LM22676MR-5.0

LM22676/LM22676Q 42V, 3A SIMPLE SWITCHER® Step-Down Voltage Regulator with Features General Description Features The LM22676 switching regulator provides all of the functions necessary to implement an efficient high voltage step-down (buck) regulator using a minimum of external components. This easy to use regulator incorporates a 42V N-channel MOSFET switch capable of providing up to 3A of load current. Excellent line and load regulation along with high efficiency (>90%) are featured. Voltage mode control offers short minimum on-time, allowing the widest ratio between input and output voltages. Internal loop compensation means that the user is free from the tedious task of calculating the loop compensation components. Fixed 5V output and adjustable output voltage options are available. A switching frequency of 500 kHz allows for small external components and good transient response. A precision enable input allows simplification of regulator control and system power sequencing. In shutdown mode the regulator draws only 25 µA (typ.). Built in softstart (500µs, typ) saves external components. The LM22676 also has built in thermal shutdown, and current limiting to protect against accidental overloads. The LM22676 is a member of Texas Instruments' SIMPLE SWITCHER™ family. The SIMPLE SWITCHER™ concept provides for an easy to use complete design using a minimum number of external components and the TI WEBENCH® design tool. TI's WEBENCH® tool includes features such as external component calculation, electrical simulation, thermal simulation, and Build-It boards for easy design-in. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide input voltage range: 4.5V to 42V Internally compensated voltage mode control Stable with low ESR ceramic capacitors 120 mΩ N-channel MOSFET TO-263 THIN package 100 mΩ N-channel MOSFET PSOP-8 package Output voltage options: -ADJ (outputs as low as 1.285V) -5.0 (output fixed to 5V) ±1.5% feedback reference accuracy Switching frequency of 500 kHz -40°C to 125°C operating junction temperature range Precision enable pin Integrated boot-strap diode Integrated soft-start Fully WEBENCH® enabled LM22676Q is an Automotive Grade product that is AECQ100 grade 1 qualified (-40°C to +125°C operating junction temperature) Package ■ PSOP-8 (Exposed Pad) ■ TO-263 THIN (Exposed Pad) Applications ■ ■ ■ ■ Industrial Control Telecom and Datacom Systems Embedded Systems Conversions from Standard 24V, 12V and 5V Input Rails Simplified Application Schematic 30076501 © 2012 Texas Instruments Incorporated 300765 SNVS587J www.ti.com LM22676/LM22676Q 42V, 3A SIMPLE SWITCHER® Step-Down Voltage Regulator with Features June 22, 2012 LM22676/LM22676Q Connection Diagrams 30076540 8-Lead Plastic PSOP-8 Package TI Package Number MRA08B 30076502 7-Lead Plastic TO-263 THIN Package TI Package Number TJ7A Ordering Information Output Voltage Order Number Package Type TI Package Drawing Supplied As ADJ LM22676MR-ADJ PSOP-8 Exposed Pad MRA08B 95 Units in Rails ADJ LM22676MRE-ADJ 250 Units in Tape and Reel ADJ LM22676MRX-ADJ 2500 Units in Tape and Reel ADJ LM22676QMR-ADJ ADJ LM22676QMRE-ADJ PSOP-8 Exposed Pad MRA08B 250 Units in Tape and Reel ADJ LM22676QMRX-ADJ 2500 Units in Tape and Reel ADJ LM22676TJE-ADJ ADJ LM22676TJ-ADJ ADJ LM22676QTJE-ADJ 250 Units in Tape and Reel ADJ LM22676QTJ-ADJ 1000 Units in Tape and Reel TO-263 THIN Exposed Pad TJ7A 95 Units in Rails AEC-Q100 Grade 1 qualified. Automotive Grade Production Flow* 250 Units in Tape and Reel 1000 Units in Tape and Reel 5.0 LM22676MR-5.0 5.0 LM22676MRE-5.0 250 Units in Tape and Reel 5.0 LM22676MRX-5.0 2500 Units in Tape and Reel www.ti.com Features PSOP-8 Exposed Pad MRA08B 2 95 Units in Rails AEC-Q100 Grade 1 qualified. Automotive Grade Production Flow* Order Number Package Type TI Package Drawing 5.0 LM22676QMR-5.0 PSOP-8 Exposed Pad MRA08B 5.0 LM22676QMRE-5.0 250 Units in Tape and Reel 5.0 LM22676QMRX-5.0 2500 Units in Tape and Reel 5.0 LM22676TJE-5.0 TO-263 THIN Exposed Pad TJ7A Supplied As Features 95 Units in Rails AEC-Q100 Grade 1 qualified. Automotive Grade Production Flow* 250 Units in Tape and Reel 5.0 LM22676TJ-5.0 5.0 LM22676QTJE-5.0 1000 Units in Tape and Reel 250 Units in Tape and Reel 5.0 LM22676QTJ-5.0 1000 Units in Tape and Reel AEC-Q100 Grade 1 qualified. Automotive Grade Production Flow* *Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies. Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified with the letter Q. For more information go to http://www.ti.com/automotive. 3 www.ti.com LM22676/LM22676Q Output Voltage LM22676/LM22676Q Pin Descriptions Pin Numbers PSOP-8 Package Pin Numbers TO-263 THIN Package Name Description Application Information 1 3 BOOT Bootstrap input Provides the gate voltage for the high side NFET. 2, 3 5 NC Not Connected Pins are not electrically connected inside the chip. Pins do function as thermal conductor. 4 6 FB Feedback pin Feedback input to regulator. 