LM34922 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit General Description Features The LM34922 Constant On-time Step-Down Switching Regulator features all the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying up to 2A of load current. This voltage regulator contains an NChannel Buck switch, a startup regulator, current limit detection, and internal ripple control. The constant on-time regulation principle requires no loop compensation, results in fast load transient response, and simplifies circuit implementation. The operating frequency remains constant with line and load. The adjustable valley current limit detection results in a smooth transition from constant voltage to constant current mode when current limit is reached, without the use of current limit foldback. The PGD output indicates the output voltage has increased to within 5% of the expected regulation value. Additional features include: Low output ripple, VIN under-voltage lock-out, adjustable soft-start timing, thermal shutdown, gate drive pre-charge, gate drive under-voltage lock-out, and maximum duty cycle limit. ■ ■ ■ ■ ■ ■ ■ ■ ■ Input operating voltage range: 6V to 28V Absolute maximum input rating: 30V Integrated 2A N-Channel Buck Switch Adjustable current limit allows for smaller inductor Adjustable output voltage from 2.51V Minimum ripple voltage at VOUT Power Good output Switching frequency adjustable to 1MHz COT topology features: -Switching frequency remains nearly constant with load current and input voltage variations -Ultra-fast transient response -No loop compensation required -Stable operation with ceramic output capacitors -Allows for smaller output capacitor and current sense resistor ■ Adjustable Soft-Start timing ■ Thermal shutdown ■ Precision 2% feedback reference Package ■ MSOP-10EP Typical Application, Basic Step-Down Regulator 30140701 © 2012 Texas Instruments Incorporated 301407 SNVS813 www.ti.com LM34922 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit June 1, 2012 LM34922 Connection Diagram 30140702 Top View 10 Lead MSOP-EP Ordering Information www.ti.com Order Number Package Type NSC Package Drawing Supplied As LM34922MY MSOP-10EP MUC10A 1000 Units on Tape and Reel LM34922MYX MSOP-10EP MUC10A 3500 Units on Tape and Reel 2 LM34922 Pin Descriptions Pin No. Name Description 1 VIN Input supply voltage Application Information Operating input range is 6V to 28V. Transient capability is 30V. A low ESR capacitor must be placed as close as possible to the VIN and SGND pins. 2 RT On-time Control An external resistor from VIN to this pin sets the buck switch on-time, and the switching frequency. 3 PGD Power Good 4 SS Soft-Start 5 SGND Signal Ground 6 FB Feedback Internally connected to the regulation comparator. The regulation level is 2.51V. 7 CSG Current Sense Ground Ground connection for the current limit sensing circuit. Connect to ground and to the current sense resistor. 8 CS Current sense Connect to the current sense resistor and the anode of the free-wheeling diode. 9 SW Switching Node 10 BST Logic output indicates when the voltage at the FB pin has increased to above 95% of the internal reference voltage. Hysteresis is provided. An external pull-up resistor to a voltage less than 7V is required. An internal current source charges an external capacitor to provide the softstart function. Ground for all internal circuitry other than the current limit sense circuit. Internally connected to the buck switch source. Connect to the external inductor, cathode of the free-wheeling diode, and bootstrap capacitor. Bootstrap capacitor connection of the Connect a 0.1µF capacitor from SW to this pin. The capacitor is charged buck switch gate driver. during the buck switch off-time via an internal diode. 3 www.ti.com LM34922 RT to SGND -0.3V to 1V FB to SGND -0.3V to 7V ESD Rating (Note 2) Human Body Model 2kV Storage Temperature Range -65°C to +150°C For soldering specs see: http://www.ti.com/lit/an/snoa549/ snoa549.pdf Junction Temperature 150°C Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. VIN to SGND (TJ = 25°C) BST to SGND SW to SGND (Steady State) BST to SW CS to CSG CSG to SGND PGD to SGND SS to SGND 30V 37V -1.5V to 30V -0.3V to 7V -0.3V to 0.3V -0.3V to 0.3V -0.3V to 7V -0.3V to 3V Operating Ratings (Note 1) VIN Voltage Junction Temperature 6.0V to 28V –40°C to +125°C Electrical Charateristics 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, RT = 50kΩ. Symbol Parameter Conditions Input operating current Non-switching, FB = 3V VIN under-voltage lock-out threshold VIN