LM5008AMMX/NOPB

LM5008A www.ti.com SNVS583F – MARCH 2009 – REVISED MARCH 2013 100-V 350-mA Constant On-Time Buck Switching Regulator Check for Samples: LM5008A FEATURES DESCRIPTION • • • • • • • The LM5008A is a functional variant of the LM5008 COT Buck Switching Regulator. The functional differences of the LM5008A are: the minimum input operating voltage is 6 V, the on-time equation is slightly different, and the requirement for a minimum load current is removed. 1 2 • • • • • • • Operating input voltage range: 6V to 95V Integrated 100V N-Channel buck switch Internal start-up regulator No loop compensation required Ultra-fast transient response On time varies inversely with input voltage Operating frequency remains constant with varying line voltage and load current Adjustable output voltage from 2.5V Highly efficient operation Precision internal reference Low bias current Intelligent current limit 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 The LM5008A 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 control scheme using an ON time inversely proportional to VIN. This feature allows the operating frequency to remain relatively constant. The control scheme 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 control while providing minimum foldback. Other features include: Thermal Shutdown, VCC under-voltage lockout, Gate drive under-voltage lockout, Max Duty Cycle limiter, and a pre-charge switch. Typical Application, Basic Step-Down Regulator 6V - 95V Input VIN VCC VIN C1 C3 LM5008A RT BST GND C4 L1 RT/SD VOUT SW RCL D1 SHUTDOWN RFB2 RCL RTN R3 C2 FB GND RFB1 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 © 2009–2013, Texas Instruments Incorporated LM5008A SNVS583F – MARCH 2009 – REVISED MARCH 2013 www.ti.com Connection Diagram SW BST 1 8 VIN 2 7 VCC RCL 3 6 RT/SD RTN 4 5 FB SW 1 8 VIN BST 2 7 VCC RCL 3 6 RT/SD RTN 4 5 FB Exposed Pad on Bottom Connect to Ground Figure 1. Top View 8-Lead WSON Figure 2. Top View 8-Lead VSSOP Pin Functions Table 1. Pin Descriptions Pin Name 1 SW Switching Node Power switching node. Connect to the output inductor, re-circulating diode, and bootstrap capacitor. 2 BST Boost pin (bootstrap capacitor input) An external capacitor is required between the BST and the SW pins. A 0.01 µF ceramic capacitor is recommended. An internal diode charges the capacitor from VCC during each off-time. 3 RCL Current Limit OFF time set pin A resistor between this pin and RTN sets the off-time when current limit is detected. The off-time is preset to 35 µs 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 RT/SD On time set pin A resistor between this pin and VIN sets the switch on time as a function of VIN. The minimum recommended on time is 400 ns at the maximum input voltage. This pin can be used for remote shutdown. 7 VCC Output from the internal high voltage series pass regulator This regulated voltage provides gate drive power for the internal Buck switch. An internal diode is provided between this pin and the BST pin. A local 0.47 µF decoupling capacitor is required. The series pass regulator is current limited to 9 mA. 8 VIN Input voltage Input operating range: 6V to 95V. EP Exposed Pad The exposed pad has no electrical contact. Connect to system ground plane for reduced thermal resistance. 2 Description Application Information Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A LM5008A www.ti.com SNVS583F – MARCH 2009 – 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) -1V ESD Rating, Human Body Model (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) 260°C Storage Temperature Range (1) (2) -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. The ESD rating for pin 2, pin 7, and pin 8 is 1 kV. Operating Ratings (1) VIN 6V 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 © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A 3 LM5008A SNVS583F – MARCH 2009 – 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 Vin = 48V Vin – Vcc 6V < Vin < 8.5V Vcc Bypass Threshold Vin Increasing 6.6 100 V mV 8.5 V 300 mV Vin =6V 100 Ω Vin = 10V 8.8 Ω Vin = 48V 0.8 Ω Vcc Current Limit Vin = 48V 9.2 mA Vcc UVLO Vcc Increasing 5.3 V 190 mV Vcc Bypass Hysteresis Vcc Output Impedance Vcc UVLO hysteresis Vcc UVLO filter delay 3 µs Iin Operating current FB = 3V, Vin = 48V 550 750 µA Iin Shutdown Current RT/SD = 0V 110 176 µA 1.25 2.57 Ω 3.8 4.8 Switch Characteristics Buck switch Rds(on) Itest = 200 mA Gate Drive UVLO Vbst – Vsw Rising 2.8 Gate Drive UVLO hysteresis Pre-charge switch voltage 490 At 1 mA Pre-charge switch on-time V mV 0.8 V 150 ns Current Limit Current Limit Threshold 0.41 Current Limit Response Time Iswitch Overdrive = 0.1A, Time to Switch Off TOFF-1 OFF time generator FB=0V, RCL = 100K TOFF-2 OFF time generator FB=2.3V, RCL = 100K 0.51 0.61 A 350 ns 35 µs 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) 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. Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A LM5008A www.ti.com SNVS583F – MARCH 2009 – REVISED MARCH 2013 Typical Performance Characteristics Efficiency vs. Load Current and VIN (Circuit of Figure 12) VCC vs. VIN Figure 3. Figure 4. ON-Time vs Input Voltage and RT Current Limit Off-Time vs. VFB and RCL CURRENT LIMIT OFF TIME (Ps) 35 30 25 20 15 RCL = 500k 300k 10 100k 5 50k 0 0 0.5 1.0 1.5 2.0 2.5 VFB (V) Figure 5. Figure 6. Maximum Frequency vs. VOUT and VIN ICC Current vs. Applied VCC Voltage Figure 7. Figure 8. Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A 5 LM5008A SNVS583F – MARCH 2009 – REVISED MARCH 2013 www.ti.com BLOCK DIAGRAM 6V to 95V Input C5 C1 GND 7V BIAS REGULATOR VIN VIN SENSE LM5008A VCC BYPASS UVLO SWITCH Q2 THERMAL SHUTDOWN VCC C3 RT ON TIMER START 0.7V RT FINISH RT/SD BST SHUTDOWN OVER-VOLTAGE COMPARATOR 2.875V START GD SD UVLO 300 ns MIN OFF TIMER FINISH C4 DRIVER L1 LEVEL SHIFT 2.5V SW S SET Q FB FB FINISH START CURRENT LIMIT OFF TIMER R CL R CL RTN VOUT D1 R CLR Q REGULATION COMPARATOR RCL Vin 0.51A BUCK SWITCH CURRENT SENSE PRE CHARGE R FB2 R FB1 R3 C2 FUNCTIONAL DESCRIPTION The LM5008A 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 control scheme using an on-time inversely proportional to VIN. The control scheme requires no loop compensation. Current limit is implemented with forced off-time, which is inversely proportional to VOUT. This scheme ensures short circuit control while providing minimum foldback. The LM5008A 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. Features include: Thermal Shutdown, VCC under-voltage lockout, Gate drive under-voltage lockout, Max Duty Cycle limit timer, intelligent current limit off timer, and a pre-charge switch. Control Circuit Overview The LM5008A 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 (RT). 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 LM5008A 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 ontime 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: F= 6 VOUT2 x L x 1.04 x 1020 RL x (RT)2 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A LM5008A www.ti.com SNVS583F – MARCH 2009 – REVISED MARCH 2013 where • RL = the load resistance (1) 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: F= VOUT 1.385 x 10-10 x RT (2) The output voltage (VOUT) is programmed by two external resistors as shown in the Block Diagram. The regulation point can be calculated as follows: VOUT = 2.5 x (RFB1 + RFB2) / RFB1 (3) The LM5008A regulates 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 LM5008A. In cases where the capacitor ESR is too small, additional series resistance may be required (R3 in the Block Diagram). 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. L1 SW RFB2 LM5008A R3 FB VOUT2 RFB1 C2 Figure 9. Low Ripple Output Configuration Start-Up Regulator (VCC) The high voltage bias regulator is integrated within the LM5008A. The input pin (VIN) can be connected directly to line voltages between 6V and 95V, with transient capability to 100V. Referring to the block diagram and the graph of VCC vs VIN, when VIN is between 6V and the bypass threshold (nominally 8.5V), the bypass switch (Q2) is on, and VCC tracks VIN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 100Ω, with inherent current limiting at approximately 100 mA. When VIN is above the bypass threshold Q2 is turned off, and VCC is regulated at 7V. The VCC regulator output current is limited at approximately 9.2 mA. When the LM5008A is shutdown using the RT/SD pin, the VCC bypass switch is shut off regardless of the voltage at VIN. When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 - 3 µs. The capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to the VCC and RTN pins. In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin to shut off the VCC regulator, thereby reducing internal power dissipation. The current required into the VCC pin is shown in the graph “ICC Current vs. Applied VCC Voltage”. