LP2957ISX

LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 LP2957/LP2957A 5V Low-Dropout Regulator for µP Applications Check for Samples: LP2957, LP2957A FEATURES DESCRIPTION • The LP2957 is a 5V micropower voltage regulator with electronic shutdown, error flag, very low quiescent current (150 µA typical at 1 mA load), and very low dropout voltage (470 mV typical at 250 mA load current). 1 2 • • • • • • • • • 5V Output within 1.4% Over Temperature (A Grade) Easily Programmed for Snap-On/Snap-Off Output Ensured 250 mA Output Current Extremely Low Quiescent Current Low Input-Output Voltage Required for Regulation Reverse Battery Protection Extremely Tight Line and Load Regulation Very Low Temperature Coefficient Current and Thermal Limiting Error Flag Signals when Output is out of Regulation APPLICATIONS • • High-Efficiency Linear Regulator Battery-Powered Regulator Output can be wired for snap-on/snap-off operation to eliminate transition voltage states where µP operation may be unpredictable. Output crowbar (50 mA typical pull-down current) will bring down the output quickly when the regulator snaps off or when the shutdown function is activated. The part has tight line and load regulation (0.04% typical) and low output temperature coefficient (20 ppm/°C typical). The accuracy of the 5V output is ensured at room temperature and over the full operating temperature range. The LP2957 is available in the five-lead TO-220 and DDPAK/TO-263 packages. Package Outline Top View Top View Figure 1. Bent, Staggered Leads 5-Lead TO-220 Package See Package Number NDH0005D Figure 2. Plastic Surface Mount Package 5-Lead DDPAK/TO-263 Package See Package Number KTT0005B Side View Figure 3. See Package Number KTT0005B These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. 1 2 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of the Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. Copyright © 1998–2013, Texas Instruments Incorporated LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com Absolute Maximum Ratings (1) (2) Operating Junction −40°C to +125°C Temperature Range −65°C to +150°C Storage Temperature Range Lead Temperature (Soldering, 5 Seconds) 260°C Power Dissipation (3) Internally Limited Input Supply Voltage −20V to +30V Shutdown Input −0.3V to +30V ESD Rating (1) 2 kV Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of its rated operating conditions. If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and specifications. The maximum allowable power dissipation is a function of the maximum junction temperature, T J(MAX), the junction-to-ambient thermal resistance, θ JA, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated (2) (3) using: Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. The junction-to-ambient thermal resistance of the TO-220 (without heatsink) is 60°C/W and 73°C/W for the DDPAK/TO-263. If the DDPAK/TO-263 package is used, the thermal resistance can be reduced by increasing the P.C. board copper area thermally connected to the package: Using 0.5 Square inches of copper area, θ JA is 50°C/W, with 1 square inch of copper area, θJA is 37°C/W; and with 1.6 or more square inches of copper area, θ JA is 32°C/W. The junction-to-case thermal resistance is 3°C/W. If an external heatsink is used, the effective junction-to-ambient thermal resistance is the sum of the junction-to-case resistance (3°C/W), the specified thermal resistance of the heatsink selected, and the thermal resistance of the interface between the heatsink and the LP2957 (see Application Hints). Electrical Characteristics Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF, VSD = 3V. Symbol Parameter Conditions Typical VO 5.0 Output Voltage (1) Output Voltage Temperature Coefficient Line Regulation Load Regulation VIN–VO (2) (3) (4) 2 Min Max Min Max 4.975 5.025 4.950 5.050 4.940 5.060 4.900 5.100 4.930 5.070 4.880 5.120 5.0 See (2) 20 100 150 0.03 0.10 0.20 0.20 0.40 0.16 0.20 0.20 0.30 100 100 150 150 300 300 420 420 400 400 520 520 600 600 800 800 VIN = 6V to 30V IL = 1 mA to 250 mA IL = 0.1 mA to 1 mA IL = 50 mA (1) LP2957I 1 mA ≤ IL ≤ 250 mA IL = 1 mA Dropout Voltage (4) LP2957AI (3) 0.04 60 240 IL = 100 mA 310 IL = 250 mA 470 Units V ppm/°C % % mV When used in dual-supply systems where the regulator load is returned to a negative supply, the output voltage must be diode-clamped to ground. Output voltage temperature coefficient is defined as the worst case voltage change divided by the total temperature range. Regulation is measured at constant junction temperature using low duty cycle pulse testing. Parts are tested separately for load regulation in the load ranges 0.1 mA–1 mA and 1 mA–250 mA. Changes in output voltage due to heating effects are covered by the thermal regulation specification. Dropout voltage is defined as the input to output voltage differential at which the output voltage drops 100 mV below the value measured with a 1V input to output differential. