LM34910

LM34910 High Voltage (40V, 1.25A) Step Down Switching Regulator February 2005 LM34910 High Voltage (40V, 1.25A) Step Down Switching Regulator General Description The LM34910 Step Down Switching Regulator features all of the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying 1.25A to the load. This buck regulator contains a 40V N-Channel Buck Switch, and is available in the thermally enhanced LLP-10 package. The hysteretic 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 current limit detection is set at 1.25A. Additional features include: VCC under-voltage lockout, thermal shutdown, gate drive under-voltage lockout, and maximum duty cycle limiter. n Ultra-Fast transient response n Operating frequency remains constant with load current and input voltage n Maximum Duty Cycle Limited During Start-Up n Adjustable output voltage n Valley Current Limit At 1.25A n Precision internal reference n Low bias current n Highly efficient operation n Thermal shutdown Typical Applications n High Efficiency Point-Of-Load (POL) Regulator n Non-Isolated Telecommunication Buck Regulator n Secondary High Voltage Post Regulator Features n n n n Integrated 40V, N-Channel buck switch Integrated start-up regulator Input Voltage Range: 8V to 36V No loop compensation required Package n LLP-10 (4 mm x 4 mm) n Exposed Thermal Pad For Improved Heat Dissipation Connection Diagram 20110902 10-Lead LLP Ordering Information Order Number LM34910SD LM34910SDX Package Type LLP-10 (4x4) LLP-10 (4x4) NSC Package Drawing SDC10A SDC10A Supplied As 1000 Units on Tape and Reel 4500 Units on Tape and Reel © 2005 National Semiconductor Corporation DS201109 www.national.com LM34910 Typical Application Circuit and Block Diagram 20110901 FIGURE 1. www.national.com 2 LM34910 Pin Description PIN 1 NAME SW DESCRIPTION Switching Node APPLICATION INFORMATION Internally connected to the buck switch source. Connect to the external inductor, diode, and boost capacitor. Connect a 0.022 µF capacitor from SW to this pin. An internal diode charges the capacitor during the off-time. Internally the current sense resistor connects from this pin to SGND. Re-circulating current flows out of this pin to the free-wheeling diode. Current limit is set at 1.25A. Re-circulating current flows into this pin to the current sense resistor. Ground for all internal circuitry other than the current limit detection. Internally connected to the regulation and over-voltage comparators. The regulation level is 2.5V. An internal 11.5 µA current source charges an external capacitor to 2.5V to provide the softstart function. An external resistor from VIN to this pin sets the buck switch on-time. Grounding this pin shuts down the regulator. Nominally regulated to 7.0V. An external voltage (8V-14V) can be connected to this pin to reduce internal dissipation. An internal diode connects VCC to VIN. Nominal input range is 8.0V to 36V. 2 BST Boost pin for boot-strap capacitor 3 ISEN Current sense input 4 5 6 7 8 SGND RTN FB SS RON/SD Sense Ground Circuit Ground Feedback Softstart On-time Control and Shutdown 9 VCC Output from the start-up regulator 10 VIN Input supply voltage 3 www.national.com LM34910 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 2) Human Body Model BST to VCC VIN to SW BST to SW 2kV 40V 40V 14V 40V 50V -1.5V VCC to GND SGND to RTN Current out of ISEN SS to RTN All Other Inputs to GND Storage Temperature Range JunctionTemperature 14V -0.3V to +0.3V See Text -0.3V to 4V -0.3 to 7V -55˚C to +150˚C 150˚C Operating Ratings (Note 1) VIN Junction Temperature 8.0V to 36V −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 = 24V, RON = 200k unless otherwise stated (Note 5). Symbol Start-Up Regulator, VCC VCCReg VCC regulated output VIN-VCC dropout voltage VCC output impedance VCC current limit (Note 3) UVLOVCC VCC under-voltage lockout threshold UVLOVCC hysteresis UVLOVCC filter delay IIN operating current IIN shutdown current Switch Characteristics Rds(on) UVLOGD Softstart Pin Pull-up voltage Internal current source Current Limit ILIM Threshold Resistance from ISEN to SGND Response time On Timer tON - 1 tON - 2 On-time On-time Shutdown threshold Threshold hysteresis Off Timer tOFF VREF Minimum Off-time FB regulation threshold FB over-voltage threshold FB bias current SS pin = steady state 2.440 280 2.5 2.875 100 2.550 ns V V nA Regulation and Over-Voltage Comparators (FB Pin) VIN = 10V, RON = 200 kΩ VIN = 36V, RON = 200 kΩ Voltage at RON/SD rising Voltage at RON/SD falling 0.35 2.1 2.75 740 0.65 40 1.1 3.6 µs ns V mV Current out of ISEN 1 1.25 130 150 1.5 A mΩ ns 2.5 11.5 V µA Buck Switch Rds(on) Gate Drive UVLO UVLOGD hysteresis ITEST = 200 mA VBST - VSW Increasing 3.0 0.45 4.3 440 0.95 5.5 Ω V mV ICC = 0 mA, VCC = VCCReg - 100 mV 0 mA ≤ ICC ≤ 5 mA VCC = 0V VCC increasing VCC decreasing 100 mV overdrive Non-switching, FB = 3V RON/SD = 0V 6.6 7 1.4 140 9 5.8 150 3 0.63 80 1 250 7.4 V V Ω mA V mV µs mA µA Parameter Conditions Min Typ Max Units www.national.com 4 LM34910 Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 24V, RON = 200k unless otherwise stated (Note 5). (Continued) Symbol Thermal Shutdown TSD Thermal shutdown temperature Thermal shutdown hysteresis 175 20 ˚C ˚C Parameter Conditions Min Typ Max Units 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 plastic LLP packages, refer to the Packaging Data Book available from National Semiconductor Corporation. Note 5: Typical specifications represent the most likely parametric norm at 25˚C operation. Typical Performance Characteristics 20110904 FIGURE 2. VCC vs VIN 20110905 FIGURE 3. ON-Time vs VIN and RON 5 www.national.com LM34910 Functional Description The LM34910 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient buck bias power converter capable of