Application Report SNVA632A – February 2012 – Revised April 2013 AN-2219 Precision Current Limiting with the LMP8646 and LP38501 ..................................................................................................................................................... ABSTRACT This application note discusses how to design the LMP8646 with the LP38501 voltage regulator and a resistive load application. Contents 1 Introduction .................................................................................................................. 2 List of Figures 1 Resistive Load Application with LP38501 Regulator ................................................................... 2 2 Plot of the Resistive Load Application with LMP8646 and LP38501 ................................................. 3 All trademarks are the property of their respective owners. SNVA632A – February 2012 – Revised April 2013 Submit Documentation Feedback AN-2219 Precision Current Limiting with the LMP8646 and LP38501 Copyright © 2012–2013, Texas Instruments Incorporated 1 Introduction 1 www.ti.com Introduction The LMP8646 is a precision current limiter used to improve the current limit accuracy of any switching or linear regulator with an available feedback node. Many regulators might have an internal current limiter, but its output accuracy is often as high as 30%. The output accuracy of the LMP8646 can be as low as 3%, making it a preferred current limiter for many regulator applications. The design procedures of the LMP8646 with the LP38501 voltage regulator and a resistive load application, as seen in Figure 1, is the focus of this application note. VCLOSE_LOOP = 2V ILIMIT = ICLOSE_LOOP = 1A VIN = 5V OUT IN C1 10 PF R3 51.1 k: RSENSE 58 m: C3 10 PF LP38501 RLOAD 2: 5V EN ENABLE ADJ RG 51.7 k: 0.6V +IN RG -IN LMP8646 V+ V- CV+ 1 PF & 10 PF CG 10 nF GND ROUT 50: RFBB 6.04 k: VOUT RFBT 19.1 k: Figure 1. Resistive Load Application with LP38501 Regulator For this example, we will let the open-loop current to be 1.25A and the close-loop current, ILIMIT, to be 1A. An open-loop occurs when the LMP8646's output is not connected to the LP38501's ADJ pin; whereas, a close-loop scenario is when the LMP8646's output is connected to the LP38051's ADJ pin. Step 1: Choose the components for the Regulator. Refer to the LP38501 evaluation board application note (literature no: SNVA339) to select the appropriate components for the LP38501 voltage regulator. The LP38501 components chosen for this example can be seen in Figure 1. Step 2: Choose the sense resistor, RSENSE RSENSE sets the voltage VSENSE between +IN and -IN and has the following equation: RSENSE = VOUT / [(ILIMIT) * (RG / 5kOhm)] (1) In general, RSENSE depends on the output voltage, limit current, and gain. Refer to LMP866 datasheet, section "Selection of the Sense Resistor, RSENSE", to choose the appropriate RSENSE value. Typically, RSENSE is a power resistor in the mOhm range. In this example, we will use 58 mOhm. Step 3: Choose the gain resistor, RG, for LMP8646 RG is chosen from the limited sense currentm, ILIMIT. As stated, VOUT = (RSENSE * ILIMIT) * (RG / 5kOhm). Since VOUT = VADJ = 0.6V, ILIMIT = 1A, and RSENSE = 58 mOhm, RG can be calculated as: RG = (VOUT * 5 kOhm) / (RSENSE * ILIMIT) RG = (0.6 * 5 kOhm) / (58 mOhm* 1.0A) = 51.7 kOhm 2 AN-2219 Precision Current Limiting with the LMP8646 and LP38501 (2) (3) SNVA632A – February 2012 – Revised April 2013 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated Introduction www.ti.com Step 4: Choose the Bandwidth Capacitance, CG. The product of CG and RG determines the bandwidth for the LMP8646. Refer to the Typical Performance Characteristics plots in the LMP8646 datasheet to see the range for the LMP8646 bandwidth and gain. Since each application is very unique, the LMP8646 bandwidth capacitance, CG, needs to be adjusted to fit the appropriate application. Bench data has been collected for this resistive load application with the LP38501 regulator, and we found that this application works best for a bandwidth of 50 Hz to 300 Hz. Operating anything larger than this recommended bandwidth might prevent the LMP8646 from quickly limiting the current. We recommend choosing a bandwidth that is in the middle of this range and using the equation: CG = 1/(2*pi*RG*Bandwidth) to find CG (this example uses a CG value of 10 nF). After this selection, capture the plot for ILIMIT and adjust CG until a desired current plot is obtained. Step 5: Choose the Output Resistor, ROUT ROUT plays a very small role in the overall system performance for the resistive load application. ROUT is used more for a supercap load because the initial current error is typically large with a capacitive load. Because current is directly proportional to voltage for a resistive load, the output current is not large at startup. The bigger the ROUT, the longer it takes for the output voltage to reach its final value. We recommend that the value for ROUT is at least 50 Ohm, which is the value we used for this example. Step 6: Adjusting the Components Capture the output current and output voltage plots and adjust the components as necessary. The most common component to adjust is CG for the bandwidth. An example plot of the output current and voltage can be seen in Figure 2. 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 0 20 40 CURRENT (A) VOLTAGE (V) Vclose_loop I_limit 60 80 100 120 140 160 TIME (ms) Figure 2. Plot of the Resistive Load Application with LMP8646 and LP38501 SNVA632A – February 2012 – Revised April 2013 Submit Documentation Feedback AN-2219 Precision Current Limiting with the LMP8646 and LP38501 Copyright © 2012–2013, Texas Instruments Incorporated 3 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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