5 7 EN Enable input Used to control regulator start-up and shutdown. See Precision Enable section of data sheet. 6 4 GND Ground input to regulator; system common System ground pin. 7 2 VIN Input Voltage Input supply to regulator 8 1 SW Switch pin Switching output of regulator EP EP EP Exposed Pad Connect to ground. Provides thermal connection to PCB. See applications information. www.ti.com 4 If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. VIN to GND EN Pin Voltage SW to GND (Note 2) BOOT Pin Voltage FB Pin Voltage Power Dissipation Junction Temperature 43V -0.5V to 6V -5V to VIN VSW + 7V -0.5V to 7V Internally Limited 150°C Operating Ratings ±2 kV -65°C to +150°C (Note 1) Supply Voltage (VIN) Junction Temperature Range 4.5V to 42V -40°C to +125°C Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the 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 TA = TJ = 25°C, and are provided for reference purposes only. Unless otherwise specified: VIN = 12V. Symbol Parameter Conditions Min (Note 5) Typ (Note 4) Max (Note 5) Units Feedback Voltage VIN = 8V to 42V 4.925/4.9 5.0 5.075/5.1 V Feedback Voltage VIN = 4.7V to 42V 1.266/1.259 1.285 1.304/1.311 V 3.4 6 mA LM22676-5.0 VFB LM22676-ADJ VFB All Output Voltage Versions IQ ISTDBY Quiescent Current VFB = 5V Standby Quiescent Current EN Pin = 0V ICL Current Limit IL Output Leakage Current RDS(ON) fO Switch On-Resistance 25 40 µA 4.2 5.3/5.5 A VIN = 42V, EN Pin = 0V, VSW = 0V 0.2 2 µA VSW = -1V 0.1 3 µA TO-263 THIN Package 0.12 0.16/0.22 Ω PSOP-8 Package 0.10 0.16/0.20 3.4/3.35 Oscillator Frequency 400 500 600 kHz TOFFMIN Minimum Off-time 100 200 300 ns TONMIN Minimum On-time 100 IBIAS Feedback Bias Current VFB = 1.3V (ADJ Version Only) VEN Enable Threshold Voltage Falling VENHYST Enable Voltage Hysteresis IEN Enable Input Current TSD Thermal Shutdown Threshold θJA Thermal Resistance θJA Thermal Resistance ns 230 1.3 1.6 nA 1.9 V 0.6 V 6 µA 150 °C TJ Package Junction to ambient thermal resistance (Note 6) 22 °C/W MR Package Junction to ambient thermal resistance (Note 7) 60 °C/W EN Input = 0V 5 www.ti.com LM22676/LM22676Q For soldering specifications, refer to the following document: www.ti.com/lit/ snoa549 ESD Rating (Note 3) Human Body Model Storage Temperature Range Absolute Maximum Ratings (Note 1) LM22676/LM22676Q Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in the recommended Operating Ratings is not implied. The recommended Operating Ratings indicate conditions at which the device is functional and should not be operated beyond such conditions. Note 2: The absolute maximum specification of the ‘SW to GND’ applies to DC voltage. An extended negative voltage limit of -10V applies to a pulse of up to 50 ns. Note 3: ESD was applied using the human body model, a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 4: Typical values represent most likely parametric norms at the conditions specified and are not guaranteed. Note 5: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate Ti's Average Outgoing Quality Level (AOQL). Note 6: The value of θJA for the TO-263 THIN (TJ) package of 22°C/W is valid if package is mounted to 1 square inch of copper. The θJA value can range from 20 to 30°C/W depending on the amount of PCB copper dedicated to heat transfer. See application note AN-1797 for more information. Note 7: The value of θJA for the PSOP-8 exposed pad (MR) package of 60°C/W is valid if package is mounted to 1 square inch of copper. The θJA value can range from 42 to 115°C/W depending on the amount of PCB copper dedicated to heat transfer. Typical Performance Characteristics Unless otherwise specified the following conditions apply: Vin = 12V, TJ = 25°C. Efficiency vs IOUT and VIN VOUT = 3.3V Normalized Switching Frequency vs Temperature 30076504 30076527 Current Limit vs Temperature Normalized RDS(ON) vs Temperature 30076508 30076503 www.ti.com 6 Normalized Enable Threshold Voltage vs Temperature 30076505 30076510 Standby Quiescent Current vs Input Voltage Normalized Feedback Voltage vs Temperature 30076507 30076506 Normalized Feedback Voltage vs Input Voltage 30076509 7 www.ti.com LM22676/LM22676Q Feedback Bias Current vs Temperature LM22676/LM22676Q Simplified Block Diagram 30076581 FIGURE 1. Simplified Block Diagram www.ti.com 8 The LM22676 incorporates a voltage mode constant frequency PWM architecture. In addition, input voltage feed-forward is used to stabilize the loop gain against variations in input voltage. This allows the loop compensation to be optimized for transient performance. The power MOSFET, in conjunction with the diode, produce a rectangular waveform at the switch pin, that swings from about zero volts to VIN. The inductor and output capacitor average this waveform to become the regulator output voltage. By adjusting the duty cycle of this waveform, the output voltage can be controlled. The error amplifier compares the output voltage with the internal reference and adjusts the duty cycle to regulate