LM34919 40V, 600 mA Step Down COT Switching Regulator General Description The LM34919 Step Down Switching Regulator features all of the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying 0.6A to the load. This buck regulator contains an N-Channel Buck Switch, and is available in a micro SMD package. The constant on-time feedback regulation scheme requires no loop compensation, results in fast load transient response, and simplifies circuit implementation. The operating frequency remains constant with line and load variations due to the inverse relationship between the input voltage and the on-time. The valley current limit results in a smooth transition from constant voltage to constant current mode when current limit is detected, reducing the frequency and output voltage, without the use of foldback. Additional features include: VCC under-voltage lockout, thermal shutdown, gate drive under-voltage lockout, and maximum duty cycle limiter. Features ■ Integrated N-Channel buck switch ■ Integrated start-up regulator ■ Input Voltage Range: 8V to 40V ■ No loop compensation required ■ Ultra-Fast transient response ■ Operating frequency remains constant with load current ■ ■ ■ ■ ■ ■ ■ ■ and input voltage Maximum switching frequency: 1.6 MHz Maximum Duty Cycle Limited During Start-Up Adjustable output voltage Valley Current Limit At 0.64A Precision internal reference Low bias current Highly efficient operation Thermal shutdown Typical Applications ■ High Efficiency Point-Of-Load (POL) Regulator ■ Non-Isolated Telecommunication Buck Regulator ■ Secondary High Voltage Post Regulator Package ■ micro SMD Basic Step Down Regulator 30004431 © 2007 National Semiconductor Corporation 300044 www.national.com LM34919 40V, 600 mA Step Down COT Switching Regulator November 1, 2007 LM34919 Connection Diagrams 30004433 Top View 30004402 Bump Side Ordering Information Order Number Package Type NSC Package Drawing Junction Temperature Range Supplied As LM34919TL 10-Bump micro SMD TLP10A1A −40°C to + 125°C 250 Units on Tape and Reel LM34919TLX 10-Bump micro SMD TLP10A1A −40°C to + 125°C 3000 Units on Tape and Reel www.national.com 2 LM34919 Pin Descriptions Pin Number Name A1 RON/SD A2 RTN A3 FB B1 SGND B3 SS C1 Description Application Information On-time control and shutdown An external resistor from VIN to this pin sets the buck switch on-time. Grounding this pin shuts down the regulator. Circuit Ground Ground for all internal circuitry other than the current limit detection. Feedback input from the regulated output Internally connected to the regulation and over-voltage comparators. The regulation level is 2.5V. Sense Ground Re-circulating current flows into this pin to the current sense resistor. Softstart An internal current source charges an external capacitor to 2.5V, providing the softstart function. ISEN Current sense The re-circulating current flows through the internal sense resistor, and out of this pin to the free-wheeling diode. Current limit is nominally set at 0.64A. C3 VCC Output from the startup regulator Nominally regulates at 7.0V. An external voltage (7V-14V) can be applied to this pin to reduce internal dissipation. An internal diode connects VCC to VIN. D1 VIN Input supply voltage Nominal input range is 8.0V to 40V. D2 SW Switching Node Internally connected to the buck switch source. Connect to the inductor, free-wheeling diode, and bootstrap capacitor. D3 BST Boost pin for bootstrap capacitor Connect a 0.022 µF capacitor from SW to this pin. The capacitor is charged from VCC via an internal diode during each off-time. 3 www.national.com LM34919 BST to SW VCC to RTN SGND to RTN SS to RTN All Other Inputs to RTN Storage Temperature Range JunctionTemperature 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 RTN BST to RTN SW to RTN (Steady State) ESD Rating (Note 2) Human Body Model BST to VCC VIN to SW 44V 52V -1.5V 14V 14V -0.3V to +0.3V -0.3V to 4V -0.3 to 7V -65°C to +150°C 150°C Operating Ratings 2kV 44V 44V (Note 1) VIN Junction Temperature 8.0V to 40V −40°C to + 125°C Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated the following conditions apply: VIN = 12V, RON = 200kΩ. See (Note 5). Symbol Parameter Conditions Min Typ Max Units 6.6 7 7.4 V Start-Up Regulator, VCC VCCReg UVLOVCC VCC regulated output VIN-VCC dropout voltage ICC = 0 mA, VCC = UVLOVCC + 250 mV 1.2 V VCC output impedance 0 mA ≤ ICC ≤ 5 mA, VIN = 8V 175 Ω VCC current limit (Note 3) VCC = 0V 9.5 mA VCC under-voltage lockout threshold VCC increasing 5.7 V UVLOVCC hysteresis VCC decreasing 150 mV UVLOVCC filter delay 100 mV overdrive IQ IIN operating current Non-switching, FB = 3V, SW = Open 0.5 3 0.8 mA µs ISD IIN shutdown current RON/SD = 0V, SW = Open 75 150 µA 0.5 1.0 Ω 4.4 5.2 Switch Characteristics Rds(on) Buck Switch Rds(on) ITEST = 200 mA UVLOGD Gate Drive UVLO VBST - VSW Increasing 3.0 V UVLOGD hysteresis 480 mV Pull-up voltage 2.5 V 10.5 µA Softstart Pin VSS Internal current source VSS = 1V Current Limit ILIM Threshold Current out of