September 2004 LM2744 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference General Description Features The LM2744 is a high-speed synchronous buck regulator controller with an externally adjustable reference voltage (between 0.5V to 1.5V). It can provide simple down conversion to output voltages as low as 0.5V. Though the control sections of the IC are rated for 3 to 6V, the driver sections are designed to accept input supply rails as high as 16V. The use of adaptive non-overlapping MOSFET gate drivers helps avoid potential shoot-through problems while maintaining high efficiency. The IC is designed for the more cost-effective option of driving only N-channel MOSFETs in both the highside and low-side positions. It senses the low-side switch voltage drop for providing a simple, adjustable current limit. The fixed-frequency voltage-mode PWM control architecture is adjustable from 50 kHz to 1 MHz with one external resistor. This wide range of switching frequency gives the power supply designer the flexibility to make better tradeoffs between component size, cost and efficiency. Power stage input voltage from 1V to 16V Control stage input voltage from 3V to 6V Output voltage adjustable down to 0.5V Power good flag and shutdown Output overvoltage and undervoltage detection External reference voltage 0.5V to 1.5V Low-side adjustable current sensing Adjustable soft-start Tracking and sequencing with shutdown and soft start pins n Switching frequency from 50 kHz to 1 MHz n TSSOP-14 package Features include soft-start, input undervoltage lockout (UVLO) and Power Good (based on both undervoltage and overvoltage detection). In addition, the shutdown pin of the IC can be used for providing startup delay, and the soft-start pin can be used for implementing precise tracking, for the purpose of sequencing with respect to an external rail. n n n n n n n n n Applications n n n n n 3.3V Buck Regulation Cable Modem, DSL and ADSL Laser Jet and Ink Jet Printers Low Voltage Power Modules DSP, ASIC, Core and I/O Typical Application 20106001 © 2004 National Semiconductor Corporation DS201060 www.national.com LM2744 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference ADVANCE INFORMATION LM2744 Connection Diagram 20106002 14-Lead Plastic TSSOP θJA = 155˚C/W NS Package Number MTC14 SS/TRACK (Pin 9) - Soft-start and tracking pin. This pin is internally connected to the non-inverting input of the error amplifier during soft-start, and in fact any time the SS/ TRACK pin voltage happens to be below the internal reference voltage. For the basic soft-start function, a capacitor of minimum value 1nF is connected from this pin to ground. To track the rising ramp of another power supply’s output, connect a resistor divider from the output of that supply to this pin as described in Application Information. FB (Pin 10) - Feedback pin. This is the inverting input of the error amplifier, which is used for sensing the output voltage and compensating the control loop. FREQ (Pin 11) - Frequency adjust pin. The switching frequency is set by connecting a resistor of suitable value between this pin and ground. The equation for calculating the exact value is provided in Application Information, but some typical values (rounded up to the nearest standard values) are 324 kΩ for 100 kHz, 97.6 kΩ for 300 kHz, 56.2 kΩ for 500 kHz, 24.9 kΩ for 1 MHz. SD (Pin 12) - IC shutdown pin. Pull this pin to VCC to ensure the IC is enabled. Connect to ground to disable the IC. Under shutdown, both high-side and low-side drives are off. This pin also features a precision threshold for power supply sequencing purposes, as well as a low threshold to ensure minimal quiescent current. VREF (Pin 13) - External reference. This goes to the noninverting input of the error amplifier. Any desired reference voltage between 0.5V to 1.5V can be connected to this pin (with appropriate filtering if necessary). HG (Pin 14) - High-gate drive pin. This is the gate drive for the high-side N-channel MOSFET. This signal is interlocked with LG (Pin 2) to avoid shoot-through. Pin Description BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the high-side gate driver. When the high-side MOSFET turns on, the voltage on this pin should be at least one gate threshold above the regulator input voltage VIN to properly turn on the MOSFET. See MOSFET Gate Drivers in the Application Information section for more details on how to select MOSFETs. LG (Pin 2) - Low-gate drive pin. This is the gate drive for the low-side N-channel MOSFET. This signal is interlocked with the high-side gate drive HG (Pin 14), so as to avoid shootthrough. PGND (Pin 3) - Power ground. This is also the ground for the low-side MOSFET driver. This pin must be connected on the PCB ground plane, which is usually also the system ground. SGND (Pin 4) - Signal ground. It should be connected appropriately to the ground plane with due regard to good layout practices in