Increasing Min Typ Max Units 1200 1600 µA 5.3 5.9 Input (VIN Pin) IIN UVLOVIN 4.6 VIN under-voltage lock-out threshold hysteresis 200 V mV Switch Characteristics RDS(ON) Buck Switch RDS(ON) ITEST = 200mA UVLOGD Gate Drive UVLO BST-SW 2.4 UVLOGD Hysteresis 0.3 0.6 Ω 3.4 4.4 V 350 mV 1.4 V Pre-charge switch on-time 120 ns VSS Pull-up voltage 2.51 V ISS Internal current source 10 µA 70 140 mV -146 -130 Pre-charge switch voltage ITEST = 10mA into SW pin Soft-Start Pin VSS-SH Shutdown Threshold Current Limit VILIM Threshold voltage at CS -115 mV CS bias current FB = 3V -120 µA CSG bias current FB = 3V -35 µA tON - 1 On-time VIN = 12V, RT = 50kΩ tON - 2 On-time (current limit) tON - 3 On Timer, RT Pin 150 200 250 ns VIN = 12V, RT = 50kΩ 100 ns On-time VIN = 12V, RT = 301kΩ 1020 ns tON - 4 On-time VIN = 9V, RT = 30.9kΩ 130 171 215 ns tON - 5 On-time VIN = 12V, RT = 30.9kΩ 105 137 170 ns tON - 6 On-time VIN = 16V, RT = 30.9kΩ 79 109 142 ns 90 150 208 ns 2.46 2.51 2.56 V Off Timer tOFF Minimum Off-time (LM34922) Regulation Comparator (FB Pin) VREF www.ti.com FB regulation threshold SS pin = steady state FB bias current FB = 3V 4 100 nA Parameter Conditions Min Typ 91 95 Max Units Power Good (PGD pin) Threshold at FB, with respect to VREF FB increasing Threshold hysteresis % 3.3 % PGDVOL Low state voltage IPGD = 1mA, FB = 0V 125 PGDLKG Off state leakage VPGD = 7V, FB = 3V 0.1 µA Thermal shutdown Junction temperature increasing 155 °C 20 °C 180 mV Thermal Shutdown TSD Thermal shutdown hysteresis Thermal Resistance θJA Junction to Ambient, 0 LFPM Air Flow (note 3) 48 °C/W θJC Junction to Case, (note 3) 10 °C/W 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: JEDEC test board description can be found in JESD 51-5 and JESD 51-7. Note 4: Current flow out of a pin is indicated as a negative number. 5 www.ti.com LM34922 Symbol LM34922 Typical Performance Characteristics Efficiency (Circuit of Figure 5) On-Time vs VIN and RT 30140705 30140703 Voltage at the RT Pin Shutdown Current into VIN 30140706 30140707 Operating Current into VIN PGD Low Voltage vs. Sink Current 30140709 30140708 www.ti.com 6 LM34922 Reference Voltage vs. Temperature Current Limit Threshold vs. Temperature 30140710 30140711 Operating Current vs. Temperature VIN UVLO vs. Temperature 30140712 30140713 SS Pin ShutdownThreshold vs. Temperature On-Time vs. Temperature 30140714 30140715 7 www.ti.com LM34922 Minimum Off-Time vs. Temperature (LM34922) 30140716 www.ti.com 8 LM34922 30140717 Block Diagram 9 www.ti.com LM34922 30140718 FIGURE 1. Startup Sequence charge, gate drive under-voltage lock-out, and maximum duty cycle limit. Functional Description The LM34922 Constant On-time Step-down Switching Regulator features all the functions needed to implement a low cost, efficient buck bias power converter capable of supplying up to 2.0A to the load. This high voltage regulator contains an N-Channel buck switch, is easy to implement, and is available in a 10-pin MSOP power enhanced package. The regulator’s operation is based on a constant on-time control principle with the on-time inversely proportional to the input voltage. This feature results in the operating frequency remaining relatively constant with load and input voltage variations. The constant on-time feedback control principle requires no loop compensation resulting in very fast load transient response. The adjustable valley current limit detection results in a smooth transition from constant voltage to constant current when current limit is reached. To aid in controlling excessive switch current due to a possible saturating inductor the on-time is reduced by ≊40% when current limit is detected. The Power Good output (PGD pin) indicates when the output voltage is within 5% of the expected regulation voltage. The LM34922 can be implemented to efficiently step-down higher voltages in non-isolated applications. Additional features include: Low output ripple, VIN under-voltage lock-out, adjustable soft-start timing, thermal shutdown, gate drive prewww.ti.com Control Circuit Overview The LM34922 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (2.51V). If the FB voltage is below the reference the internal buck switch is switched on for the one-shot timer period, which is a function of the input voltage and the programming resistor (RT). Following the on-time the switch remains off until the FB voltage falls below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the FB pin voltage falls below the reference and the offtime one-shot period expires, the buck switch is then