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LMP8646EB User’s Guide

November 2011 Page 1 of 20 SNOU010

Figure 1 – LMP8646EB

The LMP8646 Evaluation Board (LMP8646EB) is designed to ease evaluation and design-in of Texas

Instruments’ LMP8646, a Precision Current Limiter. The LMP8646 is used to detect small differential

voltages across a sense resistor in the presence of high input common mode voltages. On board with the

current limiter is the LM3102, a Step Down Switching Regulator that is capable of supplying 2.5A to

loads. This document describes super cap and resistive load applications utilizing the LM3102 voltage

regulator and the LMP8646 precision current limiter. The document also provides the schematic, layout,

and BOM for the LMP8646 evaluation board.

User’s Guide

LMP8646EB – Nov. 2011 – Revised Nov. 2011

LMP8646EB

Summary of content (21 pages)

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LMP8646EB User’s Guide User’s Guide LMP8646EB – Nov. 2011 – Revised Nov. 2011 LMP8646EB Figure 1 – LMP8646EB The LMP8646 Evaluation Board (LMP8646EB) is designed to ease evaluation and design-in of Texas Instruments’ LMP8646, a Precision Current Limiter. The LMP8646 is used to detect small differential voltages across a sense resistor in the presence of high input common mode voltages. On board with the current limiter is the LM3102, a Step Down Switching Regulator that is capable of supplying 2.
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LMP8646EB User’s Guide Table of Contents 1. EVM OVERVIEW ................................................................................................................................................... 3 1.1. 1.2. 1.3. LIST OF FEATURES .............................................................................................................................................. 3 EQUIPMENT ...................................................................................................................
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LMP8646EB User’s Guide 1. EVM Overview 1.1. List of Features The LMP8646 evaluation board consists of: 1. The LMP8646 precision current limiter and its supporting circuitries 2. Two super capacitors for the load 3. LM3102 voltage regulator and its circuitries 4. Sense resistor of 50 mOhm. 1.2. Equipment 1. 2. 3. 4. 5. LMP8646 evaluation board (NSID: LMP8646EB) 2 Power supplies to source LM3102’s VIN and LMP8646’s V+ Multimeter Oscilloscope Current probe 1.3. Additional Resources 1. 2.
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LMP8646EB User’s Guide 2. Quick Start: Supercap Application A supercap application requires a very high capacitive load to be charged. This example assumes the output capacitor is 3.3F with a limited load current at 1.5A. The LM3102 will provide the current to charge the supercap, and the LMP8646 will monitor this current to make sure it does not exceed the desired 1.5A value. This is done by connecting the LMP8646 output to the feedback pin of the LM3102, as shown in Figure 1.
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LMP8646EB User’s Guide VOUT  RSENSE I LIMIT Gain  RG RG RG   1 / transconduc tan ce 1 / 200A / V 5kOhm V 5kOhm RG  OUT RSENSE I LIMIT where Gain  0.8V 5kOhm 55 mOhm1.5 A RG  RG  50 kOhm Note: Refer to the “Selection of the Sense Resistor, RSENSE” section of the LMP8646 datasheet to select your own RSENSE if 55 mOhm is not desired. 3. Step 3: Choose the Bandwidth Capacitance, CG. The product of CG and RG determines the bandwidth for the LMP8646.
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LMP8646EB User’s Guide a) Using the formula above, calculate for the LMP8646 output accuracy knowing that VSENSE = (1.5A)(55 mOhm), RG = 50 kOhm, VOFFSET = 1 mV, and Gm_Accuracy = 2% (LMP8646 datasheet, electrical characteristics table). 50kOhm  0.825V 1 / 200V 1.5 A55mOhm  100mV 50kOhm  0.852V  1 /200V 1  2 / 100 VOUT _ THEO  1.5 A55mOhm VOUT _ CALC Output Accuracy  b) 0.825  0.852 x100  3.27% 0.