switching power regulator circuits. VCC (Pin 5) Supply rail for the control sections of the IC. PWGD (Pin 6) - Power Good pin. This is an open drain output, which is typically meant to be connected to VCC or any other low voltage source through a pull-up resistor. The voltage on this pin is thus pulled low under output fault conditions (undervoltage or overvoltage) and also under UVLO. ISEN (Pin 7) - Current limit threshold setting pin. This sources a fixed 40 µA current. A resistor of appropriate value should be connected between this pin and the drain of the low-side MOSFET (switch node). EAO (Pin 8) - Output of the error amplifier. The voltage level on this pin is compared with an internally generated ramp signal to determine the duty cycle. This pin is necessary for compensating the control loop. www.national.com 2 Infrared or Convection (20sec) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. ESD Rating (Note 3) -0.3 to 21V Supply Voltage Range (VCC) 3V to 6V -0.3 to VCC + 0.3V Junction Temperature Range (TJ) −40˚C to +125˚C BOOT Voltage All other pins 2 kV Operating Ratings -0.3 to 7V VCC 235˚C Junction Temperature 150˚C Storage Temperature −65˚C to 150˚C Thermal Resistance (θJA) 155˚C/W Voltage on FB and VREF Soldering Information Lead Temperature (soldering, 10sec) 0.5V to 1.5V 260˚C Electrical Characteristics VCC = 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA= TJ= 25˚C. Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol VON IQ_VCC Parameter UVLO Thresholds Operating VCC Current Conditions Min Rising Falling Typ 1.0 1.5 2.1 VCC = 5V, VSD = 3.3V Fsw = 600kHz 1.0 1.7 2.1 25 mA Shutdown VCC Current VCC = 3.3V, VSD = 0V 1 PWGD Pin Response Time VFB Rising 6 tPWGD2 PWGD Pin Response Time VFB Falling ISS-ON SS Pin Source Current VSS = 0V ISS-OC SS Pin Sink Current During Over Current VSS = 2.5V 10 µA µs 6 5 µs 15 90 ISEN Pin Source Current Trip Point Units V VCC = 3.3V, VSD = 3.3V Fsw = 600kHz tPWGD1 ISEN-TH Max 2.76 2.42 µA µA 20 40 60 µA -8 1 8 mV ERROR AMPLIFIER VOS GBW G Error Amplifier Input Offset Voltage Error Amplifier Unity Gain Bandwidth 9 MHz Error Amplifier DC Gain 106 dB SR Error Amplifier Slew Rate 3.2 V/µs IEAO EAO Pin Current Sourcing and Sinking Capability VEAO = 1.5, FB = 0.55V VEAO = 1.5, FB = 0.65V 2.6 9.2 mA VEA Error Amplifier Output Voltage Minimum 0 V Maximum 2 V 3 www.national.com LM2744 Absolute Maximum Ratings (Note 1) LM2744 Electrical Characteristics (Continued) VCC = 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA= TJ= 25˚C. Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol Parameter Conditions Min Typ Max Units 90 µA GATE DRIVE IQ-BOOT BOOT Pin Quiescent Current VBOOT = 12V, VSD = 0 18 RHG_UP High-Side MOSFET Driver Pull-Up ON resistance VBOOT - VSW = 9.5V@350mA 3 Ω RHG_DN High-Side MOSFET Driver Pull-Down ON resistance VBOOT - VSW = 4.5V@350mA 2 Ω RLG_UP Low-Side MOSFET Driver Pull-Up ON resistance VBOOT - VSW = 9.5V@350mA 3 Ω RLG_DN Low-Side MOSFET Driver Pull-Down ON resistance VBOOT - VSW = 4.5V@350mA 2 Ω OSCILLATOR FSW PWM Frequency RFADJ = 702.1 kΩ 50 RFADJ = 98.74 kΩ 300 RFADJ = 45.74 kΩ 475 RFADJ = 24.91 kΩ D Max High-Side Duty Cycle 600 725 kHz 1000 FSW = 300kHz FSW = 600kHz FSW = 1MHz 80 76 73 % LOGIC INPUTS AND OUTPUTS V STBY-IH Standby High Trip Point VFB = 0.575V, VBOOT = 3.3V, VSD Rising V STBY-IL Standby Low Trip Point VFB = 0.575V, VBOOT = 3.3V, VSD Falling VSD Rising 1.1 0.232 V V V SD-IH SD Pin Logic High Trip Point V SD-IL SD Pin Logic Low Trip Point VSD Falling PWGD Pin Trip Points FB Falling, VREF = 0.5V 0.3262 0.347 0.3678 FB Falling, VREF = 1.5V 0.993 1.045 1.097 V VPWGD-TH-LO 1.3 0.8 V V V VPWGD-TH-HI PWGD Pin Trip Points FB Rising, VREF = 0.5V 0.551 0.586 0.621 V FB Rising, VREF = 1.5V 1.6692 1.757 1.8448 V VPWGD-HYS PWGD Hysteresis FB Falling FB Rising 60 90 mV Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions for which the device operates correctly. Opearting Ratings do not imply guaranteed performance limits. Note 2: The power MOSFETs can run on a separate 1V to 16V rail (Input voltage, VIN). Practical lower limit of VIN depends on selection of the external MOSFET. Note 3: The human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin. www.national.com 4 Efficiency (VOUT = 2.5V, VREF = 0.8V) VCC = 3.3V, FSW = 300kHz Efficiency (VOUT = 1.2V, VREF = 0.6V) VCC = 3.3V, FSW = 300kHz 20106040 20106057 Efficiency (VOUT = 3.3V, VREF = 1.2V) VCC = 5V, FSW = 300kHz Output Voltage vs Temperature 20106090 20106041 BOOT Pin Current vs Temperature for BOOT Voltage = 3.3V FSW = 300kHz, FDS6898A FET, No-Load VCC Operating Current plus BOOT Current vs Frequency FDS6898A FET (TA = 25˚C) 20106045 20106043 5 www.national.com LM2744 Typical Performance Characteristics LM2744 Typical Performance Characteristics (Continued) BOOT Pin Current vs Temperature for BOOT Voltage = 12V FSW = 300kHz, FDS6898A FET, No-Load BOOT Pin Current vs Temperature for BOOT Voltage = 5V FSW = 300kHz, FDS6898A