LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 High-Voltage (100V) Step-Down Switching Regulator Check for Samples: LM5008 FEATURES DESCRIPTION • • • • • • The LM5008 Step-Down Switching Regulator features all of the functions needed to implement a low cost, efficient, Buck bias regulator. This high voltage regulator contains an 100 V N-Channel Buck Switch. The device is easy to implement and is provided in the VSSOP-8 and the thermally enhanced WSON-8 packages. The regulator is based on a hysteretic control scheme using an ON time inversely proportional to VIN. This feature allows the operating frequency to remain relatively constant. The hysteretic control requires no loop compensation. An intelligent current limit is implemented with forced OFF time, which is inversely proportional to Vout. This scheme ensures short circuit protection while providing minimum foldback. Other protection features include: Thermal Shutdown, VCC undervoltage lockout, Gate drive under-voltage lockout, and Max Duty Cycle limiter 1 2 • • • • • • • Integrated 100V, N-Channel buck switch Internal VCC regulator No loop compensation required Ultra-Fast transient response On time varies inversely with line voltage Operating frequency remains constant with varying line voltage and load current Adjustable output voltage Highly efficient operation Precision internal reference Low bias current Intelligent current limit protection Thermal shutdown VSSOP-8 and WSON-8 (4mm x 4mm) Packages APPLICATIONS • • • Non-Isolated Telecommunication Buck Regulator Secondary High Voltage Post Regulator +42V Automotive Systems Connection Diagram 1 8 SW VIN BST VCC RCL RON/SD RTN FB 2 3 7 6 4 5 Figure 1. 8-Lead VSSOP or WSON 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 © 2004–2013, Texas Instruments Incorporated LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 www.ti.com Typical Application Circuit and Block Diagram 7V SERIES REGULATOR 9.5 -95V Input LM5008 VCC 7 8 VIN C1 SD C5 ON TIMER START RON C3 THERMAL SHUTDOWN UVLO COMPLETE 6 SD / RON BST Ron START OVER-VOLTAGE COMPARATOR SHUTDOWN + - 2.875V UVLO 300 ns MIN OFF TIMER VIN SD C4 DRIVER COMPLETE LEVEL SHIFT 2.5V SW SET + - 5 R REGULATION COMPARATOR FB RCL CLR 4 VOUT1 Q R1 COMPLETE RCL + - START CURRENT LIMIT OFF TIMER RCL L1 1 Q S FB 3 2 0.50A BUCK SWITCH CURRENT SENSE R3 VOUT2 D1 RTN R2 C2 Figure 2. Pin Functions Table 1. Pin Descriptions 2 Pin Name 1 SW Switching node Description Power switching node. Connect to the output inductor, re-circulating diode, and bootstrap capacitor. Application Information 2 BST Boost pin (bootstrap capacitor input) An external capacitor is required between the BST and the SW pins. A 0.01µF ceramic capacitor is recommended. An internal diode charges the capacitor from VCC. 3 RCL Current limit OFF time set pin Toff = 10-5 / (0.285 + (FB / 6.35 x 10− 6 x RCL)) A resistor between this pin and RTN sets the off-time when current limit is detected. The off-time is preset to 35µs if FB = 0V. 4 RTN Ground pin Ground for the entire circuit. 5 FB Feedback input from regulated output This pin is connected to the inverting input of the internal regulation comparator. The regulation threshold is 2.5V. 6 RON/SD On time set pin Ton = 1.25 x 10-10 RON / VIN A resistor between this pin and VIN sets the switch on time as a function of VIN. The minimum recommended on time is 400ns at the maximum input voltage. This pin can be used for remote shutdown. 7 VCC Output from the internal high voltage series pass If an auxiliary voltage is available to raise the voltage on this pin, regulator. Regulated at 7.0 V. above the regulation set point (7V), the internal series pass regulator will shutdown, reducing the IC power dissipation. Do not exceed 14V. This voltage provides gate drive power for the internal Buck switch. An internal diode is provided between this pin and the BST pin. A local 0.1µF decoupling capacitor is recommended. Series pass regulator is current limited to 10mA. 8 VIN Input voltage Recommended operating range: 9.5V to 95V. EP Exposed Pad The exposed pad has no electrical contact. Connect to system ground plane for reduced thermal resistance. