measured by an LTC2971, LTC2972, or LTC2975. It is straightforward to measure IIN with the LTC2971/ LTC2972/LTC2975, as these devices have native capability to connect pins to a sense resistor RSNS in the current path of VIN. Direct wiring of the IIN_SNS pins is limited to VIN supplies that are <15 V for the LTC2972/LTC2975, and <60 V for the LTC2971.

一個(gè)電源系統(tǒng)可能有一個(gè)輸入電源，其為多個(gè)下游穩(wěn)壓器供電。輸入電源電流可由LTC2971、LTC2972或LTC2975進(jìn)行測(cè)量。使用LTC2971/LTC2972/LTC2975測(cè)量IIN非常簡(jiǎn)單，因?yàn)檫@些器件原本就有將引腳連接到VIN電流路徑中的檢測(cè)電阻RSNS的能力。IIN_SNS引腳的直接接線以VIN電源為限，對(duì)于LTC2972/LTC2975而言，該值<15V；對(duì)于LTC2971而言，該值<60V。

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Figure 10. VIN current and voltage sensing.
圖10.VIN電流和電壓檢測(cè)

Whether measuring output current or input supply current, there is a user-programmable PMBus register that translates the sense voltage to a current. In the case of input supply current, the PMBus register MFR_IIN_CAL_GAIN is used. The input supply current can then be read from the READ_IIN register.

無論是測(cè)量輸出電流還是輸入電源電流，都有一個(gè)用戶可編程PMBus寄存器可將檢測(cè)電壓轉(zhuǎn)換為電流。測(cè)量輸入電源電流時(shí)，使用PMBus寄存器MFR_IIN_CAL_GAIN，然后便可從READ_IIN寄存器讀取輸入電源電流。

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Figure 11. PMBus registers for VIN current measurement.
圖11.用于VIN電流測(cè)量的PMBus寄存器

We can measure not only the current but the voltage as well. The PMBus commands are READ_IIN and READ_VIN. With current, voltage, and a time base, the LTC2971/LTC2972/LTC2975 can also compute power and energy delivered to the system. The energy accumulator is described in the next section.

我們不僅可以測(cè)量電流，還可以測(cè)量電壓。PMBus命令分別為READ_IIN和READ_VIN。利用電流、電壓和時(shí)基，LTC2971/LTC2972/LTC2975還能計(jì)算輸送給系統(tǒng)的功率和電能。蓄能器將在下一節(jié)中描述。

The LTC2971 is capable of sensing input supply current on a 60 V rail. The IIN_SNS pins may be directly connected to a sense resistor on the supply’s input. For supply voltages above 24 V, we recommend using a buck regulator to power the LTC2971 via the VPWR pin. This saves power and avoids self-heating the LTC2971. Power is dissipated due to VPWR × IPWR and can cause the die temperature to increase higher than desired. The ADP2360 has a fixed 5 V option that offers a low cost, small footprint solution for the buck regulator.

LTC2971能夠檢測(cè)60V電源軌上的輸入電源電流。IIN_SNS引腳可以直接連到電源輸入上的檢測(cè)電阻。對(duì)于24V以上的電源電壓，建議使用降壓穩(wěn)壓器通過VPWR引腳為LTC2971供電，這樣可節(jié)省功率并避免LTC2971自發(fā)熱。由于VPWR × IPWR會(huì)產(chǎn)生功耗，可能導(dǎo)致芯片溫度升高到預(yù)期以上。ADP2360具有一個(gè)固定5 V選項(xiàng)，可為降壓穩(wěn)壓器提供低成本、小尺寸的解決方案。

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Figure 12. High voltage VIN current and voltage sensing with the LTC2971.
圖12.使用LTC2971檢測(cè)高壓VIN電流和電壓

Energy Metering
電能計(jì)量
It may be important that the energy usage be monitored. Whether the input supply is a switching regulator, solar panel output, or battery source, it may be useful to know the total energy consumed by the system. The LTC2971/LTC2972/ LTC2975 are capable of high-side current sensing of the input power supply. This feature allows the manager to measure input supply current. LTpowerPlay is very useful for exploring the features related to input supply current and energy reading. Once you select the READ_EIN command, the telemetry window will display a real-time plot of energy accumulated.