the output at the desired value. The internal loop compensation of the -ADJ option is optimized for outputs of 5V and below. If an output voltage of 5V or greater is required, the -5.0 option can be used with an external voltage divider. The minimum output voltage is equal to the reference voltage; 1.285V (typ.). The functional block diagram of the LM22676 is shown in Figure 1 . Where Voff is the input voltage where the regulator shuts off, and Von is the voltage where the regulator turns on. Due to the 6 µA pull-up, the current in the divider should be much larger than this. A value of 20 kΩ, for RENB is a good first choice. Also, a zener diode may be needed between the EN pin and ground, in order to comply with the absolute maximum ratings on this pin. Precision Enable and UVLO The precision enable input (EN) is used to control the regulator. The precision feature allows simple sequencing of multiple power supplies with a resistor divider from another supply. Connecting this pin to ground or to a voltage less than 1.6V (typ.) will turn off the regulator. The current drain from the input supply, in this state, is 25 µA (typ.) at an input voltage of 12V. The EN input has an internal pull-up of about 6 µA. Therefore this pin can be left floating or pulled to a voltage greater than 2.2V (typ.) to turn the regulator on. The hysteresis on this input is about 0.6V (typ.) above the 1.6V (typ.) threshold. When driving the enable input, the voltage must never exceed the 6V absolute maximum specification for this pin. Although an internal pull-up is provided on the EN pin, it is good practice to pull the input high, when this feature is not used, especially in noisy environments. This can most easily be done by connecting a resistor between VIN and the EN pin. The resistor is required, since the internal zener diode, at the EN pin, will conduct for voltages above about 6V. The current in this zener must be limited to less than 100 µA. A resistor of 470 kΩ will limit the current to a safe value for input voltages as high 42V. Smaller values of resistor can be used at lower input voltages. The LM22676 also incorporates an input under voltage lockout (UVLO) feature. This prevents the regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The rising threshold is 4.3V (typ.) while the falling threshold is 3.9V (typ.). In some cases these thresholds may be too low to provide good system performance. The solution is to use the EN input as an external UVLO to disable the part when the input voltage falls below a lower boundary. This is often used to prevent excessive battery discharge or early turn-on during start-up. This method is also recommended to prevent abnormal device operation in applications where the input voltage falls below the minimum of 4.5V. Figure 2 shows the connections to implement this method of UVLO. The following equations can be used to determine the correct resistor values: 30076574 FIGURE 2. External UVLO Connections Duty-Cycle Limits Ideally the regulator would control the duty cycle over the full range of zero to one. However due to inherent delays in the circuitry, there are limits on both the maximum and minimum duty cycles that can be reliably controlled. This in turn places limits on the maximum and minimum input and output voltages that can be converted by the LM22676. A minimum ontime is imposed by the regulator in order to correctly measure the switch current during a current limit event. A minimum offtime is imposed in order the re-charge the bootstrap capacitor. The following equation can be used to determine the approximate maximum input voltage for a given output voltage: Where Fsw is the switching frequency and TON is the minimum on-time; both found in the Electrical Characteristics table. The worst case occurs at the lowest output voltage. If the input voltage, found in the above equation, is exceeded, the regulator will skip cycles, effectively lowering the switching frequency. The consequences of this are higher output voltage ripple and a degradation of the output voltage accuracy. The second limitation is the maximum duty cycle before the output voltage will "dropout" of regulation. The following equation can be used to approximate the minimum input voltage before dropout occurs: 9 www.ti.com LM22676/LM22676Q Detailed Operating Description The safe operating area, when in short circuit mode, is shown in Figure 3 . Operating points below and to the right of the curve represent safe operation. 45 Current Limit 40 The LM22676 has current limiting to prevent the switch current from exceeding safe values during an accidental overload on the output. This peak current limit is found in the Electrical Characteristics table under the heading of ICL. The maximum load current that can be provided, before current limit is reached, is determined from the following equation: 35 INPUT VOLTAGE (v) LM22676/LM22676Q The values of TOFF and RDS(ON) are found in the Electrical Characteristics table. The worst case here occurs at the highest load. In this equation, RL is the D.C. inductor resistance. Of course, the lowest input voltage to the regulator must not be less than 4.5V (typ.). 