LM34919TL

LM34919 Ultra Small 40V, 600 mA Constant On-Time Buck Switching Regulator June 2, 2008 LM34919 Ultra Small 40V, 600 mA Constant On-Time Buck 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. ■ No loop compensation required ■ Ultra-Fast transient response ■ Operating frequency remains constant with load current ■ ■ ■ ■ ■ ■ ■ ■ and input voltage Maximum switching frequency: 2.0 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 Features ■ Integrated N-Channel buck switch ■ Integrated start-up regulator ■ Input Voltage Range: 8V to 40V Package ■ micro SMD Basic Step Down Regulator 30004431 © 2008 National Semiconductor Corporation 300044 www.national.com LM34919 Connection Diagrams 30004433 Top View 30004402 Bump Side Ordering Information Order Number LM34919TL LM34919TLX Package Type 10-Bump micro SMD 10-Bump micro SMD NSC Package Drawing TLP10A1A TLP10A1A Junction Temperature Range −40°C to + 125°C −40°C to + 125°C Supplied As 250 Units on Tape and Reel 3000 Units on Tape and Reel www.national.com 2 LM34919 Pin Descriptions Pin Number A1 A2 A3 B1 B3 C1 Name RON/SD RTN FB SGND SS ISEN Description On-time control and shutdown Circuit Ground Feedback input from the regulated output Sense Ground Softstart Current sense Application Information An external resistor from VIN to this pin sets the buck switch on-time. Grounding this pin shuts down the regulator. 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. Re-circulating current flows into this pin to the current sense resistor. An internal current source charges an external capacitor to 2.5V, providing the softstart function. 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. 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. Nominal input range is 8.0V to 40V. Internally connected to the buck switch source. Connect to the inductor, free-wheeling diode, and 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. C3 VCC Output from the startup regulator D1 D2 D3 VIN SW BST Input supply voltage Switching Node Boost pin for bootstrap capacitor 3 www.national.com LM34919 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 2kV 44V 44V BST to SW VCC to RTN SGND to RTN SS to RTN All Other Inputs to RTN Storage Temperature Range JunctionTemperature 14V 14V -0.3V to +0.3V -0.3V to 4V -0.3 to 7V -65°C to +150°C 150°C (Note 1) 8.0V to 40V −40°C to + 125°C Operating Ratings VIN Junction Temperature 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 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 IQ ISD Rds(on) UVLOGD Softstart Pin VSS 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 FB = 3V SS pin = steady state 2.440 155 2.5 2.9 1 2.550 ns V V nA Regulation and Over-Voltage Comparators (FB Pin) VIN = 10V, RON = 200 kΩ VIN = 40V, RON = 200 kΩ Voltage at RON/SD rising Voltage at RON/SD 0.45 2.1 2.77 700 0.8 25 1.2 3.5 µs ns V mV Current out of ISEN 0.52 0.64 140 150 0.76 A mΩ ns VSS = 1V 2.5 10.5 V µA IIN operating current IIN shutdown current Buck Switch Rds(on) Gate Drive UVLO UVLOGD hysteresis ICC = 0 mA, VCC = UVLOVCC + 250 mV 0 mA ≤ ICC ≤ 5 mA, VIN = 8V VCC = 0V VCC increasing VCC decreasing 100 mV overdrive Non-switching, FB = 3V, SW = Open RON/SD = 0V, SW = Open ITEST = 200 mA VBST - VSW Increasing 3.0 6.6 7 1.2 175 9.5 5.7 150 3 0.5 75 0.5 4.4 480 0.8 150 1.0 5.2 7.4 V V Ω mA V mV µs mA µA Ω V mV Parameter Conditions Min Typ Max Units Switch Characteristics www.national.com 4 LM34919 Symbol Thermal Shutdown TSD Parameter Thermal shutdown temperature Thermal shutdown hysteresis Conditions Min Typ 175 20 61 Max Units °C °C °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 Typical Performance Characteristics Efficiency at 800 kHz Efficiency at 2 MHz 30004441 30004442 VCC vs VIN VCC vs ICC 30004405 30004404 ICC vs Externally Applied VCC ON-TIME vs VIN and RON 30004435 30004436 www.national.com 6 LM34919 Voltage at the RON/SD Pin Shutdown and Operating Current into VIN 30004437 30004438 Typical Application Circuit and Block Diagram 30004401 7 www.national.com LM34919 30004434 FIGURE 1. Start Up Sequence Functional Description 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. www.national.com 8 Control Circuit Overview 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- LM34919 tion mode is one-half the inductor’s ripple current amplitude. The operating frequency is approximately: 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 (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: 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. (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 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 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 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