LM5008MMX

LM5008 High Voltage (100V) Step Down Switching Regulator April 21, 2009 LM5008 High Voltage (100V) Step Down Switching Regulator General 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 MSOP-8 and the thermally enhanced LLP-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 under-voltage lockout, Gate drive under-voltage lockout, and Max Duty Cycle limiter Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 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 Typical Applications ■ Non-Isolated Telecommunication Buck Regulator ■ Secondary High Voltage Post Regulator ■ +42V Automotive Systems Package ■ MSOP - 8 ■ LLP - 8 (4mm x 4mm) Connection Diagram 20097902 8-Lead MSOP, LLP Ordering Information Order Number LM5008MM LM5008MMX LM5008SD LM5008SDX LM5008SDC LM5008SDCX LLP-8 SDC08B Package Type MSOP-8 NSC Package Drawing MUA08A SDC08A Supplied As 1000 Units on Tape and Reel 3500 Units on Tape and Reel 1000 Units on Tape and Reel 4500 Units on Tape and Reel 1000 Units on Tape and Reel 4500 Units on Tape and Reel © 2009 National Semiconductor Corporation 200979 www.national.com LM5008 Typical Application Circuit and Block Diagram 20097901 www.national.com 2 FIGURE 1. LM5008 Pin Descriptions Pin 1 2 Name SW BST Switching Node Boost Pin (Boot–strap capacitor input) Description Application Information Power switching node. Connect to the output inductor, re-circulating diode, and bootstrap capacitor. 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. 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. Ground for the entire circuit. This pin is connected to the inverting input of the internal regulation comparator. The regulation threshold is 2.5V. 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. If an auxiliary voltage is available to raise the voltage on this pin, above the regulation setpoint (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. Recommended operating range: 9.5V to 95V. The exposed pad has no electrical contact. Connect to system ground plane for reduced thermal resistance. 3 RCL Current Limit OFF time set pin Toff = 10-5 / (0.285 + (FB / 6.35 x 10− 6 x RCL)) Ground pin Feedback input from Regulated Output 4 5 RTN FB 6 RON/SD On time set pin Ton = 1.25 x 10-10 RON / VIN 7 VCC Output from the internal high voltage series pass regulator. Regulated at 7.0V. 8 VIN EP Input voltage Exposed Pad 3 www.national.com LM5008 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to GND BST to GND SW to GND (Steady State) ESD Rating (Note 5) Human Body Model BST to VCC -0.3V to 100V -0.3V to 114V -1V 2kV 100V BST to SW VCC to GND All Other Inputs to GND Lead Temperature (Soldering 4 sec) Storage Temperature Range 14V 14V -0.3 to 7V 260°C -55°C to +150°C Operating Ratings VIN Operating Junction Temperature (Note 1) 9.5V to 95V −40°C to + 125°C 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 (Note 3). Symbol VCC Supply VCC Reg VCC Regulator Output VCC Current Limit VCC undervoltage Lockout Voltage (VCC increasing) VCC Undervoltage Hysteresis VCC UVLO Delay (filter) IIN Operating Current IIN Shutdown Current Switch Characteristics Buck Switch Rds(on) Gate Drive UVLO Gate Drive UVLO Hysteresis Current Limit Current Limit Threshold Current Limit Response Time OFF time generator (test 1) OFF time generator (test 2) On Time Generator TON - 1 TON - 2 Remote Shutdown Threshold Remote Shutdown Hysteresis Vin = 10V Ron = 200K Vin = 95V Ron = 200K Rising 2.15 200 0.40 2.77 300 0.70 35 3.5 420 1.05 µs ns V mV Iswitch Overdrive = 0.1A Time to Switch Off FB=0V, RCL = 100K FB=2.3V, RCL = 100K 0.41 0.51 400 35 2.56 0.61 A ns µs µs ITEST = 200mA, (Note 6) VBST − VSW Rising 3.4 1.15 4.5 430 2.47 5.5 Ω V mV 100mV overdrive Non-Switching, FB = 3V RON/SD = 0V (Note 4) 6.6 7 9.5 6.3 200 10 485 76 675 150 7.4 V mA V mV µs µA µA Parameter Conditions Min Typ Max Units www.national.com 4 LM5008 Symbol Minimum Off Time Parameter Minimum Off Timer Conditions FB = 0V Internal reference Trip point for switch ON Trip point for switch OFF Min Typ 300 Max Units ns Regulation and OV Comparators FB Reference Threshold FB Over-Voltage Threshold FB Bias Current Thermal Shutdown Tsd Thermal Shutdown Temp. Thermal Shutdown Hysteresis Thermal Resistance θJA Junction to Ambient MUA Package SDC Package 200 40 °C/W °C/W 165 25 °C °C 2.445 2.5 2.875 100 2.550 V V nA Note 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 guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: For detailed information on soldering plastic MSOP and LLP packages, refer to the Packaging Data Book available from National Semiconductor Corporation. Note 3: All limits are guaranteed. All electrical characteristics having room temperature limits are tested during production with TA = TJ = 25°C. All hot and cold limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control. Note 4: The VCC output is intended as a self bias for the internal gate drive power and control circuits. Device thermal limitations limit external loading. Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 6: For devices procured in the LLP-8 package the Rds(on) limits are guaranteed by design characterization data only. 5 www.national.com LM5008 Typical Performance Characteristics 20097909 20097912 FIGURE 2. ICC Current vs Applied VCC Voltage FIGURE 5. Current Limit Off-Time vs VFB and RCL 20097923 20097910 FIGURE 3. ON-Time vs Input Voltage and RON FIGURE 6. Efficiency vs VIN (Circuit of Figure 13) 20097911 FIGURE 4. Maximum Frequency vs VOUT and VIN www.national.com 6 LM5008 20097927 20097924 FIGURE 7. Efficiency vs Load Current vs VIN (Circuit of Figure 13) 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 MSOP-8 and the thermally enhanced LLP-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 1. 