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 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. Absolute Maximum Ratings (1) VIN to GND -0.3V to 100V BST to GND -0.3V to 114V SW to GND (Steady State) ESD Rating, Human Body Model -1V (2) 2kV BST to VCC 100V BST to SW 14V VCC to GND 14V All Other Inputs to GND -0.3 to 7V Lead Temperature (Soldering 4 sec) (3) 260°C Storage Temperature Range (1) (2) (3) -55°C to +150°C Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For specifications and test conditions, see the Electrical Characteristics. The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. For detailed information on soldering plastic VSSOP and WSON packages, refer to the Packaging Data Book. Operating Ratings (1) VIN 9.5V to 95V −40°C to + 125°C Operating Junction Temperature (1) Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For specifications and test conditions, see the Electrical Characteristics. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 3 LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 www.ti.com Electrical Characteristics Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 48V, unless otherwise stated (1). Symbol Parameter Conditions Min Typ Max 7 7.4 Unit VCC Supply VCC Reg VCC Regulator Output 6.6 V VCC Current Limit (2) 9.5 mA VCC undervoltage Lockout Voltage (VCC increasing) 6.3 V 200 mV VCC Undervoltage Hysteresis VCC UVLO Delay (filter) 100mV overdrive 10 IIN Operating Current Non-Switching, FB = 3V 485 675 µA µs IIN Shutdown Current RON/SD = 0V 76 150 µA 1.15 2.47 Ω 4.5 5.5 Switch Characteristics (3) Buck Switch Rds(on) ITEST = 200mA, Gate Drive UVLO VBST − VSW Rising 3.4 Gate Drive UVLO Hysteresis 430 V mV Current Limit Current Limit Threshold 0.41 0.51 0.61 A Current Limit Response Time Iswitch Overdrive = 0.1A, Time to Switch Off 400 ns OFF time generator (test 1) FB=0V, RCL = 100K 35 µs OFF time generator (test 2) FB=2.3V, RCL = 100K 2.56 µs On Time Generator TON - 1 Vin = 10V, Ron = 200K 2.15 2.77 3.5 µs TON - 2 Vin = 95V, Ron = 200K 200 300 420 ns Remote Shutdown Threshold Rising 0.40 0.70 1.05 Remote Shutdown Hysteresis V 35 mV 300 ns Minimum Off Time Minimum Off Timer FB = 0V Regulation and OV Comparators FB Reference Threshold Internal reference, Trip point for switch ON FB Over-Voltage Threshold Trip point for switch OFF 2.445 2.5 2.550 V 2.875 V 100 nA Thermal Shutdown Temperature 165 °C Thermal Shutdown Hysteresis 25 °C VSSOP Package 200 °C/W WSON Package 40 °C/W FB Bias Current Thermal Shutdown Tsd Thermal Resistance θJA (1) (2) (3) 4 Junction to Ambient All electrical characteristics having room temperature limits are tested during production with TA = TJ = 25°C. All hot and cold limits are specified by correlating the electrical characteristics to process and temperature variations and applying statistical process control. The VCC output is intended as a self bias for the internal gate drive power and control circuits. Device thermal limitations limit external loading. For devices procured in the WSON-8 package the Rds(on) limits are specified by design characterization data only. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 Typical Performance Characteristics 2.0 8 RON = 500k 6 5 1.6 TON (Ps) ICC INPUT CURRENT (mA) 7 1.8 1.4 300k 4 3 2 1.2 100k 1 1.0 0 8 9 10 11 12 13 14 0 20 40 EXTERNALLY APPLIED VCC (V) 80 100 VIN (V) Figure 3. ICC Current vs Applied VCC Voltage Figure 4. ON-Time vs Input Voltage and RON 700 35 48V 60V 80V 600 Max VIN = 30V 500 95V 400 300 200 100 CURRENT LIMIT OFF TIME (Ps) MAXIMUNM FREQUENCY (kHz) 60 30 25 20 15 RCL = 500k 300k 10 100k 5 50k 0 0 0 2.5 5.0 10 15 20 0 0.5 1.0 1.5 2.0 2.5 VFB (V) VOUT (V) Figure 5. Maximum Frequency vs VOUT and VIN Figure 6. Current Limit Off-Time vs VFB and RCL 100 100 90 90 80 80 EFFICIENCY (%) EFFICIENCY (%) VIN = 15V 70 60 IOUT = 300 mA 50 20 40 VIN = 95V 70 60 50 40 0 VIN = 48V 60 80 100 VIN (V) 40 100 200 300 LOAD CURRENT (mA) Figure 7. Efficiency