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than VIN. The turn-on sequence is shown in Figure 10. During the initial delay (t1) VCC ramps up at a rate determined by its current limit and C3 while internal circuitry stabilizes. When VCC reaches the upper threshold of its undervoltage lock-out (UVLO, typically 5.3V) the buck switch is enabled. The inductor current increases to the current limit threshold (ILIM) and during t2 VOUT increases as the output capacitor charges up. When VOUT reaches the intended voltage the average inductor current decreases (t3) to the nominal load current (IO). Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A 7 LM5008A SNVS583F – MARCH 2009 – REVISED MARCH 2013 VIN www.ti.com t1 7V UVLO VCC Vin SW Pin 0V I LIM Inductor Current IO t2 t3 VOUT Figure 10. Startup Sequence 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. 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 LM5008A is determined by the RT 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 LM5008A is: TON = 1.385 x 10-10 x RT / VIN 8 (4) Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A LM5008A www.ti.com SNVS583F – MARCH 2009 – REVISED MARCH 2013 RT 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. The LM5008A can be remotely disabled by taking the RT/SD pin to ground. See Figure 11. The voltage at the RT/SD pin is between 1.5 and 3.0 volts, depending on Vin and the value of the RT resistor. Input Voltage VIN LM5008A RT RT/SD STOP RUN Figure 11. Shutdown Implementation Current Limit The LM5008A contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.51A 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 the graph Current Limit Off-Time vs. VFB and RCL). 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. (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 LM5008A 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 0.01 µF 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. The internal pre-charge switch at the SW pin is turned on for ≊150 ns during the minimum off-time period, ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. This feature helps prevent operating problems which can occur during very light load conditions, involving a long off-time, during which the voltage across the bootstrap capacitor could otherwise reduce below the Gate Drive UVLO threshold. The precharge switch also helps prevent startup problems which can occur if the output voltage is pre-charged prior to turn-on. After current limit detection, the pre-charge switch is turned on for the entire duration of the forced offtime . Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A 9 LM5008A SNVS583F – MARCH 2009 – REVISED MARCH 2013 www.ti.com Thermal Protection The LM5008A should be operated so the junction temperature does not exceed 125°C during normal operation. An internal Thermal Shutdown circuit is provided to shutdown the LM5008A 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 by disabling the buck switch. This feature prevents catastrophic failures from accidental device overheating. When the junction temperature reduces below 140°C (typical hysteresis = 25°C) normal operation is resumed. 10 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A LM5008A www.ti.com SNVS583F – MARCH 2009 – 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 the Block Diagram. The following steps will configure the LM5008A for: • Input voltage range (Vin): 12V to 95V • Output voltage (VOUT1): 10V • Load current (for continuous conduction mode): 100 mA to 300 mA RFB1, RFB2: VOUT = VFB x (RFB1 + RFB2) / RFB1, and since VFB = 2.5V, the ratio of RFB2 to RFB1 calculates as 3:1. Standard values of 3.01 kΩ and 1.00 kΩ are chosen. Other values could be used as long as the 3:1 ratio is maintained. Fs and RT: The recommended operating frequency range for the LM5008A is 50 kHz to 1.1 MHz. 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 400 ns) (6) For this exercise, Fmax = 263 kHz. From Equation 2, RT calculates to 274 kΩ. A standard value 324 kΩ resistor will be used to allow for tolerances in Equation 2, resulting in a frequency of 223 kHz. 