LMP8646 Precision Current Limiter General Description Features 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. The LMP8646 accepts input signals with a common mode voltage ranging from -2V to 76V. It has a variable gain which is used to adjust the sense current. The gain is configured with a single external resistor, RG, providing a high level of flexibility and accuracy up to 2%. The adjustable bandwidth, which allows the device to be used with a variety of applications, is configurable with a single external capacitor in parallel with RG. In addition, the output is buffered in order to provide a low output impedance. The LMP8646 is an ideal choice for industrial, automotive, telecommunications, and consumer applications where circuit protection and improved precision systems are required. The LMP8646 is available in a 6-pin TSOT package and can operate at temperature range of −40°C to 125°C. ■ ■ ■ ■ ■ ■ ■ Applications ■ ■ ■ ■ ■ High-side and low-side current limit Circuit fault protection Battery and supercap charging LED constant current drive Power management Provides circuit protection and current limiting Single supply operation -2V to +76V common mode voltage range Variable gain set by external resistor Adjustable bandwidth set by external capacitor Buffered output 3% output accuracy achievable at VSENSE = 100 mV Key Specifications ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Supply voltage range Output current (source) Gain accuracy Transconductance Offset Quiescent current Input bias PSRR CMRR Temperature range 6-Pin TSOT Package 2.7V to 12V 0 to 5 mA 2.0% (max) 200 µA/V ±1 mV (max) 380 µA 12 µA (typ) 85 dB 95 dB −40°C to 125°C Typical Application 30123534 LMP™ is a trademark of National Semiconductor Corporation. © 2012 Texas Instruments Incorporated 301235 SNOSC63 www.ti.com LMP8646 Precision Current Limiter February 1, 2012 LMP8646 Ordering Information Package Part Number 6-Pin TSOT LMP8646MKX Package Marking LMP8646MK Transport Media NSC Drawing 1k Units Tape and Reel AK7A 3k Units Tape and Reel LMP8646MKE MK06A 250 Units Tape and Reel Connection Diagram 6-Pin TSOT 30123502 Top View Pin Descriptions Pin Name Description 1 VOUT Single-Ended Output Voltage 2 V- Negative Supply Voltage. This pin should be connected to ground. 3 +IN Positive Input 4 -IN Negative Input 5 RG External Gain Resistor. An external capacitance (CG) may be added in parallel with RG to limit the bandwidth. 6 V+ Positive Supply Voltage Block Diagram 30123530 www.ti.com 2 If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. ESD Tolerance (Note 2) Human Body Model For input pins: +IN and -IN For all other pins Machine Model Charge device model Supply Voltage (VS = V+ - V−) Differential voltage +IN- (-IN) Voltage at pins +IN, -IN ±4000V ±2000V 200V 1250 13.2V 6V -6V to 80V 2.7V Electrical Characteristics Operating Ratings V+ (Note 1) V−) Supply Voltage (VS = Temperature Range (Note 3) Package Thermal Resistance(Note 3) TSOT-6 2.7V to 12V -40°C to 125°C 96°C/W (Note 4) Unless otherwise specified, all limits guaranteed for at TA = 25°C, VS=(V+ – V-) = (2.7V - 0V) = 2.7 V, −2V < VCM < 76V, RG= 25kΩ, RL = 10 kΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Condition VOFFSET Input Offset Voltage TCVOS Input Offset Voltage Drift (Note 7, VCM = 2.1V Note 9) IB Input Bias Current (Note 10) VCM = 2.1V 12 eni Input Voltage Noise (Note 9) f > 10 kHz, RG = 5 kΩ 120 VSENSE Max Input Sense Voltage (Note 9) VCM = 12V, RG = 5 kΩ Gain AV Adjustable Gain Setting (Note 9) VCM = 12V Gm Transconductance = 1/RIN VCM = 2.1V Accuracy VCM = 2.1V Gm drift (Note 9) −40°C to 125°C, VCM=2.1V PSRR CMRR SR Power Supply Rejection Ratio Common Mode Rejection Ratio Slew Rate (Note 8, Note 9) VCM = 2.1V Min Typ Max (Note 6) (Note 5) (Note 6) -1 -1.7 1 1.7 mV 7 μV/°C 20 μA nV/ 600 1 100 200 -2 -3.4 VCM = 2.1V, 2.7V < V+ < 12V, 95 -2V <VCM < 2.1V, 55 VCM = 5V, CG = 4 pF, VSENSE from 25 mV mV V/V µA/V 2 3.4 % 140 ppm /°C 85 2.1V < VCM < 76V Units dB dB 0.5 V/µs to 175 mV, CL = 30 pF, RL = 1MΩ IS VOUT Supply Current VCM = 2.1V 380 610 807 VCM = −2V 2000 2500 2700 Maximum Output Voltage VCM = 2.1V, RG = 500 kΩ Minimum Output Voltage VCM = 2.1V 1.1 V 20 Maximum Output Voltage VS = VCM = 3.3V, RG = 500 kΩ Minimum Output Voltage VS = VCM = 3.3V, RG = 500 kΩ IOUT Output current (Note 9) Sourcing, VOUT= 600mV, RG = 150kΩ CLOAD Max Output Capacitance Load (Note 9) 3 uA 1.6 mV V 22 mV 5 mA 30 pF www.ti.com LMP8646 Voltage at RG pin 13.2V Voltage at OUT pin V- to V+ Storage Temperature Range -65°C to 150°C Junction Temperature (Note 3) 150°C For soldering specifications, see product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf Absolute Maximum Ratings (Note 1) LMP8646 5V Electrical Characteristics (Note 4) Unless otherwise specified, all limits guaranteed for at TA = 25°C, VS=V+-V-, V+ = 5V, V− = 0V, −2V < VCM < 76V, Rg= 25kΩ, RL = 10 kΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Condition VCM = 2.1V Min Typ Max (Note 6) (Note 5) (Note 6) VOFFSET Input Offset Voltage -1 -1.7 1 1.7 TCVOS Input Offset Voltage Drift (Note 7, VCM = 2.1V Note 9) IB Input Bias Current (Note 10) VCM = 2.1V 12.5 eni Input Voltage Noise (Note 9) f > 10 kHz, RG = 5 kΩ 120 VSENSE(MAX) Max Input Sense Voltage (Note 9) VCM = 12V, RG = 5 kΩ Gain AV Adjustable Gain Setting (Note 9) VCM = 12V Gm Transconductance = 1/RIN VCM = 2.1V Accuracy VCM = 2.1V Gm drift (Note 9) −40°C to 125°C, VCM= 2.1V PSRR Power Supply Rejection Ratio VCM = 2.1V, 2.7V < V+ < 12V, 85 CMRR Common Mode Rejection Ratio 2.1V <VCM < 76V 95 -2V < VCM < 2.1V 55 μV/°C 22 μA nV/ mV 100 200 -2 -3.4 mV 7 600 1 Units V/V µA/V 2 3.4 % 140 ppm /°C dB dB SR Slew Rate(Note 8, Note 9) VCM = 5V, CG = 4 pF, VSENSE from 100 mV 0.5 V/µs IS Supply Current VCM = 2.1V 450 660 939 VCM = −2V 2100 2800 3030 to 500 mV, CL = 30 pF, RL= 1MΩ VOUT Maximum Output Voltage VCM =5V, RG= 500 kΩ Minimum Output Voltage VCM =2.1V IOUT Output current (Note 9) Sourcing, VOUT= 1.65V, RG = 150kΩ CLOAD Max Output Capacitance Load (Note 9) www.ti.com 3.3 V 22 4 uA mV 5 mA 30 pF (Note 4) Unless otherwise specified, all limits guaranteed for at TA = 25°C, VS=V+-V-, V+ = 12V, V− = 0V, −2V < VCM < 76V, Rg= 25kΩ, RL = 10 kΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Condition VCM = 2.1V Min Typ Max (Note 6) (Note 5) (Note 6) VOFFSET Input Offset Voltage -1 -1.7 1 1.7 TCVOS Input Offset Voltage Drift (Note 7, VCM = 2.1V Note 9) IB Input Bias Current (Note 10) VCM = 2.1V 13 eni Input Voltage Noise (Note 9) f > 10 kHz, RG = 5 kΩ 120 VSENSE(MAX) Max Input Sense Voltage (Note 9) VCM =12V, RG = 5 kΩ Gain AV Adjustable Gain Setting (Note 9) VCM = 12V Gm Transconductance = 1/RIN VCM = 2.1V Accuracy VCM = 2.1V Gm drift (Note 9) −40°C to 125°C, VCM =2.1V PSRR Power Supply Rejection Ratio VCM = 2.1V, 2.7V <V+ < 12V, 85 CMRR Common Mode Rejection Ratio 2.1V <VCM < 76V 95 –2V <VCM < 2.1V 55 μV/°C 23 μA nV/ mV 100 200 -2 -3.4 mV 7 600 1 Units V/V µA/V 2 3.4 % 140 ppm /°C dB dB SR Slew Rate (Note 8, Note 9) VCM = 5V, CG = 4 pF, VSENSE from 100 mV 0.6 V/µs IS Supply Current VCM = 2.1V 555 845 1123 VCM = −2V 2200 2900 3110 to 500 mV, CL = 30 pF, RL=1MΩ VOUT Maximum Output Voltage VCM = 12V, RG= 500kΩ, Minimum Output Voltage VCM = 2.1V IOUT Output current (Note 9) Sourcing, VOUT= 5.25V, RG = 150kΩ CLOAD Max Output Capacitance Load (Note 9) 10 uA V 24 mV 5 mA 30 pF Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics Tables. Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC) FieldInduced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC). Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJ(MAX), θJA, and the ambient temperature, TA. The maximum allowable power dissipation PDMAX = (TJ(MAX) - TA)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. Note 4: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating of the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ > TA. Note 5: Typical values represent the most likely parametric norm at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material. Note 6: All limits are guaranteed by testing, design, or statistical analysis. Note 7: Offset voltage temperature drift is determined by dividing the change in VOS at the temperature extremes by the total temperature change. Note 8: The number specified is the average of rising and falling slew rates and measured at 90% to 10%. Note 9: This parameter is guaranteed by design and/or characterization and is not tested in production. Note 10: Positive Bias Current corresponds to current flowing into the device. 5 www.ti.com LMP8646 12V Electrical Characteristics Unless otherwise specified: TA = 25°C, VS=V+-V-, VSENSE= +IN - Supply Current vs. VCM 2400 3500 2184 3150 1968 2800 1752 2450 -40°C VCM = 2V 25°C 125°C -40°C VCM = -2V 25°C 125°C 1536 1320 1104 888 IS (μA) IS (μA) Supply Curent vs. Supply Voltage for VCM = 2V 3V 5V 12V 2100 1750 1400 1050 672 700 456 350 0 240 3 4 5 6 7 8 9 10 11 12 13 VS (V) -3 -1 1 3 5 7 VCM (V) 9 11 30123562 13 30123564 AC PSRR vs. Frequency AC CMRR vs. Frequency 30123513 30123512 CMRR vs. High VCM -105 -108 -111 Gain vs. Frequency (BW = 1kHz) 25 Vs = 5V Vs = 12V 18 11 -114 4 -117 GAIN (dB) CMRR (dB) LMP8646 Typical Performance Characteristics (-IN), RL = 10 kΩ. -120 -123 -17 -126 -24 -129 -31 -132 -38 -135 -45 40 44 48 52 56 60 64 68 72 76 VCM (V) Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ 10 100 1k 10k 100k FREQUENCY (Hz) 1M 30123536 30123596 www.ti.com -3 -10 6 Gain Accuracy vs. VCM 22 0.240 0.192 GAIN ACCURACY (%) 12 GAIN (dB) LMP8646 Gain vs. Frequency (BW = 35kHz) 2 Vs = 2.7V Vs = 3.3V 0.144 0.096 0.048 0.000 -0.048 -8 -0.096 -18 -28 -0.144 Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ 10 100 -0.192 -0.240 1k 10k 100k FREQUENCY (Hz) 1M -2 6 14 22 30 38 46 54 62 70 78 VCM (V) 30123537 30123578 Gain Accuracy vs. VCM VOUT vs. VSENSE 0.240 4.0 0.144 RG = 10kΩ RG = 25kΩ RG = 50kΩ 3.6 Vs = 5V Vs = 12V 3.2 0.096 2.8 0.048 2.4 VOUT (V) GAIN ACCURACY (%) 0.192 0.000 -0.048 2.0 1.6 -0.096 1.2 -0.144 0.8 -0.192 0.4 -0.240 0.0 -2 8 18 28 38 48 VCM (V) 58 68 78 0.1 0.2 0.3 0.4 VSENSE (V) 0.5 30123579 0.6 30123561 VOUT_MAX vs. Gain at Vs = 2.7V VOUT_MAX vs. Gain at Vs = 5.0V 4.0 1.3 1.2 1.1 1.0 0.9 0.8 0.7 2.8 2.4 2.0 1.6 1.2 0.8 0.6 0.4 0.5 0.4 Vcm = 0V Vcm = 5V, 12V 3.2 VOUT_MAX (V) VOUT_MAX (V) 3.6 Vcm = 0V Vcm = 5V Vcm = 12V 0.0 0 2 4 6 8 GAIN 10 12 14 0 30123573 2 4 6 8 GAIN 10 12 14 30123574 7 www.ti.com LMP8646 VOUT_MAX vs. Gain at Vs = 12V 12 1.80 VCM = 0V VCM = 5V VCM = 12V 10 VOUT_MAX (V) VOUT_MAX vs. VS at VCM = -2V 1.74 1.68 VOUT_MAX (V) 8 6 4 1.62 1.56 1.50 1.44 1.38 1.32 2 1.26 0 0 2 4 6 8 GAIN 10 12 1.20 14 0 2 4 6 8 VS (V) 10 12 30123575 14 30123576 VOUT_MAX vs. VS at VCM = 2.1V Large Step Response at BW = 1kHz 2.1 VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ VSENSE (100 mV/DIV) 2.0 VOUT_MAX (V) 1.9 1.8 1.7 1.6 1.5 1.4 VOUT (300 mV/DIV) 2.2 1.3 1.2 0 2 4 6 8 VS (V) 10 12 14 TIME (0.5 ms/DIV) 30123543 30123577 Large Step Response at BW = 35 kHz TIME (20 μs/DIV) TIME (500 μs/DIV) 30123544 www.ti.com VOUT (30 mV/DIV) VSENSE (10 mV/DIV) VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ VOUT (300 mV/DIV) VSENSE (100 mV/DIV) VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ Small Step Response at BW = 1 kHz 30123545 8 Settling Time (Rise) at 1kHz VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ VOUT (30 mV/DIV) VSENSE (10 mV/DIV) VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ VOUT (30 mV/DIV) VSENSE (10 mV/DIV) LMP8646 Small Step Response at BW = 35 kHz TIME (20 μs/DIV) TIME (100 μs/DIV) 30123546 30123547 Settling Time (Fall) at 1kHz Settling Time (Rise) at 35kHz VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ VOUT (30 mV/DIV) VSENSE (10 mV/DIV) VOUT (30 mV/DIV) VSENSE (10 mV/DIV) VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ TIME (100 μs/DIV) TIME (5 μs/DIV) 30123548 30123549 Settling Time (Fall) at 35kHz Common Mode Step Response (Rise) at 35 kHz VOUT (500 mV/DIV) VCM (5 V/DIV) VSENSE Rg = 50kΩ Rg = 25kΩ Rg = 10kΩ VOUT (200 mV/DIV) VSENSE (10 mV/DIV) VOUT VCM TIME (5 μs/DIV) TIME (0.2 ms/DIV) 30123550 30123551 9 www.ti.com LMP8646 Common Mode Step Response (Fall) at 35 kHz VCM (5 V/DIV) VOUT (500 mV/DIV) VOUT VCM TIME (0.2 ms/DIV) 30123552 www.ti.com 10 MAXIMUM OUTPUT VOLTAGE, VOUT_MAX The maximum output voltage, VOUT_MAX, depends on the supply voltage, V S = V + - V -, and on the common mode voltage, VCM = (+IN + -IN) / 2. The following subsections show three cases to calculate for VOUT_MAX. GENERAL The LMP8646 is a single supply precision current limiter with variable gain selected through an external resistor (RG) and a variable bandwidth selected through an external capacitor (CG) in parallel with RG. Its common-mode of operation is -2V to +76V, and the LMP8646 has an buffered output to provide a low output impedance. More details of the LMP8646's functional description can be seen in the following subsections. Case 1: −2V < VCM < 1.8V, and VS > 2.7V If VS ≥ 5 V, then VOUT_MAX = 1.3V. Else if Vs = 2.7V, then VOUT_MAX = 1.1V. THEORY OF OPERATION As seen from Figure 1, the sense current flowing through RSENSE develops a voltage drop equal to VSENSE. The high impedance inputs of the amplifier does not conduct this current and the high open loop gain of the sense amplifier forces its non-inverting input to the same voltage as the inverting input. In this way the voltage drop across RIN matches VSENSE. The current IIN flowing through RIN has the following equation: Case 2: 1.8V < VCM < VS, and VS > 3.3V In this case, VX is a fixed value that depends on the supply voltage. VX has the following values: If VS = 12V, then VX = 10V. Else if VS = 5V, then VX = 3.3V . Else if VS = 2.7V, then VX = 1.1V. If VX ≤ (VCM - VSENSE - 0.25) , then VOUT_MAX = VX. Else, VOUT_MAX = (VCM - VSENSE - 0.25). IIN = VSENSE/ RIN = RSENSE*ISENSE/RIN where RIN = 1/Gm = 1/(200 µA/V) = 5 kOhm IIN flows entirely across the external gain resistor RG to develop a voltage drop equal to: VRG = IIN*RG = (VSENSE/RIN) *RG = [(RSENSE*ISENSE) / RIN]*RG For example, if VCM = 4V, VS = 5V (and thus VX = 3.3V), VSENSE = 0.1 V, then VOUT_MAX = 3.3V because 3.3V ≤ (4 0.1 - 0.25). This voltage is buffered and showed at the output with a very low impedance allowing a very easy interface of the LMP8646 with the feedback of many voltage regulators. This output voltage has the following equation: Case 3: VCM > VS, and VS > 2.7V If VS = 12V, then VOUT_MAX = 10V. Else if VS = 5V, then VOUT_MAX = 3.3V . Else if VS = 2.7V, then VOUT_MAX = 1.1V. VOUT = VRG = [(RSENSE*ISENSE) / RIN]*RG VOUT = VSENSE* RG/RIN VOUT = VSENSE* RG/(5 kOhm) VOUT = VSENSE* Gain, where Gain = RG/RIN 30123503 FIGURE 1. Current monitor 11 www.ti.com LMP8646 FUNCTIONAL DESCRIPTION SELECTION OF THE SENSE RESISTOR, RSENSE The accuracy of the current measurement also depends on the value of the shunt