LM34930TLX

LM34930 Ultra Small 33V, 1A Constant On-Time Buck Switching Regulator with Intelligent Current Limit June 23, 2008 LM34930 Ultra Small 33V, 1A Constant On-Time Buck Switching Regulator with Intelligent Current Limit General Description The LM34930 constant On-Time Step Down Switching Regulator features all the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying in excess of 1A load current. This high voltage regulator contains an N-Channel Buck Switch, and is available in a µSMD bumped package. 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 valley current limit results in a smooth transition from constant voltage to constant current mode when current limit is detected without the use of current limit foldback. To reduce the possibility of saturating the inductor the valley current limit threshold reduces as the input voltage increases, and the on-time is reduced when current limit is detected. Additional features include: Over-voltage indicator, Input over-voltage shutdown, Vcc under-voltage lock-out, thermal shutdown, and maximum duty cycle limiting. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Operating Input Voltage Range: 8V to 33V Input over-voltage shutdown at 36V Input absolute maximum rating of 44V Integrated 1A N-Channel buck switch Adjustable output voltage from 2.5V Switching frequency adjustable to 2 MHz Switching frequency remains nearly constant with load current and input voltage Ultra-fast transient response No loop compensation required Adjustable soft-start timing Thermal shutdown Precision 2% feedback reference Input Over-Voltage indicator at 19V Current limit scheme helps prevent inductor from saturation in load fault conditions Package ■ Micro SMD –12, 1.77 mm x 2.1 mm Typical Application, Basic Step-Down Regulator 30060801 © 2008 National Semiconductor Corporation 300608 www.national.com LM34930 Connection Diagrams 30060803 30060802 Bump Side Top View Ordering Information Order Number LM34930TL LM34930TLX Package Type Micro SMD12 Micro SMD12 NSC Package Drawing TLA12LDA TLA12LDA Supplied As 250 Units on Tape and Reel 3000 Units on Tape and Reel Pin Descriptions Pin No. A1 A2 A3 B1 B2 B3 C1, C2 Name GND nOV FB ISEN RT SS VIN Description Ground Input over-voltage indicator Output voltage feedback Current sense On-time control Soft-Start Input supply voltage Application Information Ground for all internal circuitry Open drain output switches low when Vin exceeds the overvoltage indicator threshold Internally connected to the regulation comparator. The regulation level is 2.52V. The re-circulating current flows out of this pin to the freewheeling diode. An external resistor from VIN to this pin sets the buck switch on-time, and the switching frequency. An internal current source charges an external capacitor to provide the soft-start function. Operating input range is 8V to 33V, with over-voltage shutdown internally set at 36V. Absolute maximum transient capability is 44V. Nominally regulated at 7V. Internally connected to the buck switch source. Connect to the external inductor, free wheeling diode, and bootstrap capacitor. Connect a 0.022 µF capacitor from SW to this pin. The capacitor is charged during the buck switch off-time via an internal diode. C3 D1, D2 VCC SW Output of the internal bias regulator Switching node D3 BST Bootstrap capacitor connection of the buckswitch gate driver www.national.com 2 LM34930 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) BST to SW VCC to GND All Other Inputs to GND 44V 52V -1.5V to 44V 14V -0.3V to 8V -0.3 to 7V Current out of ISEN ESD Rating (Note 2) Human Body Model Storage Temperature Range JunctionTemperature (See text) 2kV -65°C to +150°C 150°C (Note 1) 8V to 33V −40°C to + 125°C Operating Ratings VIN Voltage Junction Temperature Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply over the Operating Junction Temperature (TJ) range of −40°C to + 125°C. 