30 25 SAFE OPERATING AREA 20 15 10 5 Where L is the value of the power inductor. When the LM22676 enters current limit, the output voltage will drop and the peak inductor current will be fixed at ICL at the end of each cycle. The switching frequency will remain constant while the duty cycle drops. The load current will not remain constant, but will depend on the severity of the overload and the output voltage. For very severe overloads ("short-circuit"), the regulator changes to a low frequency current foldback mode of operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor current "run-away", and is associated with very low output voltages when in overload. The following equation can be used to determine what level of output voltage will cause the part to change to low frequency current foldback: 0.0 0.2 0.4 0.6 0.8 1.0 SHORT CIRCUIT VOLTAGE (v) 1.2 30076590 FIGURE 3. SOA Soft-Start The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time is fixed and can not be changed. Soft-start is reset any time the part is shut down or a thermal overload event occurs. Boot-Strap Supply The LM22676 incorporates a floating high-side gate driver to control the power MOSFET. The supply for this driver is the external boot-strap capacitor connected between the BOOT pin and SW. A good quality 10 nF ceramic capacitor must be connected to these pins with short, wide PCB traces. One reason the regulator imposes a minimum off-time is to ensure that this capacitor recharges every switching cycle. A minimum load of about 5 mA is required to fully recharge the bootstrap capacitor in the minimum off-time. Some of this load can be provided by the output voltage divider, if used. Where Fsw is the normal switching frequency and Vin is the maximum for the application. If the overload drives the output voltage to less than or equal to Vx, the part will enter current foldback mode. If a given application can drive the output voltage to ≤Vx, during an overload, then a second criterion must be checked. The next equation gives the maximum input voltage, when in this mode, before damage occurs: Thermal Protection Internal thermal shutdown circuitry protects the LM22676 should the maximum junction temperature be exceeded. This protection is activated at about 150°C, with the result that the regulator will shutdown until the temperature drops below about 135°C. Where Vsc is the value of output voltage during the overload and Fsw is the normal switching frequency. If the input voltage should exceed this value, while in foldback mode, the regulator and/or the diode may be damaged. It is important to note that the voltages in these equations are measured at the inductor. Normal trace and wiring resistance will cause the voltage at the inductor to be higher than that at a remote load. Therefore, even if the load is shorted with zero volts across its terminals, the inductor will still see a finite voltage. It is this value that should be used for Vx and Vsc in the calculations. In order to return from foldback mode, the load must be reduced to a value much lower than that required to initiate foldback. This load "hysteresis" is a normal aspect of any type of current limit foldback associated with voltage regulators. www.ti.com Internal Compensation The LM22676 has internal loop compensation designed to provide a stable regulator over a wide range of external power stage components. The internal compensation of the -ADJ option is optimized for output voltages below 5V. If an output voltage of 5V or greater is needed, the -5.0 option with an external resistor divider can be used. Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The LM22676 stability can be verified using the WEBENCH® Designer on10 In general, hand calculations or simulations can only aid in selecting good power stage components. Good design practice dictates that load and line transient testing should be done to verify the stability of the application. Also, Bode plot measurements should be made to determine stability margins. Application note AN-1889 shows how to perform a loop transfer function measurement with only an oscilloscope and function generator. Application Information TYPICAL BUCK REGULATOR APPLICAION Figure 5 shows an example of converting an input voltage range of 5.5V to 42V, to an output of 3.3v at 3A. See the application note for the LM22670, AN-1885, for more information. Alternatively, this pole should be placed between 1.5kHz and 15kHz and is given by the equation shown below: The Q factor depends on the parasitic resistance of the power stage components and is not typically in the control of the designer. Of course, loop compensation is only one consideration when selecting power stage components; see the