ISEN 0.52 0.64 0.76 A Resistance from ISEN to SGND 140 mΩ Response time 150 ns On Timer tON - 1 On-time VIN = 10V, RON = 200 kΩ tON - 2 On-time VIN = 40V, RON = 200 kΩ Shutdown threshold Voltage at RON/SD rising Threshold hysteresis Voltage at RON/SD 2.1 2.77 3.5 700 0.45 0.8 µs ns 1.2 V 25 mV 155 ns Off Timer tOFF Minimum Off-time Regulation and Over-Voltage Comparators (FB Pin) VREF FB regulation threshold SS pin = steady state FB over-voltage threshold FB bias current www.national.com FB = 3V 4 2.440 2.5 2.550 V 2.9 V 1 nA Parameter Conditions Min Typ Max Units Thermal Shutdown TSD Thermal shutdown temperature 175 °C Thermal shutdown hysteresis 20 °C 61 °C/W Thermal Resistance θJA Junction to Ambient 0 LFPM Air Flow 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: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading Note 4: For detailed information on soldering micro SMD package, refer to the Application Note AN-1112. Note 5: Typical specifications represent the most likely parametric norm at 25°C operation. 5 www.national.com LM34919 Symbol LM34919 Typical Performance Characteristics VCC vs VIN VCC vs ICC 30004405 30004404 ICC vs Externally Applied VCC ON-TIME vs VIN and RON 30004435 30004436 Voltage at the RON/SD Pin Shutdown and Operating Current into VIN 30004437 www.national.com 30004438 6 LM34919 Typical Application Circuit and Block Diagram 30004401 7 www.national.com LM34919 30004434 FIGURE 1. Start Up Sequence Functional Description Control Circuit Overview The LM34919 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient buck bias power converter capable of supplying at least 0.6A to the load. This high voltage regulator contains an N-Channel buck switch, is easy to implement, and is available in micro SMD package. The regulator’s operation is based on a constant ontime control scheme, where the on-time is determined by VIN. This feature allows the operating frequency to remain relatively constant with load and input voltage variations. The feedback control requires no loop compensation resulting in very fast load transient response. The valley current limit detection circuit, internally set at 0.64A, holds the buck switch off until the high current level subsides. This scheme protects against excessively high current if the output is short-circuited when VIN is high. The LM34919 can be applied in numerous applications to efficiently regulate down higher voltages. Additional features include: Thermal shutdown, VCC under-voltage lockout, gate drive under-voltage lockout, and maximum duty cycle limiter. The LM34919 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below the reference the buck switch is turned on for a time period determined by the input voltage and a programming resistor (RON). Following the on-time the switch remains off until the FB voltage falls below the reference but not less than the minimum off-time. The buck switch then turns on for another on-time period. Typically, during start-up, or when the load current increases suddenly, the off-times are at the minimum. Once regulation is established, the off-times are longer. When in regulation, the LM34919 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous conduction mode current always flows through the inductor, never reaching zero during the off-time. In this mode the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduc- www.national.com 8 VOUT = 2.5 x (R1 + R2) / R2 Output voltage regulation is based on ripple voltage at the feedback input,normally obtained from the output voltage ripple through the feedback resistors. The LM34919 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases where the capacitor’s ESR is insufficient additional series resistance may be required (R3). (1) The buck switch duty cycle is approximately equal to: Start-Up Regulator, VCC The start-up regulator is integral to the LM34919. The input pin (VIN) can be connected directly to line voltage up to 40V, with transient capability to 44V. The VCC output regulates at 7.0V, and is current limited at 9.5 mA. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC pin reaches the undervoltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is released to allow the Softstart capacitor (C6) to charge up. The minimum input voltage is determined by the regulator’s dropout voltage, the VCC UVLO falling threshold (≊5.55V), and the frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the output. If VCC is externally loaded, the minimum input voltage increases. To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the VCC pin. Setting the auxiliary voltage to between 7V and 14V shuts off the internal regulator, reducing internal power dissipation. The sum of the auxiliary voltage and the input voltage (VCC + VIN) cannot exceed 52V. Internally, a diode connects VCC to VIN. See Figure 2. (2) In discontinuous conduction mode current through the inductor ramps up from zero 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 reference - until then the inductor current remains zero, and the load current is supplied by the output capacitor. In this mode the operating frequency is lower than in continuous conduction mode, and varies with load current. Conversion efficiency is maintained at light loads since the switching losses decrease with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: (3) where RL = the load resistance. The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as follows: 30004411 FIGURE 2. Self Biased Configuration 9 www.national.com LM34919 tion mode is one-half the inductor’s ripple current amplitude. The operating frequency is approximately: LM34919 Regulation Comparator Current Limit The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). 