resistor RSENSE. Its value depends on the application and is a compromise between small-signal accuracy and maximum permissible voltage loss in the load line. RSENSE is directly proportional to VSENSE through the equation RSENSE = (V SENSE) / (I SENSE). If V SENSE is small, then there is a smaller voltage loss in the load line, but the output accuracy is worse because the LMP8646 offset error will contribute more. Therefore, high values of RSENSE provide better output accuracy by minimizing the effects of offset, while low values of RSENSE minimize the voltage loss in the load line. For most applications, best performance is obtained with an RSENSE value that provides a VSENSE of 100 mV to 200 mV. OUTPUT ACCURACY The output accuracy is the device error contributed by the LMP8646 based on its offset and gain errors. The LMP8646 output accuracy has the following equations: 30123538 RSENSE Consideration for System Error The output accuracy described in the previous section talks about the error contributed just by the LMP8646. The system error, however, consists of the errors contributed by the LMP8646 as well as other external resistors such as RSENSE and RG. Let's rewrite the output accuracy equation for the system error assuming that RSENSE is non-ideal and RG is ideal. This equation can be seen as: FIGURE 2. Output Accuracy Equations For example, assume VSENSE = 100 mV, RG = 10 kOhm, and it is known that VOFFSET = 1 mV and Gm_Accuracy = 2% (Electrical Characteristics Table), then the output accuracy can be calculated as: 30123554 30123539 FIGURE 3. Output Accuracy Example FIGURE 5. System Error Equation Assuming RSENSE is Non-ideal and RG is Ideal In fact, as VSENSE decreases, the output accuracy worsens as seen in Figure 4. These equations provide a valuable tool to estimate how the LMP8646 affects the overall system performance. Knowing this information allows the system designer to pick the appropriate external resistances (RSENSE and RG) to adjust for the tolerable system error. Examples of this tolerable system error can be seen in the next sections. Continuing from the previous output accuracy example, we can calculate for the system error assuming that RSENSE = 100 mOhm (with 1% tolerance), ISENSE = 1A, and RG = 10 kOhm. From the Electrical Characteristics Table, it is also known that VOFFSET = 1 mV and Gm_Accuracy = 2%. 10.0 9.2 OUTPUT ACCURACY (%) LMP8646 APPLICATIONS INFORMATION 8.4 7.6 6.8 30123555 6.0 5.2 FIGURE 6. System Error Example Assuming RSENSE is Non-ideal and RG is Ideal 4.4 3.6 Because an RSENSE tolerance will increase the system error, we recommend selecting an RSENSE resistor with low tolerance. 2.8 2.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 VSENSE (V) 30123570 FIGURE 4. Output Accuracy vs. VSENSE www.ti.com 12 LMP8646 SELECTION OF THE GAIN RESISTOR, RG For the LMP8646, the gain is selected through an external resistor connected to the RG pin. The voltage at this RG pin is equal to VOUT, which has the equation VOUT = VRG = VSENSE* RG/(5 kOhm). In fact, RG must be chosen such that the VOUT does not exceed its maximum ratings (VOUT_MAX) as described in the MAXIMUM OUTPUT VOLTAGE, VOUT_MAX section. Using this VOUT_MAX and the equation RG_MAX = (VOUT_MAX * 5kOhm) / (V SENSE), a plot of RG_MAX vs. V SENSE can be seen for three cases below. Use these plots to help select the appropriate RG value so that VSENSE and VOUT stay within the recommended operating ratings. Since these plots are for RG_MAX, all of the combinations of RG below the curve are allowed. Case 3: VCM > VS, and VS > 3.3V 500 VS = 3.3V VS = 5.0V VS = 12.0V RG_MAX (kΩ) 400 300 200 100 0 Case 1: −2V < VCM < 1.8V, and VS > 3.3V 0.0 0.1 0.2 0.3 VSENSE (V) 