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 = 50 kΩ. Symbol VCCReg Parameter VCC regulated voltage VIN - VCC dropout voltage VCC output impedance VCC current limit UVLOVCC VCC under-voltage lockout threshold UVLOVCC hysteresis UVLOVCC filter delay IQ Rds(on) UVLOGD Softstart Pin VSS ISS VSS-SH ILIM Pull-up voltage Internal current source Shutdown Threshold Threshold Resistance from ISEN to SGND Over-Voltage Indicator nOVTH nOVHYS nOVVOL nOVLKG On Timer tON - 1 tON - 2 tON - 3 Off Timer tOFF Minimum Off-time 90 ns On-time On-time On-time (current limit) VIN = 10V, RT = 50 kΩ VIN = 33V, RT = 50 kΩ VIN = 10V, RT = 50 kΩ 190 292 127 150 430 ns ns ns Threshold voltage at VIN Threshold hysteresis Output low voltage Off state leakage InoV = 1 mA, VIN = 22V VnoV = 7V VIN increasing 17.5 19 1.95 100 0.1 200 20.0 V V mV µA VIN = 8V VIN = 30V 0.95 0.90 SS open 2.52 10 70 1.15 1.1 98 1.35 1.30 mΩ V µA mV A IIN operating current Buck Switch Rds(on) Gate Drive UVLO UVLOGD hysteresis Switch Characteristics ITEST = 200 mA 2.7 0.33 3.7 300 0.7 4.5 Ω V mV ICC = 0 mA, VCC = UVLOVCC + 250 mV VIN = 8V VCC = 0V VCC increasing VCC decreasing 100 mV overdrive Non-switching, FB = 3V Conditions Min 6.6 Typ 7.0 1.3 155 15 5.25 150 2 0.8 1.5 Max 7.4 Units V V Ω mA V mV µs mA Start-Up Regulator, VCC (Note 3) Current Limit 3 www.national.com LM34930 Symbol VREF Parameter FB regulation threshold FB bias current Conditions SS Pin = steady state Min 2.470 Typ 2.52 1 Max 2.575 Units V nA Regulation Comparator (FB Pin) Input Over-voltage Shutdown VIN(OV) VIN(OV)-HYS TSD Threshold voltage at VIN Hysteresis Thermal shutdown Thermal shutdown hysteresis Thermal Resistance θJA Junction to Ambient 0 LFPM Air Flow JEDEC 4 layer board (Note 4) 65 °C/W TJ increasing VIN increasing 34.0 36 0.4 155 20 38.3 V V °C °C Thermal Shutdown 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 1.5 kΩ 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: JEDEC test board description can be found in JESD 51-7. Note 5: For detailed information on soldering micro SMD packages, refer to the Application Note AN-1112. www.national.com 4 LM34930 Typical Performance Characteristics Efficiency at 1.5 MHz Efficiency at 2 MHz 30060836 30060843 VCC vs VIN VCC vs ICC 30060804 30060805 ON-TIME vs VIN and RT Voltage at the RT Pin 30060806 30060807 5 www.national.com LM34930 Shutdown Current into VIN Operating Current into VIN 30060808 30060839 Current Limit Valley Threshold vs VIN nOV Low Voltage vs Sink Current 30060809 30060810 Reference Voltage vs Temperature Current Limit Threshold vs Temperature 30060840 30060841 www.national.com 6 LM34930 VCC Voltage vs Temperature On-Time vs Temperature 30060842 30060844 7 www.national.com LM34930 Typical Application Circuit and Block Diagram 30060811 www.national.com 8 LM34930 30060812 FIGURE 1. Start Up Sequence Functional Description The LM34930 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 at least 1.0A to the load. This high voltage regulator contains an N-Channel buck switch, is easy to implement, and is available in a 12 bump µSMD package. The regulator’s operation is based on a constant on-time control principle where the ontime is inversely proportional to the input voltage. This feature results in the operating frequency remaining relatively constant with load and input voltage variations. The constant ontime feedback control principle requires no loop compensation resulting in very fast load transient response. The 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 valley current limit threshold reduces as the input voltage increases, and the on-time is reduced by ≊50% when current limit is detected. The LM34930 can be applied in numerous applications to efficiently step down higher voltages in non-isolated applications. Additional features include: Thermal shutdown, VCC under-voltage lock-out, gate driver under-voltage lock-out, maximum duty cycle limiting, input over-voltage shutdown, and input over-voltage indicator. Control Circuit Overview The LM34930 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 (2.52V). If the FB voltage is below the reference the 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 off-time oneshot period expires, the buckswitch is then turned on for another on-time one-shot period. When in regulation, the LM34930 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 amplitude. The approximate operating frequency is calculated as follows: (1) 9 www.national.com LM34930 The buck switch duty cycle is approximately equal to: (2) In discontinuous conduction mode, the inductor’s current reaches zero during the off-time because of the longer-thannormal off-time. 