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 stays on for the programmed on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch stays off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 100 nA over temperature. Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling diode (D1). Referring to the Block Diagram, when the buck switch is turned off the inductor current flows through the load, into SGND, through the sense resistor, out of ISEN and through D1. If that current exceeds 0.64A the current limit comparator output switches to delay the start of the next on-time period. The next on-time starts when the current out of ISEN is below 0.64A and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 0.64A during each on-time, that is detected at the beginning of each off-time. The operating frequency is lower due to longer-than-normal off-times. Figure 4 illustrates the inductor current waveform. During normal operation the load current is Io, the average of the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches 0.64A. During the Current Limited portion of Figure 4, the current ramps down to 0.64A during each off-time, initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an amount equal to: Over-Voltage Comparator The voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time pulse is immediately terminated. This condition can occur if the input voltage or the output load changes suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V. ON-Time Timer, and Shutdown The on-time is determined by the RON resistor and the input voltage (VIN), and is calculated from: ΔI = (VIN - VOUT) x tON / L1 During this time the LM34919 is in a constant current mode, with an average load current (IOCL) equal to 0.64A + ΔI/2. Generally, in applications where the switching frequency is higher than ≊300 kHz and uses a small value inductor, the higher dl/dt of the inductor's ripple current results in an effectively lower valley current limit threshold due to the response time of the current limit detection circuit. However, since the small value inductor results in a relatively high ripple current amplitude (ΔI in Figure 4), the load current (IOCL) at current limit is typically in excess of 640 mA. (4) The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific continuous conduction mode switching frequency (FS), the RON resistor is determined from the following: (5) In high frequency applicatons the minimum value for tON is limited by the maximum duty cycle required for regulation and the minimum off-time of (155 ns, ±15%). The minimum offtime limits the maximum duty cycle achievable with a low voltage at VIN. At high values of VIN, the minimum on-time is limited to ≊ 120 ns. The LM34919 can be remotely shut down by taking the RON/ SD pin below 0.8V. See Figure 3. In this mode the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD pin allows normal operation to resume. The voltage at the RON/SD pin is between 1.4V and 4.0V, depending on VIN and the RON resistor. 30004413 FIGURE 3. Shutdown Implementation www.national.com 10 LM34919 30004414 FIGURE 4. Inductor Current - Current Limit Operation N - Channel Buck Switch and Driver Applications Information The LM34919 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak current allowed through the buck switch is 1.5A, and the maximum allowed average current is 1A. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022 µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately -1V, and C4 charges from VCC through the internal diode. The minimum off-time forced by the LM34919 ensures a minimum time each cycle to recharge the bootstrap capacitor. EXTERNAL COMPONENTS The procedure for calculating the external components is illustrated with the following design example. Referring to the Block Diagram, the circuit is to be configured for the following specifications: - VOUT = 5V - VIN = 8V to 40V - Minimum load current = 200 mA - Maximum load current = 600 mA - Switching Frequency = 800 kHz - Soft-start time = 5 ms R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from: Softstart The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal 10.5 µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner. An internal switch grounds the SS pin if VCC is below