supplying 1.25A to the load. This high voltage regulator contains a 40V N-Channel buck switch, is easy to implement, and is available in the thermally enhanced LLP-10 package. The regulator’s operation is based on a hysteretic control scheme, and uses an on-time control which varies inversely with VIN. This feature allows the operating frequency to remain relatively constant with load and input voltage variations. The hysteretic control requires no loop compensation resulting in very fast load transient response. The valley current limit detection circuit, internally set at 1.25A, holds the buck switch off until the high current level subsides. The functional block diagram is shown in Figure 1. The LM34910 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. (1) The buck switch duty cycle is equal to : (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 (C2). 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 reduce with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: Hysteretic Control Circuit Overview The LM34910 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 for a minimum of 280 ns, and until the FB voltage falls below the reference. 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 of 280 ns. Once regulation is established, the off-times are longer. When in regulation, the LM34910 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 conduction mode is one-half the inductor’s ripple current amplitude. The operating frequency is approximately: (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: VOUT = 2.5 x (R1 + R2) / R2 Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. The LM34910 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 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 4. However, R3 slightly degrades the load regulation. 20110910 FIGURE 4. Low Ripple Output Configuration www.national.com 6 LM34910 Start-up Regulator, VCC The start-up regulator is integral to the LM34910. The input pin (VIN) can be connected directly to line voltage up to 36V, with transient capability to 40V. The VCC output regulates at 7.0V, and is current limited to 9 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 5.8V, 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 ()5.7V), 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 since the output impedance at VCC is )140Ω. See Figure 2. 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 8V 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 50V. Internally, a diode connects VCC to VIN. See Figure 5. 20110911 FIGURE 5. Self Biased Configuration Regulation Comparator 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 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. Bias current at the FB pin is nominally 100 nA. (4) See Figure 3. The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. RON should be selected for a minimum on-time (at maximum VIN) greater than 200 ns. This requirement limits the maximum frequency for each application, depending on VIN and VOUT, calculated from the following: Over-Voltage Comparator The 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 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 for the LM34910 is determined by the RON resistor and the input voltage (VIN), and is calculated from: (5) The LM34910 can be remotely shut down by taking the RON/SD pin below 0.65V. See Figure 6. 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 1.5V and 3.0V, depending on VIN and the RON resistor. 7 www.national.com LM34910 ON-Time Timer, and Shutdown (Continued) 20110913 FIGURE 6. Shutdown Implementation Current Limit Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling diode (D1). Referring to Figure 1, 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 1.25A the current limit comparator output switches to delay the start of the next on-time period if the voltage at FB is below 2.5V. The next on-time starts when the current out of ISEN is below 1.25A and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 1.25A during each on-time, that is detected at the beginning of each off-time. The operating frequency may be lower due to longer-than-normal off-times. Figure 7 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 1.25A. During the Current Limited portion of Figure 7, the current ramps down to 1.25A 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: ∆I = (VIN - VOUT) x tON / L1 During this time the LM34910 is in a constant current mode, with an average load current (IOCL) equal to 1.25A + ∆I/2. 20110914 FIGURE 7. Inductor Current - Current Limit Operation The current limit threshold can be increased by connecting an external resistor between SGND and ISEN. The external resistor will typically be less than 1Ω. The peak current out of SW and ISEN must not exceed 3.5A. The average current out of SW must be less than 3A, and the average current out of ISEN must be less than 2A. Therefore IPK in Figure 7 must not exceed 3.5A, and IOCL must not exceed 2A. at approximately -1V, and C4 charges from VCC through the internal diode. The minimum off-time of 280 ns ensures a minimum time each cycle to recharge the bootstrap capacitor. 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 11.5 µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and the noninverting 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, if a thermal shutdown occurs, or if the RON/SD pin is grounded. N - Channel Buck Switch and Driver The LM34910 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is 3A. 