vs VIN (Circuit of Figure 14) Figure 8. Efficiency vs Load Current vs VIN (Circuit of Figure 14) Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 5 LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 www.ti.com Typical Performance Characteristics (continued) 10.2 VOUT1 10.0 VOUT (V) 9.8 VOUT2 9.6 9.4 9.2 VIN = 48V 9.0 0 100 200 300 LOAD CURRENT (mA) Figure 9. Output Voltage vs Load Current (Circuit of Figure 14) 6 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 FUNCTIONAL DESCRIPTION The LM5008 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient, Buck bias power converter. This high voltage regulator contains a 100 V N-Channel Buck Switch, is easy to implement and is provided in the VSSOP-8 and the thermally enhanced WSON-8 packages. The regulator is based on a hysteretic control scheme using an on-time inversely proportional to VIN. The hysteretic control requires no loop compensation. Current limit is implemented with forced off-time, which is inversely proportional to VOUT. This scheme ensures short circuit protection while providing minimum foldback. The Functional Block Diagram of the LM5008 is shown in Figure 2. The LM5008 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator is well suited for 48 Volt Telecom and the new 42V Automotive power bus ranges. Protection features include: Thermal Shutdown, VCC under-voltage lockout, Gate drive under-voltage lockout, Max Duty Cycle limit timer and the intelligent current limit off timer. Hysteretic Control Circuit Overview The LM5008 is a Buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with line voltage (VIN). Control is based on a comparator and the on-time one-shot, with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB level is below the reference the buck switch is turned on for a fixed time determined by the line voltage and a programming resistor (RON). Following the ON period the switch will remain off for at least the minimum off-timer period of 300ns. If FB is still below the reference at that time the switch will turn on again for another on-time period. This will continue until regulation is achieved. The LM5008 operates in discontinuous conduction mode at light load currents, and continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the internal reference - until then the inductor current remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with load current. Therefore at light loads the conversion efficiency is maintained, since the switching losses reduce with the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows: VOUT2 x L x 1.28 x 1020 F= RL x (RON)2 (1) where RL = the load resistance In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero. In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively constant with load and line variations. The approximate continuous mode operating frequency can be calculated as follows: VOUT F= 1.25 x 10-10 x RON (2) The output voltage (VOUT) can be programmed by two external resistors as shown in Figure 2. The regulation point can be calculated as follows: VOUT = 2.5 x (R1 + R2) / R2 (3) All hysteretic regulators regulate the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. A minimum of 25mV to 50mV of ripple voltage at the feedback pin (FB) is required for the LM5008. In cases where the capacitor ESR is too small, additional series resistance may be required (R3 in Figure 2). For applications where lower output voltage ripple is required the output can be taken directly from a low ESR output capacitor, as shown in Figure 10. However, R3 slightly degrades the load regulation. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 7 LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 www.ti.com L1 SW LM5008 R1 R3 FB VOUT2 R2 C2 Figure 10. Low Ripple Output Configuration High Voltage Start-Up Regulator The LM5008 contains an internal high voltage startup regulator. The input pin (VIN) can be connected directly to the line voltages up to 95 Volts, with transient capability to 100 volts. The regulator is internally current limited to 9.5mA at VCC. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC pin reaches the under-voltage lockout threshold of 6.3V, the buck switch is enabled. In applications involving a high value for VIN, where power dissipation in the VCC regulator is a concern, an auxiliary voltage can be diode connected to the VCC pin. Setting the auxiliary voltage to 8.0 -14V will shut off the internal regulator, reducing internal power dissipation. See Figure 11. The current required into the VCC pin is shown in Figure 3. VCC C3 BST C4 LM5008 L1 D2 SW D1 R1 R3 FB VOUT2 R2 C2 Figure 11. Self Biased Configuration Regulation Comparator The feedback voltage at FB is compared to an internal 2.5V reference. In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch will stay on for the on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch will stay off until the FB voltage again falls below 2.5V. During start-up, the FB voltage will be below 2.5V at the end of each ontime, resulting in the minimum off-time of 300 ns. Bias current at the FB pin is nominally 100 nA. 8 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 Over-Voltage Comparator The feedback voltage at FB is compared to an internal 2.875V reference. If the voltage at FB rises above 2.875V the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load, change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V. On-Time Generator and Shutdown The on-time for the LM5008 is determined by the RON resistor, and is inversely proportional to the input voltage (Vin), resulting in a nearly constant frequency as Vin is varied over its range. Equation 4 shows the on-time equation for the LM5008. TON = 1.25 x 10-10 x RON / VIN (4) See Figure 4. RON should be selected for a minimum on-time (at maximum VIN) greater than 400 ns, for proper current limit operation. This requirement limits the maximum frequency for each application, depending on VIN and VOUT. See Figure 5. The LM5008 can be remotely disabled by taking the RON/SD pin to ground. See Figure 12. The voltage at the RON/SD pin is between 1.5 and 3.0 volts, depending on Vin and the value of the RON resistor. Input Voltage VIN RON LM5008 RON/SD STOP RUN Figure 12. Shutdown Implementation Current Limit The LM5008 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.5A the present cycle is immediately terminated, and a non-resetable OFF timer is initiated. The length of off-time is controlled by an external resistor (RCL) and the FB voltage (see Figure 6). When FB = 0V, a maximum off-time is required, and the time is preset to 35µs. This condition occurs when the output is shorted, and during the initial part of start-up. This amount of time ensures safe short circuit operation up to the maximum input voltage of 95V. In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time will be less than 35µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and the start-up time. The off-time is calculated from Equation 5. 10 TOFF = 0.285 + -5 VFB -6 (6.35 x 10 x RCL) (5) The current limit sensing circuit is blanked for the first 50-70ns of each on-time so it is not falsely tripped by the current surge which occurs at turn-on. The current surge is required by the re-circulating diode (D1) for its turnoff recovery. N-Channel Buck Switch and Driver The LM5008 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during the on-time. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 9 LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 www.ti.com During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from Vcc through the internal diode. The minimum OFF timer, set to 300ns, ensures a minimum time each cycle to recharge the bootstrap capacitor. An external re-circulating diode (D1) carries the inductor current after the internal Buck switch turns off. This diode must be of the Ultra-fast or Schottky type to minimize turn-on losses and current over-shoot. Thermal Protection The LM5008 should be operated so the junction temperature does not exceed 125°C during normal operation. An internal Thermal Shutdown circuit is provided to protect the LM5008 in the event of a higher than normal junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state, disabling the buck switch and the VCC regulator. This feature prevents catastrophic failures from accidental device overheating. When the junction temperature reduces below 140°C (typical hysteresis = 25°C), the Vcc regulator is enabled, and normal operation is resumed. 