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 are reduced with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: The maximum continuous current into the RT pin must be less than 2 mA. For high frequency applications, the maximum switching frequency is limited at the maximum input voltage by the minimum on-time one-shot period. At minimum input voltage the maximum switching frequency is limited by the minimum off-time one-shot period, which may prevent achievement of the proper duty cycle. Current Limit Current limit detection occurs during the off-time by monitoring the recirculating diode current flowing out of the ISEN pin. Referring to the Block Diagram, during the off-time the inductor current flows through the load, into the GND pin, through the internal sense resistor, out of ISEN and through D1 to the inductor. If that current exceeds the current limit threshold the current limit comparator delays the start of the next on-time period. The next on-time starts when the current out of ISEN reduces to the threshold and the voltage at FB is below 2.52V. The operating frequency is typically lower in the current limited condition due to longer-than-normal off-times. The valley current limit threshold is a function of the input voltage (VIN) as shown in the graph “Current Limit Valley Threshold vs. VIN”. This feature reduces the inductor current’s peak value at high line and load. To further reduce the inductor’s peak current, the next on-time after current limit detection is reduced by ≊50% if the voltage at the FB comparator is below its threshold when the inductor current falls below the current limit 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” interval the load current is IO1, 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 current limit threshold. During the “Current Limited” portion of Figure 2, each on-time is reduced by ≊50%, resulting in lower ripple amplitude for the inductor’s current. During this time the LM34930 is in a constant current mode with an average load current equal to the current limit threshold plus half the ripple current (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, and the ISEN pin, is 2A, and the maximum allowed average current is 1.5A. (3) where RL = the load resistance, and L1 is the circuit’s inductor. The output voltage is set by the two feedback resistors (R1, R2 in the Block Diagram). The regulated output voltage is calculated as follows: VOUT = 2.52 x (R1 + R2) / R2 Output voltage regulation requires a minimum of 25 mVp-p ripple voltage be supplied to the feedback pin (FB). In the typical application circuit shown with the Block Diagram, ripple is generated by the inductor’s ripple current passing through R3 in series with the output capacitor. The output ripple is passed to the FB pin by C6, avoiding attenuation by resistors R1 and R2. On-Time Timer The on-time for the LM34930 is determined by the RT resistor and the input voltage (VIN), calculated from: (4) 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: (5) The on-time must be chosen greater than 90 ns for proper operation. Equations 1, 4 and 5 are valid only when the regulator is not in current limit. When the LM34930 operates in current limit, the on-time is reduced by ≊50%. 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. www.national.com 10 LM34930 30060818 FIGURE 2. Inductor Current - Normal and Current Limit Operation Startup Regulator, VCC The startup bias regulator is integral to the LM34930. The input pin (VIN) can be connected directly to the main power source, and has transient capability to 44V. The VCC output is regulated at 7.0V, and is current limited to approximately 15 mA. Upon power up, the regulator sources current into the external capacitor at VCC. When the voltage on the VCC pin reaches the under-voltage lock-out (UVLO) threshold, the buck switch is enabled and the Soft-start pin is released to allow the Soft-start capacitor to charge. The minimum input voltage is determined by the regulator’s dropout voltage, the VCC UVLO falling threshold, and the switching frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the buck switch. 