FET, No-Load 20106042 20106044 Output Voltage vs Output Current (VOUT set to 1.2V nominal) Frequency vs Temperature (FSW set to 600kHz nominal) 20106060 20106056 Switch Waveforms (HG Falling) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V, IOUT = 4A, CSS = 12nF, FSW = 300kHz Switch Waveforms (HG Rising) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, FSW = 300kHz 20106047 20106046 www.national.com 6 (Continued) Start-Up (Full-Load) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, FSW = 300kHz Start-Up (No-Load) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V CSS = 12nF, FSW = 300kHz 20106048 20106049 Load Transient Response (IOUT = 0A to 4A) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V CSS = 12nF, FSW = 300kHz Shutdown (Full-Load) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, FSW = 300kHz 20106050 20106051 Load Transient Response VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V CSS = 12nF, FSW = 300kHz Load Transient Response (IOUT = 4A to 0A) VCC = 3.3V, VIN = 5V, VREF = 0.6V, VOUT = 1.2V CSS = 12nF, FSW = 300kHz 20106052 20106053 7 www.national.com LM2744 Typical Performance Characteristics LM2744 Typical Performance Characteristics (Continued) Line Transient Response (VIN = 7V to 3V) VCC = 3.3V, VREF = 0.6V, VOUT = 1.2V IOUT = 2A, FSW = 300kHz Line Transient Response (VIN = 3V to 7V) VCC = 3.3V, VREF = 0.6V, VOUT = 1.2V IOUT = 2A, FSW = 300kHz 20106054 www.national.com 20106055 8 LM2744 Block Diagram 20106003 CSS = tSS / (100 x VREF) Where CSS is in µF and tSS is in ms. During soft-start the Power Good flag is forced low and it is released when the FB pin voltage reaches 70% of VREF. At this point the chip enters normal operation mode, and the output overvoltage and undervoltage monitoring starts. Application Information THEORY OF OPERATION The LM2744 is a voltage-mode, high-speed synchronous buck regulator with a PWM control scheme. It is designed for use in set-top boxes, thin clients, DSL/Cable modems, and other applications that require high efficiency buck converters. It has output shutdown (SD), input undervoltage lock-out (UVLO) mode and power good (PWGD) flag (based on overvoltage and undervoltage detection). The overvoltage and undervoltage signals are OR-gated to drive the power good signal and provide a logic signal to the system if the output voltage goes out of regulation. Current limit is achieved by sensing the voltage VDS across the low side MOSFET. NORMAL OPERATION While in normal operation mode, the LM2744 regulates the output voltage by controlling the duty cycle of the high-side and low-side MOSFETs (see Typical Application Circuit).The equation governing output voltage is: START UP/SOFT-START When VCC exceeds 2.76V and the shutdown pin (SD) sees a logic high, the soft-start period begins. Then an internal, fixed 10 µA source begins charging the soft-start capacitor. During soft-start the voltage on the soft-start capacitor CSS is connected internaly to the non-inverting input of the error amplifier. The soft-start period lasts until the voltage on the soft-start capacitor exceeds the LM2744 reference voltage. At this point the reference voltage takes over at the noninverting error amplifier input. The capacitance of CSS determines the length of the soft-start period, and can be approximated by: The PWM frequency is adjustable between 50 kHz and 1 MHz and is set by an external resistor, RFADJ, between the FREQ pin and ground. The resistance needed for a desired frequency is approximately: Where FSW is in Hz and RFADJ is in kΩ. 9 www.national.com LM2744 Application Information (Continued) TRACKING A VOLTAGE LEVEL The LM2744 can track the output of a master power supply during soft-start by connecting a resistor divider to the SS/ TRACK pin. In this way, the output voltage slew rate of the LM2744 will be controlled by the master supply for loads that require precise sequencing. Because the output of the master supply is divided down, in order to track properly the output voltage of the LM2744 must be lower than the voltage of the master supply. When the tracking function is used no soft-start capacitor should be connected to the SS/TRACK pin. However in all other cases, a CSS value of at least 1nF between the soft-start pin and ground should be used. 20106008 FIGURE 2. Tracking with Equal Soft-Start Time TRACKING A VOLTAGE SLEW RATE The tracking feature can alternatively be used not to make both rails reach regulation at the same time but rather to have similar rise rates (in terms of output dV/dt). This method ensures that the output voltage of the LM2744 always reaches regulation before the output voltage of the master supply. In this case, the tracking resistors can be determined based on the following equation: 20106007 For the example case of VOUT1 = 5V and VOUT2 = 1.8V, with RT1 set to 150Ω as before, RT2 is calculated from the above equation to be 267Ω. A timing diagram for the