10 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 APPLICATIONS INFORMATION SELECTION OF EXTERNAL COMPONENTS A guide for determining the component values will be illustrated with a design example. Refer to Figure 2. The following steps will configure the LM5008 for: • Input voltage range (Vin): 12V to 95V • Output voltage (VOUT1): 10V • Load current (for continuous conduction mode): 100 mA to 300 mA • Maximum ripple at VOUT2: 100 mVp-p at maximum input voltage R1 and R2: From Figure 2, VOUT1 = VFB x (R1 + R2) / R2, and since VFB = 2.5V, the ratio of R1 to R2 calculates as 3:1. Standard values of 3.01 kΩ (R1) and 1.00 kΩ (R2) are chosen. Other values could be used as long as the 3:1 ratio is maintained. The selected values, however, provide a small amount of output loading (2.5 mA) in the event the main load is disconnected. This allows the circuit to maintain regulation until the main load is reconnected. Fs and RON: The recommended operating frequency range for the LM5008 is 50kHz to 600 kHz. Unless the application requires a specific frequency, the choice of frequency is generally a compromise since it affects the size of L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time of 400 ns, is calculated from: FMAX = VOUT / (VINMAX x 400ns) (6) For this exercise, Fmax = 263kHz. From Equation 2, RON calculates to 304 kΩ. A standard value 357 kΩ resistor will be used to allow for tolerances in Equation 2, resulting in a frequency of 224kHz. L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum ripple current occurs at maximum Vin. a) Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude (IOR) must be less than 200 mAp-p so the lower peak of the waveform does not reach zero. L1 is calculated using Equation 7. VOUT1 x (VIN - VOUT1) L1 = IOR x Fs x VIN (7) At Vin = 95V, L1(min) calculates to 200 µH. The next larger standard value (220 µH) is chosen and with this value IOR calculates to 181 mAp-p at Vin = 95V, and 34 mAp-p at Vin = 12V. b) Maximum load current: At a load current of 300 mA, the peak of the ripple waveform must not reach the minimum value of the LM5008’s current limit threshold (410 mA). Therefore the ripple amplitude must be less than 220 mAp-p, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum Vin and Io, the peak of the ripple will be 391 mA. While L1 must carry this peak current without saturating or exceeding its temperature rating, it also must be capable of carrying the maximum value of the LM5008’s current limit threshold (610 mA) without saturating, since the current limit is reached during startup. The DC resistance of the inductor should be as low as possible. For example, if the inductor’s DCR is one ohm, the power dissipated at maximum load current is 0.09W. While small, it is not insignificant compared to the load power of 3W. C3: The capacitor on the VCC output provides not only noise filtering and stability, but its primary purpose is to prevent false triggering of the VCC UVLO at the buck switch on/off transitions. For this reason, C3 should be no smaller than 0.1 µF. C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its ESR, ripple voltage due to its capacitance, and the nature of the load. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 11 LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 www.ti.com a) ESR and R3: A low ESR for C2 is generally desirable so as to minimize power losses and heating within the capacitor. However, a hysteretic regulator requires a minimum amount of ripple voltage at the feedback input for proper loop operation. For the LM5008 the minimum ripple required at pin 5 is 25 mVp-p, requiring a minimum ripple at VOUT1 of 100 mV. Since the minimum ripple current (at minimum Vin) is 34 mA p-p, the minimum ESR required at VOUT1 is 100mV/34mA = 2.94Ω. Since quality capacitors for SMPS applications have an ESR considerably less than this, R3 is inserted as shown in Figure 2. R3’s value, along with C2’s ESR, must result in at least 25 mVp-p ripple at pin 5. Generally, R3 will be 0.5 to 3.0Ω. b) Nature of the Load: The load can be connected to VOUT1 or VOUT2. VOUT1 provides good regulation, but with a ripple voltage which ranges from 100 mV (at Vin = 12V) to 500mV (at Vin = 95V). Alternatively, VOUT2 provides low ripple, but lower regulation due to R3. For a maximum allowed ripple voltage of 100 mVp-p at VOUT2 (at Vin = 95V), assume an ESR of 0.4Ω for C2. At maximum Vin, the ripple current is 181 mAp-p, creating a ripple voltage of 72 mVp-p. This leaves 28 mVp-p of ripple due to the capacitance. The average current into C2 due to the ripple current is calculated using the waveform in Figure 13. L1 Current 391 mA 300 mA 209 mA 0 mA 1/Freq. = Ts Ts/2 Figure 13. Inductor Current Waveform Starting when the current reaches Io (300 mA in Figure 13) half way through the on-time, the current continues to increase to the peak (391 mA), and then decreases to 300 mA half way through the off-time. The average value of this portion of the waveform is 45.5mA, and will cause half of the voltage ripple, or 14 mV. The interval is one half of the frequency cycle time, or 2.23 µs. Using the capacitor’s basic equation (see Equation 8), the minimum value for C2 is 7.2 µF. C = I x Δt / ΔV (8) The ripple due to C2’s capacitance is 90° out of phase from the ESR ripple, and the two numbers do not add directly. However, this calculation provides a practical minimum value for C2 based on its ESR, and the target spec. To allow for the capacitor’s tolerance, temperature effects, and voltage effects, a 15 µF, X7R capacitor will be used. c) In summary: The above calculations provide a minimum value for C2, and a calculation for R3. The ESR is just as important as the capacitance. The calculated values are guidelines, and should be treated as starting points. For each application, experimentation is needed to determine the optimum values for R3 and C2. RCL: When a current limit condition is detected, the minimum off-time set by this resistor must be greater than the maximum normal off-time which occurs at maximum Vin. Using Equation 4, the minimum on-time is 0.470 µs, yielding a maximum off-time of 3.99 µs. This is increased by 117 ns (to 4.11 µs) due to a ±25% tolerance of the on-time. This value is then increased to allow for: The response time of the current limit detection loop (400ns). The off-time determined by Equation 5 has a ±25% tolerance. tOFFCL(MIN) = (4.11 µs + 0.40 µs) × 1.25 = 5.64 µs (9) Using Equation 5, RCL calculates to 264kΩ (at VFB = 2.5V). The closest standard value is 267 kΩ. 12 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage drop is significant in the event the output is short-circuited as it is only this diode’s voltage which forces the inductor current to reduce during the forced off-time. For this reason, a higher voltage is better, although that affects efficiency. A good choice is an ultrafast power diode, such as the MURA110T3 from ON Semiconductor. Its reverse recovery time is 30ns, and its forward voltage drop is approximately 0.72V at 300 mA at 25°C. Other types of diodes may have a lower forward voltage drop, but may have longer recovery times, or greater reverse leakage. D1’s reverse voltage rating must be at least as great as the maximum Vin, and its current rating be greater than the maximum current limit threshold (610 mA). C1: This capacitor’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at Vin, on the assumption that the voltage source feeding Vin has an output impedance greater than zero. At maximum load current, when the buck switch turns on, the current into pin 8 will suddenly increase to the lower peak of the output current waveform, ramp up to the peak value, then drop to zero at