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 at approximately -1V, and C4 is recharged from VCC through the internal diode. The minimum off-time ensures a sufficient time each cycle to recharge the bootstrap capacitor. Soft-Start, Remote Shutdown The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, when VCC reaches its under-voltage threshold, an internal 10 µA current source charges the external capacitor at the SS pin to 2.52V (t2 in Figure 1). The ramping voltage at SS ramps the noninverting input of the regulation comparator, and the output voltage, in a controlled manner. An internal switch grounds the SS pin if VCC is below its undervoltage lockout threshold, or if the input voltage at VIN is above the Over-Voltage Shutdown threshold. The SS pin can be used to shutdown the LM34930 by grounding the pin as shown in Figure 3. Releasing the pin allows normal operation to resume. Over-Voltage Indicator The nOV pin, an open drain logic output, switches low when the voltage at VIN exceeds 19V. The over-voltage indicator comparator provides 1.95V hysteresis to reject noise and ripple on the VIN pin. A pull-up resistor is required at the nOV output pin to a voltage that does not exceed 7 volts. The pullup voltage can exceed the voltage at VIN. When nOV is low, the current into the pin must not exceed 10 mA. Input Over-Voltage Shutdown If the input voltage at VIN increases above 36V an internal comparator disables the buck switch, and grounds the softstart pin. The over-voltage shutdown comparator provides 400 mV hysteresis to reject noise and ripple on the VIN pin. Normal operation resumes when the voltage at VIN is reduced below the lower threshold. 30060819 FIGURE 3. Shutdown Implementation N - Channel Buck Switch and Driver The LM34930 integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate driver 11 www.national.com LM34930 Thermal Shutdown The LM34930 should be operated such that 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. In thermal shutdown the controller enters a low power non-switching state by disabling the buck switch. 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. L1: The main parameter controlled by the inductor is the inductor current ripple amplitude (IOR). The minimum load current is used to determine the maximum allowable ripple in order to maintain continuous conduction mode (the lower peak does not reach 0 mA). This is not a requirement of the LM34930, but serves as a guideline for selecting L1. For this example, the maximum ripple current should be less than: IOR(MAX) = 2 x IOUT(min) = 400 mAp-p (6) For applications where the minimum load current is zero, a good starting point for allowable ripple is 20% of the maximum load current. In this case substitute 20% of IOUT(max) for IOUT (min) in equation 6. The ripple amplitude calculated in Equation 6 is then used in the following equation: Applications Information EXTERNAL COMPONENTS The procedure for calculating the external components is illustrated with the following design example. Referring to the Block Diagram, the circuit is to be configured for the following specifications: - VOUT = 5V - VIN = 8V to 30V - Minimum load current for continuous conduction mode (IOUT (min)) = 200 mA - Maximum load current (IOUT(max)) = 1000 mA - Switching Frequency (FS) = 1.5 MHz - Soft-start time = 5 ms R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from: R1/R2 = (VOUT/2.52V) - 1 For this example, R1/R2 = 0.98. R1 and R2 should be chosen from standard value resistors in the range of 1.0 kΩ – 10 kΩ which satisfy the above ratio. For this example, 2.32 kΩ is chosen for R1 and 2.37 kΩ is chosen for R2. RT: This resistor sets the on-time, and (by default) the switching frequency. First check that the desired frequency does not require an on-time or off-time shorter than the minimum allowed (90 ns each). The minimum on-time occurs at the maximum VIN: (7) A standard value 10 µH inductor is chosen. The maximum ripple amplitude, which occurs at maximum VIN, calculates to 379 mAp-p, and the peak current is 1190 mA at maximum load current. Ensure the selected inductor is rated for this peak current. C2, R3 and C6: C2 should typically be no smaller than 3.3 µF, although that is dependent on the frequency and the desired output characteristics. C2 should be a low ESR good quality ceramic capacitor. 