Application Information section for more details. COMPENSATOR GAIN (dB) 40 35 -ADJ -5.0 30 25 20 15 10 5 0 100 1k 10k 100k 1M FREQUENCY (Hz) 10M 30076583 FIGURE 4. Compensator Gain 11 www.ti.com LM22676/LM22676Q line circuit simulation tool at www.ti.com. A quick start spreadsheet can also be downloaded from the online product folder. The complete transfer function for the regulator loop is found by combining the compensation and power stage transfer functions. The LM22676 has internal type III loop compensation, as detailed in Figure 4. This is the approximate "straight line" function from the FB pin to the input of the PWM modulator. The power stage transfer function consists of a D.C. gain and a second order pole created by the inductor and output capacitor(s). Due to the input voltage feedforward employed in the LM22676, the power stage D.C. gain is fixed at 20dB. The second order pole is characterized by its resonant frequency and its quality factor (Q). For a first pass design, the product of inductance and output capacitance should conform to the following equation: LM22676/LM22676Q 30076578 FIGURE 5. Typical Buck Regulator Application it can saturate resulting in damage to the LM22676 and/ or the power diode. EXTERNAL COMPONENTS The following guidelines should be used when designing a step-down (buck) converter with the LM22676. INPUT CAPACITOR The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current during switch on-time. Low ESR ceramic capacitors are preferred. Larger values of input capacitance are desirable to reduce voltage ripple and noise on the input supply. This noise may find its way into other circuitry, sharing the same input supply, unless adequate bypassing is provided. A very approximate formula for determining the input voltage ripple is shown below: INDUCTOR The inductor value is determined based on the load current, ripple current, and the minimum and maximum input voltages. To keep the application in continuous conduction mode (CCM), the maximum ripple current, IRIPPLE , should be less than twice the minimum load current. The general rule of keeping the inductor current peak-to-peak ripple around 30% of the nominal output current is a good compromise between excessive output voltage ripple and excessive component size and cost. Using this value of ripple current, the value of inductor, L, is calculated using the following formula: Where Vri is the peak-to-peak ripple voltage at the switching frequency. Another concern is the RMS current passing through this capacitor. The following equation gives an approximation to this current: where Fsw is the switching frequency and Vin should be taken at its maximum value, for the given application. The above formula provides a guide to select the value of the inductor L; the nearest standard value will then be used in the circuit. Once the inductor is selected, the actual ripple current can be found from the equation shown below: The capacitor must be rated for at least this level of RMS current at the switching frequency. All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum capacitance up to the desired value. This may also help with RMS current constraints by sharing the current among several capacitors. Many times it is desirable to use an electrolytic capacitor on the input, in parallel with the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply caused by long power leads. This method can also help to reduce voltage spikes that may exceed the maximum input voltage rating of the LM22676. It is good practice to include a high frequency bypass capacitor as close as possible to the LM22676. This small case size, low ESR, ceramic capacitor should be connected directly to the VIN and GND pins with the shortest possible PCB traces. Values in the range of 0.47 µF to 1 µF are appropriate. This Increasing the inductance will generally slow down the transient response but reduce the output voltage ripple. Reducing the inductance will generally improve the transient response but increase the output voltage ripple. The inductor must be rated for the peak current, IPK, in a given application, to prevent saturation. During normal loading conditions, the peak current is equal to the load current plus 1/2 of the inductor ripple current. During an overload condition, as well as during certain load transients, the controller may trip current limit. In this case the peak inductor current is given by ICL, found in the Electrical Characteristics table. Good design practice requires that the inductor rating be adequate for this overload condition. If the inductor is not rated for the maximum expected current, www.ti.com 12 OUTPUT CAPACITOR The output capacitor is responsible for filtering the output voltage and supplying load current during transients. Capacitor selection depends on application conditions as well as ripple and transient requirements. Best performance is achieved with a parallel combination of ceramic capacitors and a low ESR SP™ or POSCAP™ type. Very low ESR capacitors such as ceramics reduce the output ripple and noise spikes, while higher value electrolytics or polymer provide large bulk capacitance to supply transients. Assuming very low ESR, the following equation gives an approximation to the output voltage ripple: Again a value of RFBB of about 1k Ω is a good first choice. Typically, a total value of 100 µF, or greater, is recommended for output capacitance. In applications with Vout less than 3.3V, it is critical that low ESR output capacitors are selected. This will limit potential output voltage overshoots as the input voltage falls below the device normal operating range. 