turned on for another on-time one-shot period. When in regulation, the LM34922 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is always greater than zero, and 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 am- 10 operation - i.e., the circuit is not in current limit. When the LM34922 operates in current limit, the on-time is reduced by ≊40%. This feature reduces the peak inductor current which may be excessively high if the load current and the input voltage are simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the output voltage resumes its normal regulated value. The maximum continuous current into the RT pin must be less than 2mA. For high frequency applications, the maximum switching frequency is limited at the maximum input voltage by the minimum on-time one-shot period (90ns). At minimum input voltage the maximum switching frequency is limited by the minimum off-time one-shot period, which, if reached, prevents achievement of the proper duty cycle. (1) The buck switch duty cycle is approximately equal to: (2) When the load current is less than one half the inductor’s ripple current amplitude the circuit operates in discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is maintained at light loads since the switching losses are reduced with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: Current Limit Current limit detection occurs during the off-time by monitoring the voltage across the external current sense resistor RS. Referring to the Block Diagram, during the off-time the recirculating current flows through the inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the sense resistor exceeds the threshold (VILIM) the current limit comparator output switches to delay the start of the next on-time period. The next on-time starts when the recirculating current decreases such that the voltage across RS reduces to the threshold and the voltage at FB is below 2.51V. The operating frequency is typically lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by ≊40% if the voltage at the FB pin is below its threshold when the voltage across RS reduces to its threshold (VOUT is low due to current limiting). Figure 2 illustrates the inductor current waveform during normal operation and in current limit. During the first “Normal Operation” the load current is I01, the average of the inductor current waveform. As the load resistance is reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold. During the “Current Limited” portion of Figure 2, each on-time is reduced by ≊40%, resulting in lower ripple amplitude for the inductor’s current. During this time the LM34922 is in a constant current mode with an average load current equal to the current limit threshold plus half the ripple amplitude (IOCL), and the output voltage is below the normal regulated value. Normal operation resumes when the load current is reduced (to IO2), allowing VOUT and the on-time to return to their normal values. Note that in the second period of “Normal Operation”, even though the inductor’s peak current exceeds the current limit threshold during part of each cycle, the circuit is not in current limit since the inductor current falls below the current limit threshold during each off time. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is 2.0A. (3) where RL = the load resistance, and L1 is the circuit’s inductor. The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Block Diagram). The regulated output voltage is calculated as follows: VOUT = 2.51V x (RFB1 + RFB2) / RFB1 (4) Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is generated internally in the LM34922. In the LM34922 the ERM (Emulated Ripple Mode) control circuit generates the required internal ripple voltage from the ripple waveform at the CS pin. On-Time Timer The on-time for the LM34922 is determined by the RT resistor and the input voltage (VIN), calculated from: (5) 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 RT resistor is determined from the following: (6) The on-time must be chosen greater than 90ns for proper operation. Equations 1, 5 and 6 are valid only during normal 11 www.ti.com LM34922 plitude. The approximate operating frequency is calculated as follows: LM34922 30140724 FIGURE 2. Normal and Current Limit Operation itor connected between BST and SW provides the supply voltage for the driver during the on-time. During each off-time, the SW pin is at approximately -1V, and CBST is recharged from the internal 5V regulator for the next on-time. The minimum off-time ensures a sufficient time each