0.4 500 30123560 VS = 3.3V VS = 5.0V or 12.0V 400 RG_MAX (kΩ) 0.5 FIGURE 9. Allowed RG for CASE 3 300 RG Consideration for System Error The previous section discussed the system error assuming that RSENSE is non-ideal and RG is ideal. This section expands the system error equation by assuming that both RSENSE and RG are non-ideal. This system error equation can be rewritten as: 200 100 0 0.0 0.1 0.2 0.3 VSENSE (V) 0.4 0.5 30123558 FIGURE 7. Allowed RG for CASE 1 30123556 Case 2: 1.8V < VCM < VS, and VS > 3.3V 500 VS = 3.3V @ VCM = 2V VS = 5.0V @ VCM = 2.5V VS = 12V @ VCM = 6V 400 RG_MAX (kΩ) FIGURE 10. System Error Equation Assuming RSENSE and RG are Non-ideal Continuing from the previous system error equation, we can recalculate for the system error assuming that RG has a 1% tolerance. 300 200 100 30123557 0 0.0 0.1 0.2 0.3 VSENSE (V) 0.4 FIGURE 11. System Error Example Assuming RSENSE and RG are Non-ideal 0.5 30123559 Because an RG tolerance will increase the system error, we recommend selecting an RG resistor with low tolerance. FIGURE 8. Allowed RG for CASE 2 13 www.ti.com LMP8646 APPLICATION #1: CURRENT LIMITER WITH A CAPACITIVE LOAD 30123531 FIGURE 12. SuperCap Application with LM3102 Regulator Step 3: Choose the gain resistor, RG, for LMP8646 RG is chosen from the limited sense current. As stated, VOUT = (RSENSE * ILIMIT) * (RG / 5kOhm). Since VOUT = VFB = 0.8V, the limited sense current is 1.5A, and RSENSE is 55 mOhm, RG can be calculated as: A supercap application requires a very high capacitive load to be charged. This example assumes the output capacitor is 5F with a limited sense 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 12. This feedback voltage at the FB pin is compared to a 0.8V internal reference. Any voltage above this 0.8V means the output current is above the desired value of 1.5A, and the LM3102 will reduce its output current to maintain the desired 0.8V at the FB pin. The following steps show the design procedures for this supercap application. In summary, the steps consist of selecting the components for the voltage regulator, integrating the LMP8646 and selecting the proper values for its gain, bandwidth, and output resistor, and adjusting these components to yield the desired performance. RG = (VOUT * 5 kOhm) / (RSENSE * ILIMIT) RG = (0.8 * 5 kOhm) / (55 mOhm* 1.5A) = 50 kOhm (approximate) 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 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 the supercap application with the LM3102 regulator, and we found that this application works best for a bandwidth of 500 Hz to 3 kHz. Operating outside of this recommended bandwidth range might create an undesirable load current ringing. 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. For example, if the bandwidth is 1.75 kHz and RG is 50 kOhm, then CG is approximately 1.8 nF. After this selection, capture the plot for lLIMIT and adjust CG until a desired load current plot is obtained. Step 1: Choose the components for the Regulator. Refer to the LM3102 evaluation board application note (AN-1646) to select the appropriate components for the LM3102 voltage regulator. Step 2: Choose the sense resistor, RSENSE RSENSE sets the voltage VSENSE between +IN and -IN and has the following equation: Step 5: Calculate the Output Accuracy and Tolerable System Error Since the LMP8646 is a precision current limiter, the output current accuracy is extremely important. This accuracy is affected by the system error contributed by the LMP8646 device RSENSE = VOUT / [(ILIMIT) * (RG / 5kOhm)] In general, RSENSE depends on the output voltage, limit current, and gain. Refer to section SELECTION OF THE SENSE RESISTOR, RSENSE to choose