LM25011MYX

LM25011 42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit June 1, 2009 LM25011 42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit General Description The LM25011 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 high 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. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Input operating voltage range: 6V to 42V Absolute maximum input rating: 45V Integrated 2A N-Channel Buck Switch Adjustable current limit Adjustable output voltage from 2.51V Minimum ripple voltage at VOUT Power Good output Switching frequency adjustable to 2 MHz 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 Adjustable Soft-Start timing Thermal shutdown Precision 2% feedback reference Package ■ MSOP-10EP Typical Application, Basic Step-Down Regulator 30094601 © 2009 National Semiconductor Corporation 300946 www.national.com LM25011 Connection Diagram 30094602 Top View 10 Lead MSOP-EP Ordering Information Order Number LM25011MY LM25011MYX Package Type MSOP-10EP MSOP-10EP NSC Package Drawing MUC10A MUC10A Supplied As 1000 Units on Tape and Reel 3500 Units on Tape and Reel Pin Descriptions Pin No. 1 2 3 Name VIN RT PGD Description Input supply voltage On-time Control Power Good Application Information Operating input range is 6V to 42V. Transient capability is 45V. A low ESR capacitor must be placed as close as possible to the VIN and SGND pins. An external resistor from VIN to this pin sets the buck switch on-time, and the switching frequency. 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 regulation comparator. The regulation level is 2.51V. Ground connection for the current limit sensing circuit. Connect to ground and to the current sense resistor. Connect to the current sense resistor and the anode of the free-wheeling diode. Internally connected to the buck switch source. Connect to the external inductor, cathode of the free-wheeling diode, and bootstrap capacitor. 4 5 6 7 8 9 10 SS SGND FB CSG CS SW BST Soft-Start Signal Ground Feedback Current Sense Ground Current sense Switching Node Bootstrap capacitor connection of the Connect a 0.1 µF capacitor from SW to this pin. The capacitor is charged buck switch gate driver. during the buck switch off-time via an internal diode. www.national.com 2 LM25011 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 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 45V 52V -1.5V to 45V -0.3V to 7V -0.3V to 0.3V -0.3V to 0.3V -0.3V to 7V -0.3V to 3V RT to SGND FB to SGND ESD Rating (Note 2) Human Body Model Lead Temperature (soldering 4 sec) Storage Temperature Range Junction Temperature -0.3V to 1V -0.3V to 7V 2kV 260°C -65°C to +150°C 150°C Operating Ratings VIN Voltage Junction Temperature (Note 1) 6.0V to 42V –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 = 50 kΩ. Symbol Parameter Input operating current VIN under-voltage lock-out threshold VIN under-voltage lock-out threshold hysteresis Conditions Non-switching, FB = 3V VIN Increasing 4.6 Min Typ 1200 5.3 200 Max 1600 5.9 Units µA V mV Input (VIN Pin) IIN UVLOVIN Switch Characteristics RDS(ON) UVLOGD Buck Switch RDS(ON) Gate Drive UVLO UVLOGD Hysteresis Pre-charge switch voltage Pre-charge switch on-time Soft-Start Pin VSS ISS VSS-SH Current Limit VILIM Threshold voltage at CS CS bias current CSG bias current On Timer, RT Pin tON - 1 tON - 2 tON - 3 tON - 4 Off Timer tOFF VREF Minimum Off-time FB regulation threshold FB bias current SS pin = steady state FB = 3V 90 2.46 150 2.51 100 208 2.56 ns V nA Regulation Comparator (FB Pin) On-time On-time On-time (current limit) On-time VIN = 12V, RT = 50 kΩ VIN = 32V, RT = 50 kΩ VIN = 12V, RT = 50 kΩ VIN = 12V, RT = 301 kΩ 150 200 75 100 1020 250 ns ns ns ns FB = 3V FB = 3V -146 -130 -120 -35 -115 mV µA µA Pull-up voltage Internal current source Shutdown Threshold 70 2.51 10 140 V µA mV ITEST = 10 mA into SW pin ITEST = 200 mA BST-SW 2.4 0.3 3.4 350 1.4 120 0.6 4.4 Ω V mV V ns 3 www.national.com LM25011 Symbol Parameter Threshold at FB, with respect to VREF Threshold hysteresis Conditions FB increasing IPGD = 1mA, FB = 0V VPGD = 7V, FB = 3V Junction temperature increasing Min 91 Typ 95 3.3 125 0.1 155 20 48 10 Max Units % % Power Good (PGD pin) PGDVOL PGDLKG Thermal Shutdown TSD Low state voltage Off state leakage Thermal shutdown Thermal shutdown hysteresis 180 mV µA °C °C °C/W °C/W Thermal Resistance θJA θJC Junction to Ambient, 0 LFPM Air Flow (note 3) Junction to Case, (note 3) 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 100 pF 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. www.national.com 