case of equal slew rates is shown in Figure 3. FIGURE 1. Tracking Circuit One way to use the tracking feature is to design the tracking resistor divider so that the master supply’s output voltage (VOUT1) and the LM2744’s output voltage (represented symbolically in Figure 1 as VOUT2, i.e. without explicitly showing the power components) both rise together and reach their target values at the same time. For this case, the equation governing the values of the tracking divider resistors RT1 and RT2 is: The current through RT1 should be about 3-4 mA for precise tracking. The final voltage of the SS/TRACK pin should be set slightly higher than the reference voltage (say about 50mV higher as in the above equation). If the master supply voltage was 5V and the LM2744 output voltage was 1.8V, for example, then the value of RT1 needed to give the two supplies identical soft-start times would be 150Ω if VREF was set to 0.6V. A timing diagram for the equal soft-start time case is shown in Figure 2. 20106010 FIGURE 3. Tracking with Equal Slew Rates SEQUENCING The start up/soft-start of the LM2744 can be delayed for the purpose of sequencing by connecting a resistor divider from the output of a master power supply to the SD pin, as shown in Figure 4. www.national.com 10 LM2744 Application Information (Continued) 20106014 20106011 FIGURE 4. Sequencing Circuit FIGURE 5. Delay for Sequencing A desired delay time tDELAY between the startup of the master supply output voltage and the LM2744 output voltage can be set based on the SD pin low-to-high threshold VSD-IH and the slew rate of the voltage at the SD pin, SRSD: tDELAY = VSD-IH / SRSD Note again, that in Figure 4, the LM2744’s output voltage has been represented symbolically as VOUT2, i.e. without explicitly showing the power components. VSD-IH is typically 1.08V and SRSD is the slew rate of the SD pin voltage. The values of the sequencing divider resistors RS1 and RS2 set the SRSD based on the master supply output voltage slew rate, SROUT1, using the following equation: When connecting a resistor divider to the SD pin of the LM2744 some care has to be taken. Once the SD voltage goes above VSD-IH, a 17 µA pull-up current is activated as shown in Figure 6. This current is used to create the internal hysteresis ()170mV); however, high external impedances will affect the SD pin logic thresholds as well. The external impedance used for the sequencing divider network should preferably be a small fraction of the impedance of the SD pin for good performance (around 1kΩ). SD PIN IMPEDANCE For example, if the master supply output voltage slew rate was 1V/ms and the desired delay time between the startup of the master supply and LM2744 output voltage was 5ms, then the desired SD pin slew rate would be (1.08V/5ms) = 0.216V/ms. Due to the internal impedance of the SD pin, the maximum recommended value for RS2 is 1kΩ. To achieve the desired slew rate, RS2 would then be 274Ω. A timing diagram for this example is shown in Figure 5. 20106006 FIGURE 6. SD Pin Logic MOSFET GATE DRIVERS The LM2744 has two gate drivers designed for driving N-channel MOSFETs in a synchronous mode. Note that unlike most other synchronous controllers, the bootstrap capacitor of the LM2744 provides power not only to the driver of the upper MOSFET, but the lower MOSFET driver 11 www.national.com LM2744 Application Information powers both the VCC and the bootstrap circuit, providing efficient drive for logic level MOSFETs. An example of this circuit is shown in Figure 8. (Continued) too (both drivers are ground referenced, i.e. no floating driver). To fully turn the top MOSFET on, the BOOT voltage must be at least one gate threshold greater than VIN when the high-side drive goes high. This bootstrap voltage is usually supplied from a local charge pump structure. But looking at the Typical Application schematic, this also means that the difference voltage VCC - VD1, which is the voltage the bootstrap capacitor charges up to, must be always greater than the maximum tolerance limit of the threshold voltage of the upper MOSFET. Here VD1 is the forward voltage drop across the bootstrap diode D1. This therefore may place restrictions on the minimum input voltage and/or type of MOSFET used. The most basic charge bootstrap pump circuit can be built using one Schottky diode and a small capacitor, as shown in Figure 7. The capacitor CBOOT serves to maintain enough voltage between the top MOSFET gate and source to control the device even when the top MOSFET is on and its source has risen up to the input voltage level. The charge pump circuitry is fed from VCC, which can operate over a range from 3.0V to 6.0V. Using this basic method the voltage applied to the gates of both high-side and low-side MOSFETs is VCC - VD. This method works well when VCC is 5V ± 10%, because the gate drives will get at least 4.0V of drive voltage during the worst case of VCC-MIN = 4.5V and VD-MAX = 