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 www.national.com 8 LM34910 Thermal Shutdown The LM34910 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 and the on-timer, and grounding the Softstart pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below 155˚C (typical hysteresis = 20˚C), the Softstart pin is released and normal operation resumes. smaller value should then be used for L1. L1 must be rated for the peak value of the current waveform (IPK in Figure 7). C3: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents 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, and should be a good quality, low ESR, ceramic capacitor. C2, and R3: Since the LM34910 requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the required ripple at VOUT1 is increased by R1 and R2. This necessary ripple is created by the inductor ripple current acting on C2’s ESR + R3. The minimum ripple current is calculated using equation 7, rearranged to solve for IOR at minimum VIN. The minimum ESR for C2 is then equal to: Applications Information EXTERNAL COMPONENTS The following guidelines can be used to select the external components. R1 and R2: The ratio of these resistors is calculated from: R1/R2 = (VOUT/2.5V) - 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. RON: The minimum value for RON is calculated from: (10) If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Figure 1. Generally R3 is less than 1Ω. C2 should generally be no smaller than 3.3 µF, although that is dependent on the frequency and the allowable ripple amplitude at VOUT1. 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. 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 mainly this diode’s voltage (plus the voltage across the current limit sense resistor) which forces the inductor current to decrease during the off-time. For this reason, a higher voltage is better, although that affects efficiency. A reverse recovery time of )30 ns, and a forward voltage drop of )0.75V are preferred. The reverse leakage specification is important as that can significantly affect efficiency. D1’s reverse voltage rating must be at least as great as the maximum VIN, and its current rating must equal or exceed IPK Figure 7. 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. If the source’s dynamic impedance is high (effectively a current source), it supplies the average input current, but not the ripple current. At maximum load current, when the buck switch turns on, the current into VIN suddenly increases to the lower peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average current during the on-time is the load current. For a worst case calculation, C1 must supply this average load current during the maximum on-time. C1 is calculated from: Equation 1 can be used to select RON if a specific frequency is desired as long as the above limitation is met. L1: The main parameter affected by the inductor is the output current ripple amplitude (IOR). The limits for IOR must be determined at both the minimum and maximum nominal load currents. a) If the maximum load current is less than the current limit threshold (1.25A), the minimum load current is used to determine the maximum allowable ripple. To maintain continuous conduction mode the lower peak should not reach 0 mA. For this case, the maximum ripple current is: (6) IOR(MAX1) = 2 x IO(min) The ripple calculated in Equation 6 is then used in the following equation: (7) where VIN is the maximum input voltage and Fs is determined from equation 1. This provides a minimum value for L1. The next larger standard value should be used, and L1 should be rated for the IPK current level. b) If the maximum load current is greater than the current limit threshold (1.25A), the LM34910 ensures the lower peak reaches 1.25A each cycle, requiring that IOR be at least twice the difference. The upper peak, however, must not exceed 3.5A. For this case, the ripple limits are: (8) IOR(MAX2) = 2 x (3.5A - IO(max)) and (9) IOR(MIN1) = 2 x (IO(max) - 1.25A) The lesser of Equations 8 and 9 is then used in Equation 7. If IOR(MAX2) is used, the maximum VIN is used in Equation 7. The next larger value should then be used for L1. If IOR(MIN1) is used, the minimum VIN is used in Equation 7. The next 9 where Io is the load current, tON is the maximum on-time, and ∆V is the allowable ripple voltage at VIN. C5’s purpose is to help avoid transients and ringing due to long lead inductance at VIN. A low ESR, 0.1 µF ceramic chip capacitor is recommended, located close to the LM34910 . www.national.com LM34910 Applications Information (Continued) 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 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 reference voltage at the regulation comparator, and the output voltage, to reach their final value. The time is determined from the following: 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 current loop formed by D1, L1, C2 and the SGND and ISEN pins should be as small as possible. The ground connection from C2 to C1 should be as short and direct as possible. If it is expected that the internal dissipation of the LM34910 will produce 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 IC package can be soldered to a ground plane, and that plane should extend out from beneath the IC, and be connected to ground plane on the board’s other side with several vias, to help dissipate the heat. The exposed pad is internally connected to the IC substrate. Additionally 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. PC BOARD LAYOUT The LM34910 regulation, over-voltage, and current limit comparators are very fast, and respond to short duration noise pulses. Layout considerations are therefore critical for www.national.com 10 LM34910 High Voltage (40V, 1.25A) Step Down Switching Regulator Physical Dimensions inches (millimeters) unless otherwise noted 10-Lead LLP Package NS Package Number SDC10A National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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