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 nonvarying load. Ripple voltage is created at VOUT as the inductor’s ripple current passes through R3 into C2. That ripple voltage is AC coupled directly to the FB pin by C6 without the attenuation of R1 and R2, allowing the minimum ripple at VOUT to be set at 25 mVp-p. The minimum inductor ripple current occurs at minimum VIN, and is calculated by re-arranging equation 7 to the following: (8) The minimum value for R3 is then equal to 25 mV/125 mA = 0.2Ω. The next larger standard value resistor should be used for R3 to allow for tolerances. The minimum value for C6 is equal to: The minimum off-time occurs at the minimum VIN. For this example (9) The next larger standard value capacitor should be used for C6. C1 and C7: The purpose of C1 is to supply most of the switch current during the on-time, and limit the voltage ripple at VIN, since it is assumed the voltage source feeding VIN has some amount of source impedance. 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, then ramps up to the upper peak, then drops to zero at turnoff. The average current during the on-time is the average load current. For a worst case calculation, C1 must supply this average load current during the maximum on-time, without letting the voltage at the VIN pin drop below a minimum operating level of 7.5V. The minimum value for C1 is calculated from: This off-time is acceptable since it is significantly greater than the 90 ns minimum off-time. The RT resistor is calculated from equation 5 using the minimum input voltage: A standard value 60.4 kΩ resistor is selected, resulting in a nominal frequency of 1.50 MHz. The minimum on-time calculates to 152 ns at Vin = 30V, which is acceptably longer than the minimum allowed 90 ns. The maximum on-time calculates to 416 ns at Vin = 8V. www.national.com 12 LM34930 where tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN (0.5V at VIN = 8V). The purpose of C7 is to minimize transients and ringing due to long lead inductance leading to the VIN pin. A low ESR 0.1 µF ceramic chip capacitor is recommended, and C7 must be located close to the VIN and GND pins. C3: The capacitor at the VCC pin provides noise filtering and stability for the VCC regulator. C3 should be no smaller than 0.1 µF, and should be a good quality, low ESR ceramic capacitor. The value of C3, and the VCC current limit, determine a portion of the turn-on-time (t1 in Figure 1). 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 each turn-on. A low ESR also helps ensure a complete recharge during each off-time. C5: The capacitor at the SS pin determines the soft-start time, i.e. the time for the output voltage to reach its final value (t2 in Figure 1). For soft-start time of 5 ms, the capacitor value is determined from the following: D1: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high speed transitions at the SW pin may affect the regulator’s operation due to the diode’s reverse recovery transients. The diode must be rated for the maximum input voltage, the maximum load current, and the peak current which occurs when the current limit and maximum ripple current are reached simultaneously. The diode’s average power dissipation is calculated from: PD1 = VF x IOUT x (1-D) where VF is the diode’s forward voltage drop, and D is the ontime duty cycle. FINAL CIRCUIT The final circuit is shown in Figure 4, and its performance is shown in Figure 5 and Figure 6. The current limit measured approximately 1.28A at Vin = 8V, and 1.18A at Vin = 30V. The output voltage ripple amplitude measured 32 mVp-p at Vin = 8V, and 87 mVp-p at Vin = 30V. 30060828 FIGURE 4. Example Circuit 13 www.national.com LM34930 30060836 FIGURE 5. Efficiency vs. Load Current and VIN (Circuit of Figure 4) 30060837 FIGURE 6. Frequency vs. VIN (Circuit of Figure 4) ALTERNATE OUTPUT RIPPLE CONFIGURATIONS For applications which require lower levels of ripple at VOUT, or for those which can accept higher levels of ripple while using one less capacitor, the following two alternatives are available. a) Minimum ripple configuration: If the application requires a lower value of ripple at VOUT (