the appropriate RSENSE value; this example uses 55 mOhm. www.ti.com 14 Next, use the formula below to calculate for ROUT: 30123533 FIGURE 13. ROUT Equation For example, assume the minimum LM3102 output voltage, VO_REG_MIN, is 0.6V, then ROUT can be calculated as ROUT = [1.575A * 55 mOhm * (49.9k / 5k) - 0.8] / [ (0.8 / 2k) - (0.6 0.8) / 10k] = 153.6 Ohm. Populate ROUT with a resistor that is as close as possible to 153.6 Ohm (this application uses 160 Ohm). If the limited sense current has a gain error and is not 1.5A at any point in time, then adjust this ROUT value to obtain the desired limit current. We recommend that the value for ROUT is at least 50 Ohm. Step 6: Choose the output resistor, ROUT At startup, the capacitor is not charged yet and thus the output voltage of the LM3102 is very small. Therefore, at startup, the output current is at its maximum (IMAX). When the output voltage is at its nominal, then the output current will settle to the desired limited value. Because a large current error is not desired, ROUT needs to be chosen to stabilize the loop with minimal initial startup current error. Follow the equations and example below to choose the appropriate value for ROUT to minimize this initial error. As discussed in step 4, the allowable IERROR is 5%, where IERROR = (IMAX - ILIMIT)/IMAX (%). Therefore, the maximum allowable current is calculated as: IMAX = ILIMIT (1+ IERROR) = 1.5A * (1 + 5/100) = 1.575 A. Step 7: Adjusting Components Capture the output current and output voltage plots and adjust the components as necessary. The most common components to adjust are CG to decrease the current ripple and ROUT to get a low current error. An example output current and voltage plot can be seen in Figure 14 . 5 5 Vo_load I_limit 4 3 3 I_max I_limit 2 2 1 1 Vo_reg_min 0 -10 0 10 20 TIME (s) CURRENT (A) 4 VOLTAGE (V) LMP8646 error and other errors contributed by external resistances, such as RSENSE and RG. In this application, VSENSE = ILIMIT * RSENSE = 1.5A * 55 mOhm = 0.0825V, and RG = 50 kOhm. From the Electrical Characteristics Table, it is known that VOFFSET = 1 mV and Gm_Accuracy = 2%. Using the equations shown in Figure 2, the output accuracy can be calculated as 3.24%. After figuring out the LMP8646 output accuracy, choose a tolerable system error or the output current accuracy that is bigger than the LMP8646 output accuracy. This tolerable system error will be labeled as IERROR, and it has the equation IERROR = (IMAX - ILIMIT)/IMAX (%). In this example, we will choose an IERROR of 5%, which will be used to calculate for ROUT shown in the next step. 0 30 40 30123540 FIGURE 14. SuperCap Application with LM3102 Regulator Plot 15 www.ti.com LMP8646 APPLICATION #2: CURRENT LIMITER WITH A RESISTIVE LOAD 30123532 FIGURE 15. Resistive Load Application with LMZ12003 Regulator This subsection describes the design process for a resistive load application with the LMZ12003 voltage regulator as seen in Figure 15. 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 LMZ12003’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. 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 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 LMZ12003 regulator, and we found that this application works best for a bandwidth of 2 kHz to 30 kHz. Operating anything less 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 0.1nF). After this selection, capture the load current plot and adjust CG until a desired output current plot is obtained. Step 1: Choose the components for the Regulator. Refer to the LMZ12003 application note (AN-2031) to select the appropriate components for the LMZ12003. 