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. L1 = VOUT x (VIN - VOUT) 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 182 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 LM5008A’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 LM5008A’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. C3 should be no smaller than 0.47 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. ESR and R3: A low ESR for C2 is generally desirable so as to minimize power losses and heating within the capacitor. However, the regulator requires a minimum amount of ripple voltage at the feedback input for proper loop operation. For the LM5008A the minimum ripple required at pin 5 is 25 mVp-p, requiring a minimum ripple at VOUT of 100 mV. Since the minimum ripple current (at minimum Vin) is 34 mA p-p, the minimum ESR required at VOUT is 100 mV/34 mA = 2.94Ω. Since quality capacitors for SMPS applications have an ESR considerably less than this, R3 is inserted as shown in the Block Diagram. 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. Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A 11 LM5008A SNVS583F – MARCH 2009 – REVISED MARCH 2013 www.ti.com RCL: When current limit is detected, the minimum off-time set by this resistor must be greater than the maximum normal off-time, which occurs at maximum input voltage. Using Equation 4, the minimum on-time is 472 ns, yielding an off-time of 4 µs (at 223 kHz). Due to the 25% tolerance on the on-time, the off-time tolerance is also 25%, yielding a maximum off-time of 5 µs. Allowing for the response time of the current limit detection circuit (350 ns) increases the maximum off-time to 5.35 µs. This is increased an additional 25% to 6.7 µs to allow for the tolerances of Equation 5. Using Equation 5, RCL calculates to 325 kΩ at VFB = 2.5V. A standard value 332 kΩ resistor will be used. 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 a Schottky power diode, such as the DFLS1100. 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 3.74 Ps = 0.56 PF 2.0V (8) 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., RT = 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 LM5008A. FINAL CIRCUIT The final circuit is shown in Figure 12. The circuit was tested, and the resulting performance is shown in Figure 13 and Figure 14. PC BOARD LAYOUT The LM5008A 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 D1 to C1 should be as short and direct as possible. If the internal dissipation of the LM5008A 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. 12 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A LM5008A www.ti.com SNVS583F – MARCH 2009 – REVISED MARCH 2013 12 - 95V Input VCC VIN 7 8 C1 1.0 PF C3 0.47 PF C5 0.1 PF BST RT 2 324k RT/SD C4 0.01 PF LM5008A 6 L1 220 PH 10.0V SW VOUT 1 SHUTDOWN D1 RCL RFB2 3.01k R3 RFB1 1.0k C2 22 PF 3.0 3 RCL 332k FB RTN 5 4 GND Figure 12. LM5008A Example Circuit Table 2. Bill of Materials Item Description Part Number Value C1 Ceramic Capacitor TDK C4532X7R2A105M 1 µF, 100V C2 Ceramic Capacitor TDK C4532X7R1E226M 22 µF, 25V C3 Ceramic Capacitor Kemet C1206C474K5RAC 0.47 µF, 50V C4 Ceramic Capacitor Kemet C1206C103K5RAC 0.01 µF, 50V C5 Ceramic Capacitor TDK C3216X7R2A104M 0.1 µF, 100V D1 Schottky Power Diode Diodes Inc. DFLS1100 100V, 1A L1 Power Inductor COILTRONICS DR125-221-R or TDK SLF10145T-221MR65 220 µH RFB2 Resistor Vishay CRCW12063011F 3.01 kΩ RFB1 Resistor Vishay CRCW12061001F 1.0 kΩ R3 Resistor Vishay CRCW12063R00F 3.0 Ω RT Resistor Vishay CRCW12063243F 324 kΩ RCL Resistor Vishay CRCW12063323F 332 kΩ U1 Switching Regulator LM5008A Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM5008A 13 LM5008A SNVS583F – MARCH 2009 – REVISED MARCH 2013 www.ti.com Figure 13. Efficiency vs. Load Current and VIN Figure 14. Efficiency vs. VIN LOW OUTPUT RIPPLE CONFIGURATIONS For applications where low output ripple is required, the following options can be used to reduce or nearly eliminate the ripple. a) Reduced ripple configuration: In Figure 15, Cff is added across RFB2 to AC-couple the ripple at VOUT directly to the FB pin. This allows the ripple at VOUT to be reduced to a minimum of 25 mVp-p by reducing R3, since the ripple at VOUT is not attenuated by the feedback resistors. The minimum value for Cff is determined from: 3 x tON (max) Cff = (RFB1//RFB2) where • tON(max) is the maximum on-time, which occurs at VIN(min). The next larger standard value capacitor should be used for Cff. (9) L1 SW VOUT Cff LM5008A RFB2 R3 FB RFB1 C2 Figure 15. Reduced Ripple Configuration b) Minimum ripple configuration: If the application requires a lower value of ripple (