0.5V. Logic level MOSFETs generally specify their on-resistance at VGS = 4.5V. When VCC = 3.3V ± 10%, the gate drive at worst case could go as low as 2.5V. Logic level MOSFETs are not guaranteed to turn on, or may have much higher on-resistance at 2.5V. Sub-logic level MOSFETs, usually specified at VGS = 2.5V, will work, but are more expensive, and tend to have higher on-resistance. The circuit in Figure 7 works well for input voltages ranging from 1V up to 16V and VCC = 5V ± 10%, because the drive voltage depends only on VCC. 20106013 FIGURE 8. LP8340 Feeding Basic Charge Pump Figure 9 shows a second possibility for bootstrapping the MOSFET drives using a doubler. This circuit provides an equal voltage drive of VCC - 3VD + VIN to both the high-side and low-side MOSFET drives. This method should only be used in circuits that use 3.3V for both VCC and VIN. Even with VIN = VCC = 3.0V (10% lower tolerance on 3.3V) and VD = 0.5V both high-side and low-side gates will have at least 4.5V of drive. The power dissipation of the gate drive circuitry is directly proportional to gate drive voltage, hence the thermal limits of the LM2744 IC will quickly be reached if this circuit is used with VCC or VIN voltages over 5V. 20106019 20106012 FIGURE 9. Charge Pump with Added Gate Drive FIGURE 7. Basic Charge Pump (Bootstrap) All the gate drive circuits shown in the above figures typically use 100nF ceramic capacitors in the bootstrap locations. Note that the LM2744 can be paired with a low cost linear regulator like the LP8340 to run from a single input rail between 6.0 and 16V. The 5V output of the linear regulator www.national.com 12 LM2744 Application Information (Continued) POWER GOOD SIGNAL The Power Good signal is an OR-gated flag which takes into account both output overvoltage and undervoltage conditions. If the feedback pin (FB) voltage is 18% above its nominal value or falls 28% below that value the Power Good flag goes low. The Power Good flag can be used to signal other circuits that the output voltage has fallen out of regulation, however the switching of the LM2744 continues regardless of the state of the Power Good signal. The Power Good flag will return to logic high whenever the feedback pin voltage is between 72% and 118% of VREF. UVLO The 2.76V turn-on threshold on VCC has a built in hysteresis of about 300mV. If VCC drops below 2.42V, the chip enters UVLO mode. UVLO consists of turning off the top and bottom MOSFETS and remaining in that condition until VCC rises above 2.76V. As with shutdown, the soft-start capacitor is discharged through an internal MOSFET, ensuring that the next start-up will be controlled by the soft-start circuitry. 20106088 FIGURE 10. Current Limit Threshold CURRENT LIMIT Current limit is realized by sensing the voltage across the low-side MOSFET while it is on. The RDSON of the MOSFET is a known value; hence the current through the MOSFET can be determined as: VDS = IOUT * RDSON The current through the low-side MOSFET while it is on is also the falling portion of the inductor current. The current limit threshold is determined by an external resistor, RCS, connected between the switching node and the ISEN pin. A constant current of 40 µA is forced through RCS, causing a fixed voltage drop. This fixed voltage is compared against VDS and if the latter is higher, the current limit of the chip has been reached. RCS can be found by using the following equation: RCS = RDSON x ILIM / 40 µA For example, a conservative 15A current limit in a 10A design with a minimum RDSON of 10mΩ would require a 3.74kΩ resistor. Because current sensing is done across the low-side MOSFET, no minimum high-side on-time is necessary. The LM2744 enters current limit mode if the inductor current exceeds the current limit threshold at the point where the high-side MOSFET turns off and the low-side MOSFET turns on. (The point of peak inductor current, see Figure 10). Note that in normal operation mode the high-side MOSFET always turns on at the beginning of a clock cycle. In current limit mode, by contrast, the high-side MOSFET on-pulse is skipped. This causes inductor current to fall. Unlike a normal operation switching cycle, however, in a current limit mode switching cycle the high-side MOSFET will turn on as soon as inductor current has fallen to the current limit threshold. The LM2744 will continue to skip high-side MOSFET pulses until the inductor current peak is below the current limit threshold, at which point the system resumes normal operation. Unlike a high-side MOSFET current sensing scheme, which limits the peaks of inductor current, low-side current sensing is only allowed to limit the current during the converter off-time, when inductor current is falling. Therefore in a typical current limit plot the valleys are normally well defined, but the peaks are variable, according to the duty cycle. The PWM error amplifier and comparator control the off-pulse of