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. FIGURE 8. Output Voltage vs Load Current (Circuit of Figure 13) 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: 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: 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 oneshot, 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 (1) The output voltage (VOUT) can be programmed by two external resistors as shown in Figure 1. The regulation point can be calculated as follows: VOUT = 2.5 x (R1 + R2) / R2 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 1). 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 9. However, R3 slightly degrades the load regulation. 7 www.national.com LM5008 20097905 FIGURE 9. 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 10. The current required into the VCC pin is shown in Figure 2. 20097906 FIGURE 10. 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 on-time, resulting in the minimum offtime of 300 ns. Bias current at the FB pin is nominally 100 nA. 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. www.national.com 8 LM5008 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. The on-time equation for the LM5008 is: TON = 1.25 x 10-10 x RON / VIN (2) See Figure 3. 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 4. The LM5008 can be remotely disabled by taking the RON/SD pin to ground. See Figure 11. 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. 20097907 FIGURE 11. 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 5). 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 the following equation: TOFF = 10-5 / (0.285 + (VFB / 6.35 x 10-6 x RCL)) (3) 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 turn-off recovery. 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. Applications Information SELECTION OF EXTERNAL COMPONENTS A guide for determining the component values will be illustrated with a design example. Refer to Figure 1. 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 1, 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 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. 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. 9 www.national.com LM5008 the switching losses. The maximum allowed frequency, based on a minimum on-time of 400 ns, is calculated from: FMAX = VOUT / (VINMAX x 400ns) For this exercise, Fmax = 263kHz. From equation 1, RON calculates to 304 kΩ. A standard value 357 kΩ resistor will be used to allow for tolerances in equation 1, 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 mA p-p so the lower peak of the waveform does not reach zero. L1 is calculated using the following equation: 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 mA p-p at Vin = 95V, and 34 mA p-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 guaranteed value of the LM5008’s current limit threshold (410 mA). Therefore the ripple amplitude must be less than 220 mA p-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 guaranteed 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. 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 mV p-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 1. R3’s value, along with C2’s ESR, must result in at least 25 mV p-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 (@ Vin = 12V) to 500mV (@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 (@ 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 12. 20097926 FIGURE 12. Inductor Current Waveform Starting when the current reaches Io (300 mA in Figure 12) 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: C = I x Δt / ΔV the minimum value for C2 is 7.2 µF. The ripple due to C2’s capacitance is 90° out of phase from the ESR ripple, and the www.national.com 10 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. LM5008 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 2, the minimum on-time is 0.470 µs, yielding a maximum offtime 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 3 has a ±25% tolerance, tOFFCL(MIN) = (4.11 µs + 0.40µs) x 1.25 = 5.64 µs Using equation 3, RCL calculates to 264kΩ (at VFB = 2.5V). The closest standard value is 267 kΩ. 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: 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 on-time 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 startup, 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 13. The circuit was tested, and the resulting performance is shown in Figure 6 through Figure 8. 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 LLP-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. 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 11 www.national.com LM5008 20097922 FIGURE 13. LM5008 Example Circuit Bill of Materials (Circuit of Figure 13) Item C1 C2 C3 C4 C5 D1 L1 R1 R2 R3 RON RCL U1 Description Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor Ceramic Capacitor UltraFast Power Diode Power Inductor Resistor Resistor Resistor Resistor Resistor Switching Regulator Part Number TDK C4532X7R2A105M TDK C4532X7R1E156M Kemet C1206C104K5RAC Kemet C1206C103K5RAC TDK C3216X7R2A104M ON Semi MURA110T3 Coilcraft DO3316-224 or TDK SLF10145T-221MR65 Vishay CRCW12063011F Vishay CRCW12061001F Vishay CRCW12062R00F Vishay CRCW12063573F Vishay CRCW12062673F National Semiconductor LM5008 3.01 kΩ 1.0 kΩ 2.0 Ω 357 kΩ 267 kΩ Value 1µF, 100V 15µF, 25V 0.1µF, 50V 0.01µF, 50V 0.1µF, 100V 100V, 1A 220 µH www.national.com 12 LM5008 Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead MSOP Package NS Package Number MUA08A 8-Lead LLP Package NS Package Number SDC08A 13 www.national.com LM5008 8-Lead LLP Package NS Package Number SDC08B www.national.com 14 LM5008 Notes 15 www.national.com LM5008 High Voltage (100V) Step Down Switching Regulator Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage Reference PowerWise® Solutions Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/AU Quality and Reliability Feedback/Support Design Made Easy Solutions Mil/Aero SolarMagic™ Analog University® THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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