4 LM25011 Typical Performance Characteristics Efficiency (Circuit of Figure 5) Efficiency at 2 MHz 30094603 30094604 On-Time vs VIN and RT Voltage at the RT Pin 30094605 30094606 Shutdown Current into VIN Operating Current into VIN 30094607 30094608 5 www.national.com LM25011 PGD Low Voltage vs. Sink Current Reference Voltage vs. Temperature 30094609 30094610 Current Limit Threshold vs. Temperature Operating Current vs. Temperature 30094611 30094612 VIN UVLO vs. Temperature SS Pin ShutdownThreshold vs. Temperature 30094613 30094614 www.national.com 6 LM25011 On-Time vs. Temperature Minimum Off-Time vs. Temperature 30094615 30094616 7 www.national.com LM25011 Block Diagram www.national.com 8 30094617 LM25011 30094618 FIGURE 1. Startup Sequence Functional Description The LM25011 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 LM25011 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 pre9 charge, gate drive under-voltage lock-out, and maximum duty cycle limit. Control Circuit Overview The LM25011 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 LM25011 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- www.national.com LM25011 plitude. The approximate operating frequency is calculated as follows: (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: The on-time must be chosen greater than 90 ns for proper operation. Equations 1, 5 and 6 are valid only during normal operation - i.e., the circuit is not in current limit. When the LM25011 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 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 (90 ns). 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. 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 LM25011 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 by the LM25011’s ERM (Emulated Ripple Mode) control block. The ERM circuit generates the required internal ripple voltage from the ripple waveform at the CS pin during each off-time. This feature eliminates the need for ripple at VOUT, allowing output ripple to be kept to a minimum. Output ripple is therefore a function of the inductor’s ripple current and the characteristics of the output capacitor. On-Time Timer The on-time for the LM25011 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) www.national.com 10 LM25011 30094624 FIGURE 2. Normal and Current Limit Operation 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 LM25011 requires a minimum of 10 mVp-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 3. 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 10 µA 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 1000 pF. The LM25011 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 1 mA. 30094625 FIGURE 3. CS Pin Waveform 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 LM25011 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 0.1 µF capacitor 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 11 Shutdown Function The SS pin can be used to shutdown the LM25011 by grounding the SS pin as shown in Figure 4. Releasing the pin allows normal operation to resume. www.national.com LM25011 RFB2/RFB1 = (VOUT/2.51V) - 1 (7) 30094626 FIGURE 4. Shutdown Implemetation Power Good Output (PGD) 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 LM25011’s status to other circuitry. When PGD is low, the pin’s voltage is determined by the current into the pin. See the graph “PGD Low Voltage vs. Sink Current”. Upon powering up the LM25011, 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 LM25011 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. For this example, RFB2/RFB1 = 0.992. RFB1 and RFB2 should be chosen from standard value resistors in the range of 1.0 kΩ – 10 kΩ which satisfy the above ratio. For this example, 4.99 kΩ is chosen for both resistors, providing a 5.02V output. 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 values (90 ns and 150, respectively). The minimum ontime occurs at the maximum input voltage. For this example: The minimum off-time occurs at the minimum input voltage. For this example: Both the on-time and off-time are acceptable since they are significantly greater than the minimum value for each. The RT resistor is calculated from equation 6 using the minimum input voltage: Thermal Shutdown The LM25011 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. A standard value 118 kΩ resistor is selected. The minimum on-time calculates to 152 ns at Vin = 36V, and the maximum on-time calculates to 672 ns at Vin = 8V L1: The parameters controlled by the inductor are the inductor current ripple amplitude (IOR), and the ripple voltage amplitude across the current sense resistor RS. 