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)] Step 5: Choose the output resistor, ROUT, for the LMP8646 ROUT plays a very small role in the overall system performance for the resistive load application. ROUT was important in the supercap application because it affects the initial current error. 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 chosen value for this example. In general, RSENSE depends on the output voltage, limit current, and gain. Refer to section SELECTION OF THE SENSE RESISTOR, RSENSE to choose the appropriate RSENSE value; this example uses 50 mOhm. Step 3: Choose the gain resistor, RG, for LMP8646 RG is chosen from ILIMIT. As stated, VOUT = (RSENSE * ILIMIT) * (RG / 5kOhm). Since VOUT = VFB = 0.8V, ILIMIT = 1A, and RSENSE = 50 mOhm , RG can be calculated as: RG = (VOUT * 5 kOhm) / (RSENSE * ILIMIT) RG = (0.8 * 5 kOhm) / (50 mOhm* 1A) = 80 kOhm Step 6: Adjusting Components Capture the output current and output voltage plots and adjust the components as necessary. The most common compowww.ti.com 16 LMP8646 nent to adjust is CG for the bandwidth. An example of the output current and voltage plot can be seen in Figure 16. 1.98 VOLTAGE (V) 1.76 2.0 I_limit Vclose_loop 1.8 1.6 1.54 1.4 1.32 1.2 1.10 1.0 0.88 0.8 0.66 0.6 0.44 0.4 0.22 0.2 0.00 0.0 CURRENT (A) 2.20 -0.030 -0.018 -0.006 0.006 0.018 0.030 TIME (s) 30123541 FIGURE 16. Plot for the Resistive Load Application with LMZ12003 Regulator Plot 17 www.ti.com LMP8646 APPLICATION #3: CURRENT LIMITER WITH A LOW-DROPOUT REGULATOR AND RESISTIVE LOAD 30123535 FIGURE 17. Resistive Load Application with LP38501 Regulator This next example is the same as the last example, except that the regulator is now a low-dropout regulator, the LP38501, as seen in Figure 17. For this example, we will let the open-loop current to be 1.25A and the close-loop current, ILIMIT, to be 1A. 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 ISENSE and adjust CG until a desired sense current plot is obtained. Step 1: Choose the components for the Regulator. Refer to the LP38501 application note (AN-1830) to select the appropriate components for the LP38501. 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)] Step 5: Choose the output resistor, ROUT, for the LMP8646 ROUT plays a very small role in the overall system performance for the resistive load application. ROUT was important in the supercap application because it affects the initial current error. 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. In general, RSENSE depends on the output voltage, limit current, and gain. Refer to section SELECTION OF THE SENSE RESISTOR, RSENSE to choose the appropriate RSENSE value; this example uses 58 mOhm. Step 3: Choose the gain resistor, RG, for LMP8646 RG is chosen from ILIMIT. As stated, VOUT = (RSENSE * ILIMIT) * (RG / 5kOhm). Since VOUT = ADJ = 0.6V, ILIMIT = 1A, and RSENSE = 58 mOhm , RG can be calculated as: RG = (VOUT * 5 kOhm) / (RSENSE * ILIMIT) Step 6: Adjusting 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 18. RG = (0.6 * 5 kOhm) / (58 mOhm* 1A) = 51.7 kOhm 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 to see the range for the LMP8646 bandwidth and gain. www.ti.com 18 2.5 Vclose_loop I_limit 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 CURRENT (A) VOLTAGE (V) LMP8646 2.5 -10 10 30 50 70 90 110 130 150 170 TIME (ms) 30123542 FIGURE 18. Plot for the Resistive Load Application with the LP38501 LDO Regulator 19 www.ti.com LMP8646 Physical Dimensions inches (millimeters) unless otherwise noted TSOT-6 NS Package Number MK06A www.ti.com 20 LMP8646 Notes 21 www.ti.com LMP8646 Precision Current Limiter Notes www.ti.com 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. 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