turn-off. The average input current during this on-time is the load current (300 mA). For a worst case calculation, C1 must supply this average load current during the maximum on-time. To keep the input voltage ripple to less than 2V (for this exercise), C1 calculates to: C1 = I x tON 'V = 0.3A x 3.72 Ps = 0.56 PF 2.0V (10) Quality ceramic capacitors in this value have a low ESR which adds only a few millivolts to the ripple. It is the capacitance which is dominant in this case. To allow for the capacitor’s tolerance, temperature effects, and voltage effects, a 1.0 µF, 100V, X7R capacitor will be used. C4: The recommended value is 0.01µF for C4, as this is appropriate in the majority of applications. A high quality ceramic capacitor, with low ESR is recommended as C4 supplies the surge current to charge the buck switch gate at turn-on. A low ESR also ensures a quick recharge during each off-time. At minimum Vin, when the ontime is at maximum, it is possible during start-up that C4 will not fully recharge during each 300 ns off-time. The circuit will not be able to complete the start-up, and achieve output regulation. This can occur when the frequency is intended to be low (e.g., RON = 500K). In this case C4 should be increased so it can maintain sufficient voltage across the buck switch driver during each on-time. C5: This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at VIN. A low ESR, 0.1µF ceramic chip capacitor is recommended, located close to the LM5008. FINAL CIRCUIT The final circuit is shown in Figure 14. The circuit was tested, and the resulting performance is shown in Figure 7 through Figure 9. MINIMUM LOAD CURRENT A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level, the bootstrap capacitor may discharge during the long off-time, and the circuit will either shutdown, or cycle on and off at a low frequency. If the load current is expected to drop below 1 mA in the application, the feedback resistors should be chosen low enough in value so they provide the minimum required current at nominal Vout. PC BOARD LAYOUT The LM5008 regulation and over-voltage comparators are very fast, and as such will respond to short duration noise pulses. Layout considerations are therefore critical for optimum performance. The components at pins 1, 2, 3, 5, and 6 should be as physically close as possible to the IC, thereby minimizing noise pickup in the PC tracks. The current loop formed by D1, L1, and C2 should be as small as possible. The ground connection from C2 to C1 should be as short and direct as possible. If the internal dissipation of the LM5008 produces excessive junction temperatures during normal operation, good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad on the bottom of the WSON-8 package can be soldered to a ground plane on the PC board, and that plane should extend out from beneath the IC to help dissipate the heat. Additionally, the use of wide PC board traces, where possible, can also help conduct heat away from the IC. Judicious positioning of the PC board within the end product, along with use of any available air flow (forced or natural convection) can help reduce the junction temperatures. Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 13 LM5008 SNVS280G – APRIL 2004 – REVISED MARCH 2013 12 - 95V Input www.ti.com VCC VIN 7 8 C1 1.0 PF C3 0.1 PF C5 0.1 PF BST RON 357k 2 RON / SD 6 LM5008 C4 0.01 PF L1 220 PH 10.0V SW VOUT1 1 SHUTDOWN D1 RCL R1 R3 3.01k 2.0 3 RCL 267k RTN VOUT2 FB R2 5 1.0k 4 C2 15 PF GND Figure 14. LM5008 Example Circuit Table 2. Bill of Materials (Circuit of Figure 14) Item Description Part Number Value C1 Ceramic Capacitor TDK C4532X7R2A105M 1µF, 100V C2 Ceramic Capacitor TDK C4532X7R1E156M 15µF, 25V C3 Ceramic Capacitor Kemet C1206C104K5RAC 0.1µF, 50V C4 Ceramic Capacitor Kemet C1206C103K5RAC 0.01µF, 50V C5 Ceramic Capacitor TDK C3216X7R2A104M 0.1µF, 100V D1 UltraFast Power Diode ON Semi MURA110T3 100V, 1A L1 Power Inductor Coilcraft DO3316-224 or 220 µH TDK SLF10145T-221MR65 R1 Resistor Vishay CRCW12063011F 3.01 kΩ R2 Resistor Vishay CRCW12061001F 1.0 kΩ R3 Resistor Vishay CRCW12062R00F 2.0 Ω RON Resistor Vishay CRCW12063573F 357 kΩ RCL Resistor Vishay CRCW12062673F 267 kΩ U1 Switching Regulator Texas Instruments LM5008 14 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 LM5008 www.ti.com SNVS280G – APRIL 2004 – REVISED MARCH 2013 REVISION HISTORY Changes from Revision F (March 2013) to Revision G • Page Changed layout of National Data Sheet to TI format .......................................................................................................... 