the high-side MOSFET, even during current limit mode, meaning that peak inductor current can exceed the current limit threshold. Assuming that the output inductor does not saturate, the maximum peak inductor current during current limit mode can be calculated with the following equation: where TOSC is the inverse of switching frequency FSW. The 200ns term represents the minimum off-time of the duty cycle, which ensures enough time for correct operation of the current sensing circuitry. In order to minimize the time period in which peak inductor current exceeds the current limit threshold, the IC also discharges the soft-start capacitor through a fixed 90 µA sink. The output of the LM2744 internal error amplifier is limited by the voltage on the soft-start capacitor. Hence, discharging the soft-start capacitor reduces the maximum duty cycle D of the controller. During severe current limit this reduction in duty cycle will reduce the output voltage if the current limit conditions last for an extended time. Output inductor current will be reduced in turn to a flat level equal to the current limit threshold. The third benefit of the soft-start capacitor discharge is a smooth, controlled ramp of output voltage when the current limit condition is cleared. SHUTDOWN If the shutdown pin is pulled low, (below 0.8V) the LM2744 enters shutdown mode, and discharges the soft-start capacitor through a MOSFET switch. The high and low-side MOSFETs are turned off. The LM2744 remains in this state as long as VSD sees a logic low (see the Electrical Characteristics table). To assure proper IC start-up the shutdown pin 13 www.national.com LM2744 Application Information (Continued) should not be left floating. For normal operation this pin should be connected directly to VCC or to another voltage between 1.3V to VCC (see the Electrical Characteristics table). L = 1.6µH Here we have plugged in the values for output current ripple, input voltage, output voltage, switching frequency, and assumed a 40% peak-to-peak output current ripple. This yields an inductance of 1.6 µH. The output inductor must be rated to handle the peak current (also equal to the peak switch current), which is (IOUT + 0.5*∆IOUT) 4.8A for a 4A design. The Coilcraft DO3316P-222P is 2.2 µH, is rated to 7.4A peak, and has a direct current resistance (DCR) of 12mΩ. DESIGN CONSIDERATIONS The following is a design procedure for all the components needed to create the Typical Application Circuit shown on the front page. This design converts 3.3V (VIN) to 1.2V (VOUT) at a maximum load of 4A with an efficiency of 89% and a switching frequency of 300kHz. The same procedures can be followed to create many other designs with varying input voltages, output voltages, and load currents. After selecting an output inductor, inductor current ripple should be re-calculated with the new inductance value, as this information is needed to select the output capacitor. Re-arranging the equation used to select inductance yields the following: Input Capacitor The input capacitors in a Buck converter are subjected to high stress due to the input current trapezoidal waveform. Input capacitors are selected for their ripple current capability and their ability to withstand the heat generated since that ripple current passes through their ESR. Input rms ripple current is approximately: VIN(MAX) is assumed to be 10% above the steady state input voltage, or 3.6V. The actual current ripple will then be 1.2A. Peak inductor/switch current will be 4.6A. where duty cycle D = VOUT/VIN. The power dissipated by each input capacitor is: Output Capacitor The output capacitor forms the second half of the power stage of a Buck switching converter. It is used to control the output voltage ripple (∆VOUT) and to supply load current during fast load transients. In this example the output current is 4A and the expected type of capacitor is an aluminum electrolytic, as with the input capacitors. Other possibilities include ceramic, tantalum, and solid electrolyte capacitors, however the ceramic type often do not have the large capacitance needed to supply current for load transients, and tantalums tend to be more expensive than aluminum electrolytic. Aluminum capacitors tend to have very high capacitance and fairly low ESR, meaning that the ESR zero, which affects system stability, will be much lower than the switching frequency. The large capacitance means that at the switching frequency, the ESR is dominant, hence the type and number of output capacitors is selected on the basis of ESR. One simple formula to find the maximum ESR based on the desired output voltage ripple, ∆VOUT and the designed output current ripple, ∆IOUT, is: where n is the number of capacitors, and ESR is the equivalent series resistance of each capacitor. The equation above indicates that power loss in each capacitor decreases rapidly as the number of input capacitors increases. The worst-case