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LMP8646EB User’s Guide I RFBB  I RFBT  I ROUT V  VFB  VO _ 8646  VFB  VFB  O _ 3102 _ min  RFBB RFBT ROUT V  VFB  I max RSENSEGain  VFB  VFB  O _ 3102 _ min  RFBB RFBT ROUT I max RSENSEGain  VFB   VFB  VO _ 3102 _ min  VFB  ROUT RFBB RFBT  I max RSENSE Gain  VFB  ROUT  V  VFB  VFB  O _ 3102 _ min RFBB RFBT 1.57555mOhm 10  0.8 ROUT  0.8 0.6  0.8  2k 10k RFB 3  157.74  160 ohm This equation provides an initial value for ROUT.
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LMP8646EB User’s Guide 3. Quick Start: Resistive Load Application Now, let’s look at the design process for a resistive load application as seen in Figure 4. To see the current limiting capability of the LMP8646, the open-loop current must be greater than the close-loop current. An open-loop occurs when the LMP8646 output is not connected the LM3102’s feedback pin. For this example, we will let the open-loop current to be 1.5A and the close-loop current, ILIMIT, to be 1A.
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LMP8646EB User’s Guide Figure 5 – Recommended Components for the LM3102 3. Step 2: Choose the gain resistor, Rg, for LMP8646 If Rsense = 55 mOhm, then use the equation below to calculate for the appropriate gain resistor, Rg, for a close-loop output current of 1A. VFB  I LIMIT RSENSE Gain LMP 8646  where Gain  RG RG RG   1 / transconduc tan ce 1 / 200A / V 5kOhm  Rg  VFB  I LIMIT RSENSE    5kOhm  VFB 5k  Rg  I LIMIT RSENSE  Rg  0.
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LMP8646EB User’s Guide For example, if Rg = 73 kOhm and the chosen bandwidth is 2 kHz, then the initial Cg value can be calculated as: 1 2 Rg Bandwidth 1 Cg  2 73 kOhm2k Hz  Cg  0.109 nF Cg  Once Cg is chosen, capture the output regulator current plot and adjust Cg to get the desired value. 5. Step 4: Choose the output resistor, ROUT, for LMP8646 For the resistive load application, we found that ROUT plays a very small role in the performance.
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LMP8646EB User’s Guide 4. Powering the LMP8646EB Source V+ with an external voltage between 3.3V and 12V. Do this by connecting the external source to banana connector J2, VP, and J1, GND. To activate the LM3102, source its VIN with external voltage of 4.5V to 42V. This can be done by connecting the external source to J5, VIN, and J8, GND.
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CIN4 NS TP2 GND GND CIN1 10 uF GND CIN2 10 uF CIN3 0.1 uF TP1_LM3102_IN GND GND 1 JP2 EN CSS1 10nF GND 2 GND 1 2 1 GND 1 2 J5 VIN 1 1 1 2 RON1 51.
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LMP8646EB User’s Guide 6.
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LMP8646EB User’s Guide Figure 9 - Layout Top Layer November 2011 Page 14 of 20 SNOU010
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LMP8646EB User’s Guide Figure 10 - Layout Layer #2: Power November 2011 Page 15 of 20 SNOU010
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LMP8646EB User’s Guide Figure 11 - Layout Layer #3: Ground November 2011 Page 16 of 20 SNOU010
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LMP8646EB User’s Guide Figure 12 - Layout Bottom Layer November 2011 Page 17 of 20 SNOU010
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LMP8646EB User’s Guide 7. BOM Item Qnt Reference Value Description Package Manufacturer Manufacturer Part # 1 1 CBST1 33 nF CAP CER 33000PF 25V 10% X7R 0603 603 Murata GRM188R71E333KA01D 2 2 CSS1,CFB1 10nF CAP CER 10000PF 50V 10% X7R 0805 805 Murata GRM216R71H103KA01D 3 1 CFIL1 NS CAP CER .
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LMP8646EB User’s Guide November 2011 Page 19 of 20 SNOU010
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LMP8646EB User’s Guide Evaluation Board/Kit Important Notice Texas Instruments (TI) provides the enclosed product(s) under the following conditions: This evaluation board/kit is intended for use for ENGINEERING DEVELOPMENT, DEMONSTRATION, OR EVALUATION PURPOSES ONLY and is not considered by TI to be a finished end-product fit for general consumer use. Persons handling the product(s) must have electronics training and observe good engineering practice standards.
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IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.