30076594 FIGURE 6. Resistive Feedback Divider A maximum value of 10 kΩ is recommended for the sum of RFBB and RFBT to maintain good output voltage accuracy for the -ADJ option. A maximum of 2 kΩ is recommended for the -5.0 option. For the -5.0 option, the total internal divider resistance is typically 9.93 kΩ. In all cases the output voltage divider should be placed as close as possible to the FB pin of the LM22676; since this is a high impedance input and is susceptible to noise pick-up. BOOT-STRAP CAPACITOR The bootstrap capacitor between the BOOT pin and the SW pin supplies the gate current to turn on the N-channel MOSFET. The recommended value of this capacitor is 10 nF and should be a good quality, low ESR ceramic capacitor. In some cases it may be desirable to slow down the turn-on of the internal power MOSFET, in order to reduce EMI. This can be done by placing a small resistor in series with the Cboot capacitor. Resistors in the range of 10Ω to 50Ω can be used. This technique should only be used when absolutely necessary, since it will increase switching losses and thereby reduce efficiency. POWER DIODE A Schottky type power diode is required for all LM22676 applications. Ultra-fast diodes are not recommended and may result in damage to the IC due to reverse recovery current transients. The near ideal reverse recovery characteristics and low forward voltage drop of Schottky diodes are particularly important for high input voltage and low output voltage applications common to the LM22676. The reverse breakdown rating of the diode should be selected for the maximum VIN, plus some safety margin. A good rule of thumb is to select a diode with a reverse voltage rating of 1.3 times the maximum input voltage. Select a diode with an average current rating at least equal to the maximum load current that will be seen in the application. OUTPUT VOLTAGE DIVIDER SELECTION For output voltages between about 1.285V and 5V, the -ADJ option should be used, with an appropriate voltage divider as shown in Figure 6. The following equation can be used to calculate the resistor values of this divider: A good value for RFBB is 1k Ω. This will help to provide some of the minimum load current requirement and reduce susceptibility to noise pick-up. The top of RFBT should be connected directly to the output capacitor or to the load for remote sensing. If the divider is connected to the load, a local highfrequency bypass should be provided at that location. For output voltages of 5V, the -5.0 option should be used. In this case no divider is needed and the FB pin is connected to the output. The approximate values of the internal voltage divider are as follows: 7.38kΩ from the FB pin to the input of the error amplifier and 2.55kΩ from there to ground. Both the -ADJ and -5.0 options can be used for output voltages greater than 5V, by using the correct output divider. As mentioned in the Internal Loop Compensation section, the -5.0 option is optimized for output voltages of 5V. However, Circuit Board Layout Board layout is critical for the proper operation of switching power supplies. First, the ground plane area must be sufficient for thermal dissipation purposes. Second, appropriate guidelines must be followed to reduce the effects of switching noise. Switch mode converters are very fast switching devices. In such cases, the rapid increase of input current combined with the parasitic trace inductance generates unwanted L di/dt noise spikes. The magnitude of this noise tends to increase as the output current increases. This noise may turn into electromagnetic interference (EMI) and can also cause problems in device performance. Therefore, care must be taken in layout to minimize the effect of this switching noise. 