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 LM25011, 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) = 600 mA p-p (8) 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 8. The ripple amplitude calculated in Equation 8 is then used in the following equation: Applications Information EXTERNAL COMPONENTS The procedure for calculating the external components is illustrated with a design example. Referring to the Block Diagram, the circuit is to be configured for the following specifications: • VOUT = 5V • VIN = 8V to 36V • Minimum load current for continuous conduction mode (IOUT(min) = 300 mA • Maximum load current (IOUT(max) = 1.5 A • Switching frequency (FS) = 1.0 MHz • Soft-start time = 5 ms RFB2 and RFB1: These resistors set the output voltage, and their ratio is calculated from: www.national.com 12 A standard value 10 µH inductor is chosen. Using this inductor value, the maximum ripple current amplitude, which occurs at maximum VIN, calculates to 472 mAp-p, and the peak current is 1736 mA at maximum load current. Ensure the selected inductor is rated for this peak current. The minimum ripple current, which occurs at minimum VIN, calculates to 200 mApp. RS: The minimum current limit threshold is calculated at maximum load current, using the minimum ripple current calculated above. The current limit threshold is the lower peak of the inductor current waveform when in current limit (see Figure 2). ILIM = 1.5A – (0.2 A/2) = 1.4A LM25011 Current limit detection occurs when the voltage across the sense resistor (RS) reaches the current limit threshold. To allow for tolerances, the sense resistor value is calculated using the minimum threshold specification: RS = 115 mV/1.4A = 82 mΩ The next smaller standard value, 80 mΩ, is selected. The next step is to ensure that sufficient ripple voltage occurs across RS with this value sense resistor. As mentioned in the Ripple Requirements section, a minimum of 10mVp-p voltage ripple is required across the RS sense resistor during the off-time to ensure the regulation circuit operates properly. The ripple voltage is the product of the inductor ripple current amplitude and the sense resistor value. In this case, the minimum ripple voltage calculates to: VRIPPLE = ΔI x RS = 200 mA x 0.080Ω = 16 mV If the ripple voltage had calculated to less than 10 mVp-p the inductor value would have to be reduced to increase the ripple current amplitude. This would have required a recalculation of ILIM and RS in the above equations. Since the minimum requirement is satisfied in this case no change is necessary. The nominal current limit threshold calculates to 1.63A. The minimum and maximum thresholds calculate to 1.44A and 1.83A respectively, using the minimum and maximum limits for the current limit threshold specification. The load current is equal to the threshold current plus one half the ripple current. Under normal load conditions, the maximum power dissipation in RS occurs at maximum load current, and at maximum input voltage where the on-time duty cycle is minimum. In this design example, the minimum on-time duty cycle is: at VIN, since it is assumed the voltage source feeding VIN has some amount of source impedance. 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 turn-off. The average current during the on-time is the average load current. For a worst case calculation, CIN 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 5.5V. For this exercise 0.5V is chosen as the maximum allowed input ripple voltage. Using the maximum load current, the minimum value for CIN is calculated from: (9) where tON is the maximum on-time, and ΔV is the allowable ripple voltage at VIN. The purpose of CBYP 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 CBYP must be located close to the VIN and SGND pins. CBST: The recommended value for CBST is 0.1 µF. A high quality ceramic capacitor with low ESR is recommended as CBST 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. CSS: The capacitor at the SS pin determines the soft-start time, i.e. the time for the output voltage to reach its final value (t1 in Figure 1). For a soft-start time of 5 ms, the capacitor value is determined from the following: At maximum load current, the power dissipation in RS is equal to: P(RS) = (1.5A)2 x 0.080Ω x (1 – 0.139) = 155 mW When in current limit the maximum power dissipation in RS calculates to P(RS) = (1.83A + 0.472A/4)2 x 0.080Ω = 304 mW Duty cycle is not included in this power calculation since the on-time duty cycle is typically