the under-voltage lockout threshold, or if the RON/SD pin is grounded. R1/R2 = (VOUT/2.5V) - 1 For this example, R1/R2 = 1. R1 and R2 should be chosen from standard value resistors in the range of 1.0 kΩ - 10 kΩ which satisfy the above ratio. For this example, 2.49kΩ is chosen for R1 and R2. RON: This resistor sets the on-time, and (by default) the switching frequency. The switching frequncy must be less than 1.6 MHz to ensure the minimum forced off-time does not interfere with the circuit's proper operation. The RON resistor is calculated from the following equation, using the minimum input voltage. Thermal Shutdown The LM34919 should be operated so the junction temperature does not exceed 125°C. If the junction temperature increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes the controller to a low power reset state by disabling the buck switch. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below 155°C (typical hysteresis = 20°C) normal operation resumes. Check that this value resistor does not set an on-time less than 120 ns at maximum VIN. A standard value 43.2 kΩ resistor is used, resulting in a nominal frequency of 806 kHz. The minimum on-time is ≊231 ns at Vin = 40V, and the maximum on-time is ≊875 ns at Vin = 8V. Alternately, RON can be determined using Equation 4 if a specific on-time is required. 11 www.national.com LM34919 inductor’s ripple current, ramps up to the upper peak, then drops to zero at turn-off. The average current during the ontime is the load current. For a worst case calculation, C1 must supply this average load current during the maximum on-time, without letting the voltage at VIN drop below ≊7.5V. The minimum value for C1 is calculated from: L1: The main parameter affected by the inductor is the inductor current ripple amplitude (IOR). The minimum load current is used to determine the maximum allowable ripple in order to maintain continuous conduction mode, where the lower peak does not reach 0 mA. This is not a requirement of the LM34919, but serves as a guideline for selecting L1. For this case the maximum ripple current is: IOR(MAX) = 2 x IOUT(min) = 400 mA (6) If the minimum load current is zero, use 20% of IOUT(max) for IOUT(min) in equation 6. The ripple calculated in Equation 6 is then used in the following equation: where tON is the maximum on-time, and ΔV is the allowable ripple voltage (0.5V at VIN = 8V). C5’s purpose is to minimize transients and ringing due to long lead inductance leading to the VIN pin. A low ESR, 0.1 µF ceramic chip capacitor must be located close to the VIN and RTN pins. C3: The capacitor at the VCC pin provides noise filtering and stability for the Vcc regulator. C3 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic capacitor. C3’s value, and the VCC current limit, determine a portion of the turn-on-time (t1 in Figure 1). C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended as C4 supplies a surge current to charge the buck switch gate at each turn-on. A low ESR also helps ensure a complete recharge during each off-time. C6: The capacitor at the SS pin determines the softstart time, i.e. the time for the output voltage, to reach its final value (t2 in Figure 1). The capacitor value is determined from the following: (7) A standard value 15 µH inductor is selected. The maximum ripple amplitude, which occurs at maximum VIN, calculates to 362 mA p-p, and the peak current is 781 mA at maximum load current. Ensure the selected inductor is rated for this peak current. C2 and R3: Since the LM34919 requires a minimum of 25 mVp-p ripple at the FB pin for proper operation, the required ripple at VOUT is increased by R1 and R2. This necessary ripple is created by the inductor ripple current flowing through R3, and to a lesser extent by C2 and its ESR. The minimum inductor ripple current is calculated using equation 7, rearranged to solve for IOR at minimum VIN. The minimum value for R3 is equal to: D1: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high speed transitions at the SW pin may inadvertently affect the IC’s operation through external or internal EMI. The diode should be rated for the maximum input voltage, the maximum load current, and the peak current which occurs when the current limit and maximum ripple current are reached simultaneously. The diode’s average power dissipation is calculated from: A standard value 0.39Ω resistor is used for R3 to allow for tolerances. C2 should generally be no smaller than 3.3 µF, although that is dependent on the frequency and the desired output characteristics. C2 should be a low ESR good quality ceramic capacitor. Experimentation is usually necessary to determine the minimum value for C2, as the nature of the load may require a larger value. A load which creates significant transients requires a larger value for C2 than a non-varying load. C1 and C5: C1’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 VIN suddenly increases to the lower peak of the www.national.com PD1 = VF x IOUT x (1-D) where