13 www.ti.com LM22676/LM22676Q for output voltages greater than 5V, this option may provide better loop bandwidth than the -ADJ option, in some applications. If the -5.0 option is to be used at output voltages greater than 5V, the following equation should be used to determine the resistor values in the output divider: capacitor helps to provide a low impedance supply to sensitive internal circuitry. It also helps to suppress any fast noise spikes on the input supply that may lead to increased EMI. LM22676/LM22676Q The most important layout rule is to keep the AC current loops as small as possible. Figure 7 shows the current flow in a buck converter. The top schematic shows a dotted line which represents the current flow during the FET switch on-state. The middle schematic shows the current flow during the FET switch off-state. The bottom schematic shows the currents referred to as AC currents. These AC currents are the most critical since they are changing in a very short time period. The dotted lines of the bottom schematic are the traces to keep as short and wide as possible. This will also yield a small loop area reducing the loop inductance. To avoid functional problems due to layout, review the PCB layout example. Best results are achieved if the placement of the LM22676, the bypass capacitor, the Schottky diode, RFBB, RFBT, and the inductor are placed as shown in the example. Note that, in the layout shown, R1 = RFBB and R2 = RFBT. It is also recommended to use 2oz copper boards or heavier to help thermal dissipation and to reduce the parasitic inductances of board traces. See application note AN-1229 for more information. regulator. The easiest method to determine the power dissipation within the LM22676 is to measure the total conversion losses then subtract the power losses in the diode and inductor. The total conversion loss is the difference between the input power and the output power. An approximation for the power diode loss is: Where VD is the diode voltage drop. An approximation for the inductor power is: where RL is the DC resistance of the inductor and the 1.1 factor is an approximation for the AC losses. The regulator has an exposed thermal pad to aid power dissipation. Adding multiple vias under the device to the ground plane will greatly reduce the regulator junction temperature. Selecting a diode with an exposed pad will also aid the power dissipation of the diode. The most significant variables that affect the power dissipation of the regulator are output current, input voltage and operating frequency. The power dissipated while operating near the maximum output current and maximum input voltage can be appreciable. The junction-toambient thermal resistance of the LM22676 will vary with the application. The most significant variables are the area of copper in the PC board, the number of vias under the IC exposed pad and the amount of forced air cooling provided. A large continuous ground plane on the top or bottom PCB layer will provide the most effective heat dissipation. The integrity of the solder connection from the IC exposed pad to the PC board is critical. Excessive voids will greatly diminish the thermal dissipation capacity. The junction-to-ambient thermal resistance of the LM22676 PSOP-8 package is specified in the Electrical Characteristics table. See application note AN-2020 for more information. 30076524 FIGURE 7. Current Flow in a Buck Application Thermal Considerations The components with the highest power dissipation are the power diode and the power MOSFET internal to the LM22676 www.ti.com 14 LM22676/LM22676Q PCB Layout Example for TO-263 THIN Package 30076525 15 www.ti.com LM22676/LM22676Q PCB Layout Example for PSOP-8 Package 30076541 www.ti.com 16 LM22676/LM22676Q 30076598 FIGURE 8. Inverting Regulator Application 17 www.ti.com LM22676/LM22676Q Physical Dimensions inches (millimeters) unless otherwise noted 7-Lead Plastic TO-263 THIN Package TI Package Number TJ7A 8-Lead PSOP Package TI Package Number MRA08B www.ti.com 18 LM22676/LM22676Q Notes 19 www.ti.com LM22676/LM22676Q 42V, 3A SIMPLE SWITCHER® Step-Down Voltage Regulator with Features 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. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by government requirements, testing of all parameters of each product is not necessarily performed. TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and applications using TI components. To minimize the risks associated with customer products and applications, customers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in such safety-critical applications. TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated products in automotive applications, TI will not be responsible for any failure to meet such requirements. Following are URLs where you can obtain information on other Texas Instruments products and application solutions: Products Applications Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps DSP dsp.ti.com Energy and Lighting www.ti.com/energy Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial Interface interface.ti.com Medical www.ti.com/medical Logic logic.ti.com Security www.ti.com/security Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com OMAP Mobile Processors www.ti.com/omap Wireless Connectivity www.ti.com/wirelessconnectivity TI E2E Community Home Page e2e.ti.com Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2012, Texas Instruments Incorporated