LM34910SD/NOPB

LM34910 www.ti.com SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 LM34910 High Voltage (40V, 1.25A) Step Down Switching Regulator Check for Samples: LM34910 FEATURES 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 WSON-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. 1 2 • • • • • • • Integrated 40V, N-Channel Buck Switch Integrated Start-Up Regulator Input Voltage Range: 8V to 36V No Loop Compensation Required Ultra-Fast Transient Response 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 • • • Package WSON-10 (4 mm x 4 mm) Exposed Thermal Pad For Dissipation High Efficiency Point-Of-Load (POL) Regulator Non-Isolated Telecommunication Buck Regulator Secondary High Voltage Post Regulator Improved Heat Connection Diagram SW 1 10 VIN BST 2 9 VCC ISEN 3 8 RON/SD SGND 4 7 SS RTN 5 6 FB Figure 1. 10-Lead WSON See DPR0010A Package 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 © 2004–2013, Texas Instruments Incorporated LM34910 SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 www.ti.com Typical Application Circuit and Block Diagram 7V SERIES REGULATOR 8V-36V Input LM34910 10 VIN VCC 9 C3 VCC UVLO C5 THERMAL SHUTDOWN C1 RON RON/ 8 SD 280 ns OFF TIMER ON TIMER RON + 0.7V START COMPLETE START COMPLETE BST 2 GATE DRIVE UVLO C4 VIN 2.5V 11.5 PA 7 DRIVER LOGIC SS C6 DRIVER L1 LEVEL SHIFT SW 1 VOUT1 + REGULATION COMPARATOR + OVER-VOLTAGE 2.875V COMPARATOR 6 FB D1 CURRENT LIMIT COMPARATOR R3 + - 5 RTN 62.5 mV + ISEN 3 R1 RSENSE 50 m: SGND 4 R2 VOUT2 C2 PIN DESCRIPTIONS PIN NAME DESCRIPTION APPLICATION INFORMATION 1 SW Switching Node Internally connected to the buck switch source. Connect to the external inductor, diode, and boost capacitor. 2 BST Boost pin for boot-strap capacitor Connect a 0.022 µF capacitor from SW to this pin. An internal diode charges the capacitor during the off-time. 3 ISEN Current sense input Internally the current sense resistor connects from this pin to SGND. Re-circulating current flows out of this pin to the freewheeling diode. Current limit is set at 1.25A. 4 SGND Sense Ground Re-circulating current flows into this pin to the current sense resistor. 5 RTN Circuit Ground Ground for all internal circuitry other than the current limit detection. 6 FB Feedback Internally connected to the regulation and over-voltage comparators. The regulation level is 2.5V. 7 SS Softstart An internal 11.5 µA current source charges an external capacitor to 2.5V to provide the softstart function. 8 RON/SD 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. 9 VCC Output from the start-up 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. 10 VIN Input supply voltage Nominal input range is 8.0V to 36V. 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. 2 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 LM34910 www.ti.com SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 Absolute Maximum Ratings (1) (2) VIN to GND 40V BST to GND 50V SW to GND (Steady State) -1.5V ESD Rating (3) Human Body Model 2kV BST to VCC 40V VIN to SW 40V BST to SW 14V VCC to GND 14V SGND to RTN -0.3V to +0.3V Current out of ISEN See Text SS to RTN -0.3V to 4V All Other Inputs to GND -0.3 to 7V Storage Temperature Range -55°C to +150°C JunctionTemperature 150°C (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. For detailed information on soldering plastic WSON packages, refer to the Packaging Data Book available from National Semiconductor Corporation. The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (2) (3) Operating Ratings (1) VIN 8.0V to 36V −40°C to + 125°C 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 guaranteed specifications and test conditions, see the Electrical Characteristics. 