LM34910CSDX

LM34910C High Voltage (50V, 1.25A) Step Down Switching Regulator August 2007 LM34910C High Voltage (50V, 1.25A) Step Down Switching Regulator General Description The LM34910C 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 55V 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. ■ Operating frequency remains constant with load current ■ ■ ■ ■ ■ ■ ■ and input voltage Maximum Duty Cycle Limited During Start-Up Adjustable output voltage Valley Current Limit At 1.25A 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 55V, N-Channel buck switch Integrated start-up regulator Input Voltage Range: 8V to 50V No loop compensation required Ultra-Fast transient response Package ■ LLP-10 (4 mm x 4 mm) ■ Exposed Thermal Pad For Improved Heat Dissipation Connection Diagram 30023502 10-Lead LLP Ordering Information Order Number LM34910CSD LM34910CSDX LM34910CSDE Package Type LLP-10 (4x4) LLP-10 (4x4) LLP-10 (4x4) NSC Package Drawing SDC10A SDC10A SDC10A Supplied As 1000 Units on Tape and Reel 4500 Units on Tape and Reel 250 Units on Tape and Reel © 2007 National Semiconductor Corporation 300235 www.national.com LM34910C Typical Application Circuit and Block Diagram 30023501 FIGURE 1. www.national.com 2 LM34910C Pin Descriptions Pin 1 2 3 Name SW BST ISEN Description Switching Node Boost pin for boot-strap capacitor Current sense input 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 50V. 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 LM34910C 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 55V 70V -1.5V 2kV 55V 55V 14V 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 (Note 1) 8.0V to 50V −40°C to + 125°C Operating Ratings VIN Junction Temperature Electrical Characteristics 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 Buck Switch Rds(on) Gate Drive UVLO UVLOGD hysteresis Parameter 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). Conditions Min 6.6 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 ITEST = 200 mA VBST - VSW Increasing 3.0 Typ 7 1.4 140 9 5.8 150 3 0.63 80 0.45 4.3 440 2.5 11.5 Current out of ISEN 1 1.25 130 150 VIN = 10V, RON = 200 kΩ VIN = 50V, RON = 200 kΩ Voltage at RON/SD rising Voltage at RON/SD falling 0.35 2.1 2.75 560 0.65 40 280 SS pin = steady state 2.440 2.5 2.875 100 2.550 1.1 3.6 1.5 1 250 0.95 5.5 Max 7.4 Units V V Ω mA V mV µs mA µA Ω V mV V µA A mΩ ns µs ns V mV ns V V nA 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 Regulation and Over-Voltage Comparators (FB Pin) www.national.com 4 LM34910C Symbol Thermal Shutdown TSD Parameter Thermal shutdown temperature Thermal shutdown hysteresis Conditions Min Typ 175 20 Max Units °C °C 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 30023504 FIGURE 2. VCC vs VIN 30023520 FIGURE 3. ON-Time vs VIN and RON 5 www.national.com LM34910C Functional Description The LM34910C 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 55V 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 LM34910C 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 LM34910C 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 offtimes are at the minimum of 280 ns. Once regulation is established, the off-times are longer. When in regulation, the LM34910C 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 LM34910C 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. 30023510 FIGURE 4. Low Ripple Output Configuration Start-up Regulator, VCC The start-up regulator is integral to the LM34910C. The input pin (VIN) can be connected directly to line voltage up to 50V, with transient capability to 55V. The VCC output regulates at www.national.com 6 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 under-voltage lockout threshold of 5.8V, the buck switch is enabled and the LM34910C 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 70V. Internally, a diode connects VCC to VIN. See Figure 5. 30023511 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. (5) The LM34910C 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. 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 LM34910C is determined by the RON resistor and the input voltage (VIN), and is calculated from: 30023513 (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: 7 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 www.national.com LM34910C 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