cycle to recharge the bootstrap capacitor. Ripple Requirements The LM34922 requires a minimum of 15mVp-p ripple voltage at the CS pin. That ripple voltage is generated by the decreasing recirculating current (the inductor’s ripple current) through RS during the off-time. See Figure 3a. Soft-Start The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. Upon turn-on, when VIN reaches its under-voltage lock-out threshold an internal 10µA current source charges the external capacitor at the SS pin to 2.51V (t1 in Figure 1). The ramping voltage at SS ramps the noninverting input of the regulation comparator, and the output voltage, in a controlled manner. For proper operation, the softstart capacitor should be no smaller than 1000pF. The LM34922 can be employed as a tracking regulator by applying the controlling voltage to the SS pin. The regulator’s output voltage tracks the applied voltage, gained up by the ratio of the feedback resistors. The applied voltage at the SS pin must be within the range of 0.5V to 2.6V. The absolute maximum rating for the SS pin is 3.0V. If the tracking function causes the voltage at the FB pin to go below the thresholds for the PGD pin, the PGD pin will switch low (see the Power Good Output section). An internal switch grounds the SS pin if the input voltage at VIN is below its under-voltage lock-out threshold or if the Thermal Shutdown activates. If the tracking function (described above) is used, the tracking voltage applied to the SS pin must be current limited to a maximum of 1mA. 30140725 CS Pin Waveform FIGURE 3. The ripple voltage is equal to: VRIPPLE = ΔI x RS where ΔI is the inductor current ripple amplitude, and RS is the current sense resistor at the CS pin. N-Channel Buck Switch and Driver The LM34922 integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor (CBST) and an internal high voltage diode. A 0.1µF capacwww.ti.com 12 The SS pin can be used to shutdown the LM34922 by grounding the SS pin as shown in Figure 4. Releasing the pin allows normal operation to resume. 30140726 FIGURE 4. Shutdown Implemetation Power Good Output (PGD) Thermal Shutdown The Power Good output (PGD) indicates when the voltage at the FB pin is close to the internal 2.51V reference voltage. The rising threshold at the FB pin for the PGD output to switch high is 95% of the internal reference. The falling threshold for the PGD output to switch low is approximately 3.3% below the rising threshold. The PGD pin is internally connected to the drain of an Nchannel MOSFET switch. An external pull-up resistor (RPGD), connected to an appropriate voltage not exceeding 7V, is required at PGD to indicate the LM34922’s status to other circuitry. When PGD is low, the pin’s voltage is deter- The LM34922 should be operated so the junction temperature does not exceed 125°C. If the junction temperature increases above that, an internal Thermal Shutdown circuit activates (typically) at 155°C, taking the controller to a low power reset state by disabling the buck switch and taking the SS pin to ground. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below 135°C (typical hysteresis = 20°C) normal operation resumes. 30140734 FIGURE 5. Example Circuit 13 www.ti.com LM34922 mined by the current into the pin. See the graph “PGD Low Voltage vs. Sink Current”. Upon powering up the LM34922, the PGD pin is high until the voltage at VIN reaches 2V, at which time PGD switches low. As VIN is increased PGD stays low until the output voltage takes the voltage at the FB pin above 95% of the internal reference voltage, at which time PGD switches high. As VIN is decreased (during shutdown) PGD remains high until either the voltage at the FB pin falls below ≊92% of the internal reference, or when VIN falls below its lower UVLO threshold, whichever occurs first. PGD then switches low, and remains low until VIN falls below 2V, at which time PGD switches high. If the LM34922 is used as a tracking regulator (see the Softstart section), the PGD output is high as long as the voltage at the FB pin is above the thresholds mentioned above. Shutdown Function LM34922 30140703 FIGURE 6. Efficiency (Circuit of Figure 5) 30140736 FIGURE 7. Frequency vs VIN (Circuit of Figure 5) www.ti.com 14 LM34922 Physical Dimensions inches (millimeters) unless otherwise noted 10-Lead MSSOP-EP Package NS Package Number MUC10A 15 www.ti.com LM34922 28V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit Notes www.ti.com IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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