監(jiān)視電能使用可能很重要。無論輸入電源是開關(guān)穩(wěn)壓器、太陽能電池板輸出還是電池電源，了解系統(tǒng)消耗的總電能可能很有用。LTC2971/LTC2972/LTC2975能夠檢測(cè)輸入電源的高端電流。此特性允許管理器測(cè)量輸入電源電流。LTpowerPlay對(duì)于探索與輸入電源電流和電能讀數(shù)相關(guān)的特性非常有用。選擇READ_EIN命令后，遙測(cè)窗口就會(huì)顯示電能累計(jì)結(jié)果的實(shí)時(shí)曲線。

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Figure 13. Real-time energy plot from LTpowerPlay.
圖13.LTpowerPlay繪制的實(shí)時(shí)電能圖

The energy meter also measures input supply voltage and is therefore able to report input power as well. Since energy is the product of power and time, accumulated energy is provided based on the manager’s internal time base. The meter displayed in the upper right-hand corner of the GUI provides more information. The needle is a real-time indicator of input power in watts, and the smaller five dials show the total accumulated energy similar to a home electricity meter. Digital readouts are also provided for convenience.

電表還會(huì)測(cè)量輸入電源電壓，因此也能夠報(bào)告輸入功率。由于電能是功率和時(shí)間的乘積，因此累計(jì)電能是根據(jù)管理器的內(nèi)部時(shí)基提供的。GUI右上角顯示的儀表提供了更多信息。指針是輸入功率（以瓦特為單位）的實(shí)時(shí)指示器，較小的五個(gè)刻度盤顯示總累計(jì)電能，類似于家用電表。為方便起見，還提供了數(shù)字讀數(shù)。

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Figure 14. Energy meter in LTpowerPlay.
圖14.LTpowerPlay中的電表

LTpowerPlay offers a simple and easy to understand interface that brings together input and output current, voltage, power, and energy readings.

LTpowerPlay提供一個(gè)簡(jiǎn)單易懂的界面，其中匯集了輸入和輸出電流、電壓、功率、電能讀數(shù)。

Input current, input voltage, input power, and input energy may be viewed in tabular format. These appear in the telemetry portion of the GUI. The MFR_EIN register holds the accumulated energy value in millijoules. There is also a total time that the energy accumulator has been active and is shown as the MFR_EIN_ TIME register. The GUI will automatically update the displayed SI prefix as the units change from mJ to J to kJ.

輸入電流、輸入電壓、輸入功率和輸入電能可以表格形式查看，這些值顯示在GUI的遙測(cè)部分。MFR_EIN寄存器保存累計(jì)電能值（以毫焦耳為單位）。還有一個(gè)電能累計(jì)器處于活動(dòng)狀態(tài)的總時(shí)間，顯示為MFR_EIN_TIME寄存器。當(dāng)單位從mJ變?yōu)镴再到kJ時(shí)，GUI會(huì)自動(dòng)更新所顯示的SI前綴。

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Figure 15. View of telemetry related to input supply voltage, current, power, and energy.
圖15.與輸入電源電壓、電流、功率和電能相關(guān)的遙測(cè)視圖

Table 3 is a summary of all telemetry that can be read back from the LTC297x. The registers are I2C/PMBus word reads except for the MFR_EIN register, which is a block read.

表3總結(jié)了可以從LTC297x回讀的所有遙測(cè)數(shù)據(jù)。寄存器是I2C/PMBus字讀取，但MFR_EIN寄存器除外，它是塊讀取。

Table 3. Summary of Telemetry

表3.遙測(cè)總結(jié)

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1.?? ?If the adc_hires bit is set, READ_VOUT value is returned in mV. L11 format.

如果設(shè)置了adc_hires位，則READ_VOUT值以mV為單位返回。L11格式。

2.?? ?Block read that includes energy value in mJ and elapsed time in ms.

塊讀取，包括以mJ為單位的電能值和以ms為單位的經(jīng)過時(shí)間。

About the Author
作者簡(jiǎn)介
Michael Peters is a senior applications engineer for power system management devices at Analog Devices. He has more than 30 years of experience in analog and digital circuits, including working on memory devices at previous companies. He received his B.S.E.E. degree from the University of Michigan, Ann Arbor, Michigan.?

Michael Peters是ADI公司電源系統(tǒng)管理器件方面的高級(jí)應(yīng)用工程師。他在模擬和數(shù)字電路領(lǐng)域擁有30多年的經(jīng)驗(yàn)，包括在以前的公司從事存儲(chǔ)器件工作的經(jīng)驗(yàn)。他畢業(yè)于密歇根大學(xué)安娜堡分校，獲電氣工程學(xué)士學(xué)位。