14 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM5008 15 PACKAGE OPTION ADDENDUM www.ti.com 12-Jan-2016 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan Lead/Ball Finish MSL Peak Temp (2) (6) (3) Op Temp (°C) Device Marking (4/5) LM5008MM ACTIVE VSSOP DGK 8 1000 TBD Call TI Call TI -40 to 125 SAYB LM5008MM/NOPB ACTIVE VSSOP DGK 8 1000 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 SAYB LM5008MMX NRND VSSOP DGK 8 3500 TBD Call TI Call TI -40 to 125 SAYB LM5008MMX/NOPB ACTIVE VSSOP DGK 8 3500 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 SAYB LM5008SD NRND WSON NGU 8 1000 TBD Call TI Call TI L00040B LM5008SD/NOPB NRND WSON NGT 8 1000 Green (RoHS & no Sb/Br) CU NIPDAU | CU SN Level-1-260C-UNLIM L00040B LM5008SDC/NOPB ACTIVE WSON NGU 8 1000 Green (RoHS & no Sb/Br) CU NIPDAU | CU SN Level-1-260C-UNLIM L5008SD LM5008SDCX/NOPB ACTIVE WSON NGU 8 4500 Green (RoHS & no Sb/Br) CU NIPDAU | CU SN Level-1-260C-UNLIM L5008SD LM5008SDX/NOPB NRND WSON NGT 8 4500 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM L00040B (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) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. TBD: The Pb-Free/Green conversion plan has not been defined. Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above. Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material) (3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. Addendum-Page 1 Samples PACKAGE OPTION ADDENDUM www.ti.com (4) 12-Jan-2016 There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device. (5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation of the previous line and the two combined represent the entire Device Marking for that device. (6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish value exceeds the maximum column width. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release. In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis. Addendum-Page 2 PACKAGE MATERIALS INFORMATION www.ti.com 14-Mar-2016 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant LM5008MM VSSOP DGK 8 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM5008MM/NOPB VSSOP DGK 8 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM5008MMX VSSOP DGK 8 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM5008MMX/NOPB VSSOP DGK 8 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM5008SD WSON NGU 8 1000 178.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1 LM5008SD/NOPB WSON NGT 8 1000 180.0 12.4 4.3 4.3 1.1 8.0 12.0 Q1 LM5008SDC/NOPB WSON NGU 8 1000 180.0 12.4 4.3 4.3 1.1 8.0 12.0 Q1 LM5008SDCX/NOPB WSON NGU 8 4500 330.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1 LM5008SDX/NOPB WSON NGT 8 4500 330.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1 Pack Materials-Page 1 PACKAGE MATERIALS INFORMATION www.ti.com 14-Mar-2016 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM5008MM VSSOP DGK 8 1000 210.0 185.0 35.0 LM5008MM/NOPB VSSOP DGK 8 1000 210.0 185.0 35.0 LM5008MMX VSSOP DGK 8 3500 367.0 367.0 35.0 LM5008MMX/NOPB VSSOP DGK 8 3500 367.0 367.0 35.0 LM5008SD WSON NGU 8 1000 210.0 185.0 35.0 LM5008SD/NOPB WSON NGT 8 1000 203.0 203.0 35.0 LM5008SDC/NOPB WSON NGU 8 1000 203.0 203.0 35.0 LM5008SDCX/NOPB WSON NGU 8 4500 367.0 367.0 35.0 LM5008SDX/NOPB WSON NGT 8 4500 367.0 367.0 35.0 Pack Materials-Page 2 MECHANICAL DATA NGT0008A SDC08A (Rev A) www.ti.com MECHANICAL DATA NGU0008B SDC08B (Rev A) www.ti.com 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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