VF is the diode’s forward voltage drop, and D is the ontime duty cycle. FINAL CIRCUIT The final circuit is shown in Figure 5, and its performance is shown in Figure 6 and Figure 7. Current limit measured approximately 650 mA at 8V, and 740 mA at 40V. 12 LM34919 30004421 FIGURE 5. Example Circuit 30004440 FIGURE 6. Efficiency vs. Load Current and VIN (Circuit of Figure 5) 30004423 FIGURE 7. Frequency vs. VIN (Circuit of Figure 5) 13 www.national.com LM34919 LOW OUTPUT RIPPLE CONFIGURATIONS For applications where lower ripple at VOUT is required, the following options can be used to reduce or nearly eliminate the ripple. a) Reduced ripple configuration: In Figure 8, Cff is added across R1 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: 30004427 FIGURE 9. Minimum Output Ripple Using Ripple Injection c) Alternate minimum ripple configuration: The circuit in Figure 10 is the same as that in Figure 5, except the output voltage is taken from the junction of R3 and C2. The ripple at VOUT is determined by the inductor’s ripple current and C2’s characteristics. However, R3 slightly degrades the load regulation. This circuit may be suitable if the load current is fairly constant. where tON(max) is the maximum on-time, which occurs at VIN The next larger standard value capacitor should be used for Cff. R1 and R2 should each be towards the upper end of the 2 kΩ to 10 kΩ range. (min). 30004428 30004425 FIGURE 10. Alternate Minimum Output Ripple Configuration FIGURE 8. Reduced Ripple Configuration b) Minimum ripple configuration: The circuit of Figure 9 provides minimum ripple at VOUT, determined primarily by C2’s characteristics and the inductor’s ripple current since R3 is removed. RA and CA are chosen to generate a sawtooth waveform at their junction, and that voltage is AC-coupled to the FB pin via CB. To determine the values for RA, CA and CB, use the following procedure: Minimum Load Current The LM34919 requires a minimum load current of 1 mA. If the load current falls below that level, the bootstrap capacitor (C4) 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, R1 and R2 should be chosen low enough in value so they provide the minimum required current at nominal VOUT. Calculate VA = VOUT - (VSW x (1 - (VOUT/VIN(min)))) where VSW is the absolute value of the voltage at the SW pin during the off-time (typically 1V). VA is the DC voltage at the RA/CA junction, and is used in the next equation. PC BOARD LAYOUT Refer to application note AN-1112 for PC board guidelines for the Micro SMD package. The LM34919 regulation, over-voltage, and current limit comparators are very fast, and respond to short duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be as neat and compact as possible, and all of the components must be as close as possible to their associated pins. The two major current loops have currents which switch very fast, and so the loops should be as small as possible to minimize conducted and radiated EMI. The first loop is that formed by C1, through the VIN to SW pins, L1, C2, and back to C1.The second current loop is formed by D1, L1, C2 and the SGND and ISEN pins. The power dissipation within the LM34919 can be approximated by determining the total conversion loss (PIN - POUT), and then subtracting the power losses in the free-wheeling diode and the inductor. The power loss in the diode is approximately: where tON is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the RA/CA junction, typically 100 mV. RA and CA are then chosen from standard value components to satisfy the above product. Typically CA is 3000 pF to 5000 pF, and RA is 10 kΩ to 300 kΩ. CB is then chosen large compared to CA, typically 0.1 µF. R1 and R2 should each be towards the upper end of the 2 kΩ to 10 kΩ range. PD1 = Iout x VF x (1-D) where Iout is the load current, VF is the diode’s forward voltage drop, and D is the on-time duty cycle. The power loss in the inductor is approximately: www.national.com 14 ditionally the use of wide PC board traces, where possible, can help conduct heat away from the IC. Judicious positioning of the PC board within the end product, along with the use of any available air flow (forced or natural convection) can help reduce the junction temperatures. where RL is the inductor’s DC resistance, and the 1.1 factor is an approximation for the AC losses. If it is expected that the internal dissipation of the LM34919 will produce excessive junction temperatures during normal operation, good use of the PC board’s ground plane can help to dissipate heat. Ad- 15 www.national.com LM34919 PL1 = Iout2 x RL x 1.1 LM34919 Physical Dimensions inches (millimeters) unless otherwise noted Note: X1 = 1.514 mm, ±0.030 mm X2 = 1.970 mm, ±0.030 mm X3 = 0.60 mm, ±0.075 mm 10 Bump micro SMD Package NS Package Number TLP10A1A www.national.com 16 LM34919 Notes 17 www.national.com LM34919 40V, 600 mA Step Down COT Switching Regulator Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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