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 (1). Symbol Parameter Conditions Min Typ Max Units 6.6 7 7.4 V Start-Up Regulator, VCC VCCReg VCC regulated output VIN-VCC dropout voltage ICC = 0 mA, VCC = VCCReg - 100 mV 1.4 V VCC output impedance 0 mA ≤ ICC ≤ 5 mA 140 Ω VCC current limit UVLOVCC (2) 9 mA VCC under-voltage lockout threshold VCC = 0V VCC increasing 5.8 V UVLOVCC hysteresis VCC decreasing 150 mV UVLOVCC filter delay 100 mV overdrive IIN operating current Non-switching, FB = 3V IIN shutdown current RON/SD = 0V 3 µs 0.63 1 mA 80 250 µA 0.45 0.95 Ω 4.3 5.5 V Switch Characteristics Rds(on) Buck Switch Rds(on) ITEST = 200 mA UVLOGD Gate Drive UVLO VBST - VSW Increasing UVLOGD hysteresis 3.0 440 mV Softstart Pin (1) (2) Pull-up voltage 2.5 V Internal current source 11.5 µA Typical specifications represent the most likely parametric norm at 25°C operation. VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 3 LM34910 SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 www.ti.com Electrical Characteristics (continued) 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 (1). Symbol Parameter Conditions Min Typ Max 1 1.25 1.5 Units Current Limit ILIM Threshold Current out of ISEN A Resistance from ISEN to SGND 130 mΩ Response time 150 ns On Timer tON - 1 On-time VIN = 10V, RON = 200 kΩ tON - 2 On-time VIN = 36V, RON = 200 kΩ Shutdown threshold Voltage at RON/SD rising Threshold hysteresis Voltage at RON/SD falling 2.1 2.75 3.6 740 0.35 0.65 µs ns 1.1 V 40 mV 280 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 2.440 2.5 2.550 V 2.875 V 100 nA Thermal shutdown temperature 175 °C Thermal shutdown hysteresis 20 °C FB bias current Thermal Shutdown TSD 4 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 LM34910 www.ti.com SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 Typical Performance Characteristics 7.0 7.5 RON = 400k 6.0 RON = 200k 7.0 S FS = 730 kHz 6.0 ON-TIME (Ps) kH = 19 4 6.5 F VCC (V) z FS = 100 kHz 5.0 RON = 100k 4.0 RON = 44.2k 3.0 2.0 5.5 Load Current = 500 mA 1.0 0 5.0 6.5 7.0 7.5 8.0 8.5 9.0 0 10 20 30 40 VIN (V) VIN (V) Figure 2. VCC vs VIN Figure 3. ON-Time vs VIN and RON Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 5 LM34910 SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 www.ti.com 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 NChannel buck switch, is easy to implement, and is available in the thermally enhanced WSON-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 Typical Application Circuit and Block Diagram. 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. 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: VOUT FS = 1.3 x 10-10 x RON (1) The buck switch duty cycle is equal to : VOUT tON DC = tON + tOFF = VIN (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: VOUT2 x L1 x 1.18 x 1020 FS = RL x (RON)2 where • 6 RL = the load resistance (3) Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 LM34910 www.ti.com SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 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 (4) 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 Typical Application Circuit and 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 4. However, R3 slightly degrades the load regulation. L1 SW LM34910 R1 R3 FB VOUT2 R2 C2 Figure 4. Low Ripple Output Configuration 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 under-voltage 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. VCC C3 BST C4 L1 LM34910 D2 SW VOUT1 D1 ISEN R1 R3 VOUT2 SGND R2 C2 FB Figure 5. Self Biased Configuration Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 7 LM34910 SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 www.ti.com 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. 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 ontime 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: 1.3 x 10-10 x RON tON = VIN (5) 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: VOUT FMAX = VINMAX x 200 ns (6) 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. VIN Input Voltage RON LM34910 RON/SD STOP RUN 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 Typical Application Circuit and 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 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. 8 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM34910 LM34910 www.ti.com SNVS297B – OCTOBER 2004 – REVISED MARCH 2013 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