ripple for a Buck converter occurs during full load and when the duty cycle (D) is 0.5. For this 3.3V to 1.2V design the duty cycle is 0.364. For a 4A maximum load the ripple current is 1.92A. Output Inductor The output inductor forms the first half of the power stage in a Buck converter. It is responsible for smoothing the square wave created by the switching action and for controlling the output current ripple (∆IOUT). The inductance is chosen by selecting between tradeoffs in efficiency and response time. The smaller the output inductor, the more quickly the converter can respond to transients in the load current. However, as shown in the efficiency calculations, a smaller inductor requires a higher switching frequency to maintain the same level of output current ripple. An increase in frequency can mean increasing loss in the MOSFETs due to the charging and discharging of the gates. Generally the switching frequency is chosen so that conduction loss outweighs switching loss. The equation for output inductor selection is: In this example, in order to maintain a 2% peak-to-peak output voltage ripple and a 40% peak-to-peak inductor current ripple, the required maximum ESR is 20mΩ. The Sanyo 4SP560M electrolytic capacitor will give an equivalent ESR of 14mΩ. The capacitance of 560 µF is enough to supply energy even to meet severe load transient demands. MOSFETs Selection of the power MOSFETs is governed by a tradeoff between cost, size, and efficiency. One method is to determine the maximum cost that can be endured, and then select the most efficient device that fits that price. Breaking www.national.com 14 D1 - A small Schottky diode should be used for the bootstrap. It allows for a minimum drop for both high and low-side drivers. The MBR0520 or BAT54 work well in most designs. RCS - Resistor used to set the current limit. Since the design calls for a peak current magnitude (IOUT+0.5*∆IOUT) of 4.8A, a safe setting would be 6A. (This is below the saturation current of the output inductor, which is 7A.) Following the equation from the Current Limit section, a 1.3kΩ resistor should be used. RFADJ - This resistor is used to set the switching frequency of the chip. The resistor value is calculated from equation in Normal Operation section. For 300 kHz operation, a 97.6 kΩ resistor should be used. CSS - The soft-start capacitor depends on the user requirements and is calculated based on the equation given in the section titled START UP/SOFT-START. Therefore, for a 7ms delay, a 12nF capacitor is suitable. (Continued) down the losses in the high-side and low-side MOSFETs and then creating spreadsheets is one way to determine relative efficiencies between different MOSFETs. Good correlation between the prediction and the bench is not guaranteed, however. Single-channel buck regulators that use a controller IC and discrete MOSFETs tend to be most efficient for output currents of 2-10A. Losses in the high-side MOSFET can be broken down into conduction loss, gate charging loss, and switching loss. Conduction, or I2R loss, is approximately: PC = D (IO2 x RDSON-HI x 1.3) (High-Side MOSFET) PC = (1 - D) x (IO2 x RDSON-LO x 1.3) (Low-Side MOSFET) In the above equations the factor 1.3 accounts for the increase in MOSFET RDSON due to heating. Alternatively, the 1.3 can be ignored and the RDSON of the MOSFET estimated using the RDSON Vs. Temperature curves in the MOSFET datasheets. Control Loop Compensation The LM2744 uses voltage-mode (‘VM’) PWM control to correct changes in output voltage due to line and load transients. One of the attractive advantages of voltage mode control is its relative immunity to noise and layout. However VM requires careful small signal compensation of the control loop for achieving high bandwidth and good phase margin. The control loop is comprised of two parts. The first is the power stage, which consists of the duty cycle modulator, output inductor, output capacitor, and load. The second part is the error amplifier, which for the LM2744 is a 9MHz op-amp used in the classic inverting configuration. Figure 11 shows the regulator and control loop components. Gate charging loss results from the current driving the gate capacitance of the power MOSFETs, and is approximated as: PGC = n x (VDD) x QG x FSW where ‘n’ is the number of MOSFETs (if multiple devices have been placed in parallel), VDD is the driving voltage (see MOSFET Gate Drivers section) and QGS is the gate charge of the MOSFET. If different types of MOSFETs are used, the ‘n’ term can be ignored and their gate charges simply summed to form a cumulative QG. Gate charge loss differs from conduction and switching losses in that the actual dissipation occurs in the LM2744, and not in the MOSFET itself. Switching loss occurs during the brief transition period as the high-side MOSFET turns on and off, during which both current and voltage are present in the channel of the MOSFET. It can be