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 Electrical Characteristics (continued) Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF, VSD = 3V. Symbol Parameter IGND Conditions IL = 1 mA IL = 50 mA Ground Pin Current (5) IGND IGND IO (Sink) IO (Off) ILIMIT LP2957AI Min 150 1.1 3 Max LP2957I Min Max 200 200 230 230 2 2 2.5 2.5 6 6 8 8 28 IL = 250 mA 16 28 33 33 Ground Pin Current in Shutdown (5) IL = 0 130 180 180 200 200 Ground Pin Current at Dropout (5) VIN = 4.5V 230 230 250 250 Off-State Output Pulldown Current VIN = 5.3V Output Leakage in Shutdown I(SD IN) ≥ 1 µA Current Limit Thermal Regulation en IL = 100 mA Typical Output Noise Voltage (10 Hz to 100 kHz) IL = 100 mA VSD = 0.4V 180 IL = 0.1 mA 50 VO = 5V, VSD = 0.4V 30 30 20 20 3 VIN = 30V, VOUT = 0V RL = 1Ω See (6) 400 0.05 CL = 2.2 µF 500 CL = 33 µF 320 Units µA mA µA µA mA 10 10 20 20 500 500 530 530 0.2 0.2 µA mA %/W µV RMS SHUTDOWN INPUT VSD (ON) HYST IB Output Turn-On Threshold Voltage Hysteresis Input Bias Current 1.155 1.305 1.155 1.305 1.140 1.320 1.140 1.320 6 VIN(SD) = 0V to 5V 10 V mV −30 30 −30 30 −50 50 −50 50 nA DROPOUT DETECTION COMPARATOR IOH Output “HIGH” Leakage VOL Output “LOW” Voltage VTHR (Max) VTHR (Min) HYST (5) (6) (7) Upper Threshold Voltage Lower Threshold Voltage Hysteresis VOH = 30V VIN = 4V 0.01 150 IO(COMP) = 400 µA See (7) See (7) See (7) −240 −350 1 1 2 2 250 250 400 400 −320 −150 −320 −150 −380 −100 −380 −100 −450 −230 −450 −230 −640 −160 −640 −160 60 µA mV mV mV mV Ground pin current is the regulator quiescent current. The total current drawn from the source is the sum of the load current plus the ground pin current. Thermal regulation is defined as the change in output voltage at a time T after a change in power dissipation is applied, excluding load or line regulation effects. Specifications are for a 200 mA load pulse at VIN = 20V (3W pulse) for T = 10 ms. Voltages are referenced to the nominal regulated output voltage. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 3 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com Typical Performance Characteristics Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF, V SD = 3V, TA = 25°C 4 Ground Pin Current Ground Pin Current Figure 4. Figure 5. Ground Pin Current vs Load Ground Pin Current Figure 6. Figure 7. Ground Pin Current Output Noise Voltage Figure 8. Figure 9. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF, V SD = 3V, TA = 25°C Ripple Rejection Ripple Rejection Figure 10. Figure 11. Ripple Rejection Line Transient Response Figure 12. Figure 13. Line Transient Response Output Impedance Figure 14. Figure 15. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 5 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF, V SD = 3V, TA = 25°C 6 Load Transient Response Load Transient Response Figure 16. Figure 17. Dropout Characteristics Enable Transient Figure 18. Figure 19. Enable Transient Short-Circuit Output Current and Maximum Output Current Figure 20. Figure 21. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 6V, IL = 1 mA, CL = 2.2 µF, V SD = 3V, TA = 25°C (1) Thermal Regulation Error Output Sink Current Figure 22. Figure 23. Dropout Detection Threshold Voltages Maximum Power Dissipation (DDPAK/TO-263) (1) Figure 24. Figure 25. Error Output Voltage Dropout Voltage Figure 26. Figure 27. The maximum allowable power dissipation is a function of the maximum junction temperature, T J(MAX), the junction-to-ambient thermal resistance, θ JA, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated using: Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. The junction-to-ambient thermal resistance of the TO-220 (without heatsink) is 60°C/W and 73°C/W for the DDPAK/TO-263. If the DDPAK/TO-263 package is used, the thermal resistance can be reduced by increasing the P.C. board copper area thermally connected to the package: Using 0.5 Square inches of copper area, θ JA is 50°C/W, with 1 square inch of copper area, θJA is 37°C/W; and with 1.6 or more square inches of copper area, θ JA is 32°C/W. The junction-to-case thermal resistance is 3°C/W. If an external heatsink is used, the effective junction-to-ambient thermal resistance is the sum of the junction-to-case resistance (3°C/W), the specified thermal resistance of the heatsink selected, and the thermal resistance of the interface between the heatsink and the LP2957 (see Application Hints). Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 7 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com Block Diagram Typical Application Circuits Figure 28. LP2957 Basic Application *See Application Hints Figure 29. LP2957 Application with Snap-On/Snap-Off Output 8 Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 APPLICATION HINTS EXTERNAL CAPACITORS A 2.2 µF (or greater) capacitor is required between the output pin and ground to assure stability (refer to Figure 30). Without this capacitor, the part may oscillate. Most type of tantalum or aluminum electrolytics will work here. Film types will work, but are more expensive. Many aluminum electrolytics contain electrolytes which freeze at −30°C, which requires the use of solid tantalums below −25°C. The important parameters of the capacitor are an ESR of about 5Ω or less and a resonant frequency above 500 kHz (the ESR may increase by a factor of 20 or 30 as the temperature is reduced from 25°C to −30°C). The value of this capacitor may be increased without limit. At lower values of output current, less output capacitance is required for stability. The capacitor can be reduced to 0.68 µF for currents below 10 mA or 0.22 µF for currents below 1 mA. A 1 µF capacitor should be placed from the input pin to ground if there is more than 10 inches of wire between the input and the AC filter capacitor or if a battery input is used. This capacitor may have to be increased if the regulator is wired for snap-on/snap-off output and the source impedance is high (see Snap-On/Snap-Off Operation section). SHUTDOWN INPUT A logic-level signal will shut off the regulator output when a “LOW” (< 1.2V) is applied to the Shutdown input. To prevent possible mis-operation, the Shutdown input must be actively terminated. If the input is driven from open-collector logic, a pull-up resistor (20 kΩ to 100 kΩ recommended) must be connected from the Shutdown input to the regulator input. If the Shutdown input is driven from a source that actively pulls high and low (like an op-amp), the pull-up resistor is not required, but may be used. If the shutdown function is not to be used, the cost of the pull-up resistor can be saved by tying the Shutdown input directly to the regulator input. IMPORTANT: Since the Absolute Maximum Ratings state that the Shutdown input can not go more than 0.3V below ground, the reverse-battery protection feature which protects the regulator input is sacrificed if the Shutdown input is tied directly to the regulator input. If reverse-battery protection is required in an application, the pull-up resistor between the Shutdown input and the regulator input must be used. MINIMUM LOAD It should be noted that a minimum load current is specified in several of the electrical characteristic test conditions, so the value listed must be used to obtain correlation on these tested limits. The part is parametrically tested down to 100 µA, but is functional with no load. DROPOUT VOLTAGE The dropout voltage of the regulator is defined as the minimum input-to-output voltage differential required for the output voltage to stay within 100 mV of the output voltage measured with a 1V differential. The dropout voltages for various values of load current are listed under Electrical Characteristics. If the regulator is powered from a transformer connected to the AC line, the minimum AC line voltage and maximum load current must be used to measure the minimum voltage at the input of the regulator. The minimum input voltage is the lowest voltage level including ripple on the filter capacitor . It is also advisable to verify operation at minimum operating ambient temperature , since the increasing ESR of the filter capacitor makes this a worst-case test due to increased ripple amplitude. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 9 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com HEATSINK REQUIREMENTS A heatsink may be required with the LP2957 depending on the maximum power dissipation and maximum ambient temperature of the application. Under all possible operating conditions, the junction temperature must be within the range specified under Absolute Maximum Ratings. To determine if a heatsink is required, the maximum power dissipated by the regulator, P(max), must be calculated. It is important to remember that if the regulator is powered from a transformer connected to the AC line, the maximum specified AC input voltage must be used (since this produces the maximum DC input voltage to the regulator), and the maximum load current must also be used. Figure 30 shows the voltages and currents which are present in the circuit. The formula for calculating the power dissipated in the regulator is also shown in Figure 30. PTOTAL = (VIN − 5)I L + (VIN)IG *See EXTERNAL CAPACITORS Figure 30. Basic 5V Regulator Circuit The next parameter which must be calculated is the maximum allowable temperature rise, TR(Max). This is calculated by using the formula: TR(Max) = TJ(Max) − T A(Max) where • • TJ(Max) is the maximum allowable junction temperature TA(Max) is the maximum ambient temperature (1) Using the calculated values for TR(Max) and P(Max), the required value for junction-to-ambient thermal resistance, θ (JA), can now be found: θ(JA) = TR(Max)/P(Max) (2) If the calculated value is 60°C/W or higher , the regulator may be operated without an external heatsink. If the calculated value is below 60°C/W, an external heatsink is required. The required thermal resistance for this heatsink, θ(HA), can be calculated using the formula: θ(HA) = θ(JA) − θ (JC) − θ(CH) where • • θ(JC) is the junction-to-case thermal resistance, which is specified as 3°C/W for the LP2957 θ(CH) is the case-to-heatsink thermal resistance, which is dependent on the interfacing material (see Table 1 and Table 2) (3) Typical TO-220 Case-To-Heatsink Thermal Resistance in °C/W Table 1. (From AAVID) 10 Silicone Grease 1.0 Dry Interface 1.3 Mica with Grease 1.4 Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 Table 2. (From Thermalloy) Thermasil III 1.3 Thermasil II 1.5 Thermalfilm (0.002) with Grease 2.2 θ(HA) is the heatsink-to-ambient thermal resistance. It is this specification (listed on the heatsink manufacturers data sheet) which defines the effectiveness of the heatsink. The heatsink selected must have a thermal resistance which is equal to or lower than the value of θ(HA)calculated from the above listed formula. ERROR COMPARATOR This comparator produces a logic “LOW” whenever the output falls out of regulation by more than about 5%. This figure results from the comparator's built-in offset of 60 mV divided by the 1.23V reference. An out-of-regulation condition can result from low input voltage, current limiting, or thermal limiting. Figure 31 gives a timing diagram showing the relationship between the output voltage, the ERROR output, and input voltage as the input voltage is ramped up and down to the regulator without snap-on/snap-off output. The ERROR signal becomes low at about 1.3V input. It goes high at about 5V input, where the output equals 4.75V. Since the dropout voltage is load dependent, the input voltage trip points will vary with load current. The output voltage trip point does not vary. The comparator has an open-collector output which requires an external pull-up resistor. This resistor may be connected to the regulator output or some other supply voltage. Using the regulator output prevents an invalid “HIGH” on the comparator output which occurs if it is pulled up to an external voltage while the regulator input voltage is reduced below 1.3V. In selecting a value for the pull-up resistor, note that while the output can sink 400 µA, this current adds to battery drain. Suggested values range from 100k to 1 MΩ. The resistor is not required if the output is unused. *In shutdown mode, ERROR will go high if it has been pulled up to an external supply. To avoid this invalid response, pull up to regulator output. **Exact value depends on dropout voltage, which varies with load current. Figure 31. ERROR Output Timing If a single pull-up resistor is connected to the regulator output, the error flag may briefly rise up to about 1.3V as the input voltage ramps up or down through the 0V to 1.3V region. In some cases, this 1.3V signal may be mis-interpreted as a false high by a µP which is still “alive” with 1.3V applied to it. To prevent this, the user may elect to use two resistors which are equal in value on the error output (one connected to ground and the other connected to the regulator output). If this two-resistor divider is used, the error output will only be pulled up to about 0.6V (not 1.3V) during power-up or power-down, so it can not be interpreted as a high signal. When the regulator output is in regulation (4.8V to 5V), the error output voltage will be 2.4V to 2.5V, which is clearly a high signal. OUTPUT ISOLATION The regulator output can be connected to an active voltage source (such as a battery) with the regulator input turned off, as long as the regulator ground pin is connected to ground. If the ground pin is left floating, damage to the regulator can occur if the output is pulled up by an external voltage source. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 11 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com SNAP-ON/SNAP-OFF OPERATION The LP2957 output can be wired for snap-on/snap-off operation using three external resistors: *Minimum value (increase as required for smooth turn-on characteristic). Figure 32. Snap-On/Snap-Off Output When connected as shown, the shutdown input holds the regulator off until the input voltage rises up to the turnon threshold (V ON), at which point the output “snaps on”. When the input power is shut off (and the input voltage starts to decay) the output voltage will snap off when the input voltage reaches the turn-off threshold, VOFF. Figure 33. Snap-On/Snap-Off Input and Output Voltage Diagram It is important to note that the voltage VOFF must always be lower than VON (the difference in these voltage levels is called the hysteresis). Hysteresis is required when using snap-on/snap-off output, with the minimum amount of hysteresis required for a specific application being dependent on the source impedance of whatever is supplying VIN. Caution: A type of low-frequency oscillation can occur if VON and VOFFare too close together (insufficient hysteresis). When the output snaps on, the regulator must draw sufficient current to power the load and charge up the output capacitor (in most cases, the regulator will briefly draw the maximum current allowed by its internal limiter). For this reason, it is best to assume the LP2957 may pull a peak current of about 600 mA from the source (which is the listed maximum short-circuit load current of 530 mA plus the ground pin current of 70 mA). This high peak current causes VIN to drop by an amount equal to the source impedance multiplied by the current. If V IN drops below VOFF, the regulator will turn off and stop drawing current from the source. This will allow VIN to rise back up above VON, and the cycle will start over. The regulator will stay in this oscillating mode and never come into regulation. HYSTERESIS IN TRANSFORMER-POWERED APPLICATIONS: If the unregulated DC input voltage to the regulator comes from a transformer, the required hysteresis is easily measured by loading the source with a resistive load. Figure 34. Transformer Powered Input Supply 12 Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 If the regulator is powered from a battery, the source impedance will probably be low enough that other considerations will determine the optimum values for hysteresis (see Design Example #2). For best results, the load resistance used to test the transformer should be selected to draw about 600 mA for the maximum load current test, since this is the maximum peak current the LP2957 could be expected to draw from the source. The difference in input voltage measured at no load and full load defines the amount of hysteresis required for proper snap-on/snap-off operation (the programmed hysteresis must be greater than the difference in voltages). CALCULATING RESISTOR VALUES: The values of R1, R2 and R3 can be calculated assuming the designer knows the hysteresis. In most transformer-powered applications, it can be assumed that VOFF (the input voltage at turn-off) should be set for about 5.5V, since this allows about 500 mV across the LP2957 to keep the output in regulation until it snaps off. VON (the input voltage at turn on) is found by adding the hysteresis voltage to VOFF. R1, R2 and R3 are found by solving the node equations for the currents entering the node nearest the shutdown pin (written at the turn-on and turn-off thresholds). The shutdown pin bias current (10 nA typical) is not included in the calculations: Figure 35. Turn-ON Transition - Equivalent Circuit Figure 36. Turn-OFF Transition - Equivalent Circuits (4) Since these two equations contain three unknowns (R1, R2 and R3) one resistor value must be assumed and then the remaining two values can be obtained by solving the equations. The node equations will be simplified by solving both equations for R2, and then equating the two to generate an expression in terms of R1 and R3. (5) Setting these equal to each other and solving for R1 yields: (6) The same equation solved for R3 is: (7) A value for R1 or R3 can be derived using either one of the above equations, if the designer assumes a value for one of the resistors. Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 13 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com The simplest approach is to assume a value for R3. Best results will typically be obtained using values between about 20 kΩ and 100 kΩ (this keeps the current drain low, but also generates realistic values for the other resistors). There is no limit on the minimum value of R3, but current should be minimized as it generates power that drains the source and does not power the load. SUMMARY: TO SOLVE FOR R1, R2 AND R3: 1. Assume a value for either R1 or R3. 2. Solve for the other variable using the equation for R1 or R3. 3. Take the values for R1 and R3 and plug them back into either equation for R2 and solve for this value. DESIGN EXAMPLE #1: A 5V regulated output is to be powered from a transformer secondary which is rectified and filtered. The voltage VIN is measured at zero current and maximum current (600 mA) to determine the minimum allowable hysteresis. VIN is measured using an oscilloscope (both traces are shown on the same grid for clarity): Figure 37. VIN VOLTAGE WAVEFORMS The full-load voltage waveform from a transformer-powered supply will have ripple voltage as shown. The correct point to measure is the lowest value of the waveform. The 1.2V differential between no-load and full-load conditions means that at least 1.2V of hysteresis is required for proper snap-on/snap-off operation (for this example, we will use 1.5V). As a starting point, we will assume: VOFF = 5.5V VON = V OFF + HYST = 5.5 + 1.5 = 7V R3 = 49.9k Solving for R1: (8) Solving for R2: (9) 14 Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 DESIGN EXAMPLE #2: A 5V regulated output is to be powered from a battery made up of six NiCad cells. The cell data is: cell voltage (full charged): 1.4V cell voltage (90% discharged): 1.0V The internal impedance of a typical battery is low enough that source loading during regulator turn-on is not usually a problem. In a battery-powered application, the turn-off voltage VOFF should be selected so that the regulator is shut down when the batteries are about 90% discharged (over discharge can damage rechargeable batteries). In this case, the battery voltage will be 6.0V at the 90% discharge point (since there are six cells at 1.0V each). That means for this application, VOFF will be set to 6.0V. Selecting the optimum voltage for VON requires understanding battery behavior. If a Ni-Cad battery is nearly discharged (cell voltage 1.0V) and the load is removed, the cell voltage will drift back up. The voltage where the regulator turns on must be set high enough to keep the regulator from re-starting during this time, or an on-off pulsing mode can occur. If the regulator restarts when the discharged cell voltage drifts up, the load on the battery will cause the cell voltage to fall below the turn-off level, which causes the regulator to shut down. The cell voltage will again float up and the on-off cycling will continue. For NiCad batteries, a good cell voltage to use to calculate VON is about 1.2V per cell. In this application, this will yield a value for VON of 7.2V. We can now find R1, R2 and R3 assuming: VOFF = 6.0V V ON = 7.2V R3 = 49.9k Solving for R1: (10) Solving for R2: (11) Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 15 LP2957, LP2957A SNVS102C – JUNE 1998 – REVISED APRIL 2013 www.ti.com Schematic Diagram 16 Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A LP2957, LP2957A www.ti.com SNVS102C – JUNE 1998 – REVISED APRIL 2013 REVISION HISTORY Changes from Revision B (April 2013) to Revision C • Page Changed layout of National Data Sheet to TI format .......................................................................................................... 16 Submit Documentation Feedback Copyright © 1998–2013, Texas Instruments Incorporated Product Folder Links: LP2957 LP2957A 17 PACKAGE OPTION ADDENDUM www.ti.com 10-Dec-2020 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan (2) Lead finish/ Ball material MSL Peak Temp Op Temp (°C) Device Marking (3) (4/5) (6) LP2957AISX/NOPB ACTIVE DDPAK/ TO-263 KTT 5 500 RoHS-Exempt & Green SN Level-3-245C-168 HR -40 to 125 LP2957AIS LP2957AIT/NOPB ACTIVE TO-220 NDH 5 45 RoHS & Green SN Level-1-NA-UNLIM -40 to 125 LP2957AIT LP2957ISX/NOPB ACTIVE DDPAK/ TO-263 KTT 5 500 RoHS-Exempt & Green SN Level-3-245C-168 HR -40 to 125 LP2957IS (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of