approximated as: PSW = 0.5 x VIN x IO x (tr + tf) x FSW where tR and tF are the rise and fall times of the MOSFET. Switching loss occurs in the high-side MOSFET only. For this example, the maximum drain-to-source voltage applied to either MOSFET is 3.6V. The maximum drive voltage at the gate of the high-side MOSFET is 3.1V, and the maximum drive voltage for the low-side MOSFET is 3.3V. Due to the low drive voltages in this example, a MOSFET that turns on fully with 3.1V of gate drive is needed. For designs of 5A and under, dual MOSFETs in SO-8 provide a good tradeoff between size, cost, and efficiency. Support Components CIN2 - A small (0.1 to 1 µF) ceramic capacitor should be placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET (dual MOSFETs make this easy). This capacitor should be X5R type dielectric or better. RCC, CCC- These are standard filter components designed to ensure smooth DC voltage for the chip supply. RCC should be 1-10Ω. CCC should be 1 µF, X5R type or better. CBOOT- Bootstrap capacitor, typically 100nF. RPULL-UP – This is a standard pull-up resistor for the opendrain power good signal (PWGD). The recommended value is 10 kΩ connected to VCC. If this feature is not necessary, the resistor can be omitted. 20106064 FIGURE 11. Power Stage and Error Amp One popular method for selecting the compensation components is to create Bode plots of gain and phase for the power stage and error amplifier. Combined, they make the overall bandwidth and phase margin of the regulator easy to see. Software tools such as Excel, MathCAD, and Matlab are useful for showing how changes in compensation or the power stage affect system gain and phase. 15 www.national.com LM2744 Application Information LM2744 Application Information (Continued) The power stage modulator provides a DC gain ADC that is equal to the input voltage divided by the peak-to-peak value of the PWM ramp. This ramp is 1.0VP-P for the LM2744. The inductor and output capacitor create a double pole at frequency fDP, and the capacitor ESR and capacitance create a single zero at frequency fESR. For this example, with VIN = 3.3V, these quantities are: 20106069 In the equation for fDP, the variable RL is the power stage resistance, and represents the inductor DCR plus the on resistance of the high-side MOSFET. RO is the output voltage divided by output current. The power stage transfer function GPS is given by the following equation, and Figure 12 shows Bode plots of the phase and gain in this example. a = LCO(RO + RC) b = L + CO(RORL + RORC + RCRL) c = R O + RL 20106070 FIGURE 12. Power Stage Gain and Phase The double pole at 4.5kHz causes the phase to drop to approximately -130˚ at around 10kHz. The ESR zero, at 20.3kHz, provides a +90˚ boost that prevents the phase from dropping to -180o. If this loop were left uncompensated, the bandwidth would be approximately 10kHz and the phase margin 53˚. In theory, the loop would be stable, but would suffer from poor DC regulation (due to the low DC gain) and would be slow to respond to load transients (due to the low bandwidth.) In practice, the loop could easily become unstable due to tolerances in the output inductor, capacitor, or changes in output current, or input voltage. Therefore, the loop is compensated using the error amplifier and a few passive components. For this example, a Type III, or three-pole-two-zero approach gives optimal bandwidth and phase. In most voltage mode compensation schemes, including Type III, a single pole is placed at the origin to boost DC gain as high as possible. Two zeroes fZ1 and fZ2 are placed at the double pole frequency to cancel the double pole phase lag. Then, a pole, fP1 is placed at the frequency of the ESR zero. A final pole fP2 is placed at one-half of the switching frequency. The gain of the error amplifier transfer function is selected to give the best bandwidth possible without violating the Nyquist stability criteria. In practice, a good crossover www.national.com 16 LM2744 Application Information (Continued) point is one-fifth of the switching frequency, or 60kHz for this example. The generic equation for the error amplifier transfer function is: In this equation the variable AEA is a ratio of the values of the capacitance and resistance of the compensation components, arranged as shown inFigure 12. AEA is selected to provide the desired bandwidth. A starting value of 80,000 for AEA should give a conservative bandwidth. Increasing the value will increase the bandwidth, but will also decrease phase margin. Designs with 45-60˚ are usually best because they represent a good tradeoff between bandwidth and phase margin. In general, phase margin is lowest and gain highest (worst-case) for maximum input voltage and minimum output current. One method to select AEA is to use an iterative process beginning with these worst-case conditions. 20106074 1. Increase AEA 2. Check overall bandwidth and phase margin 3. Change VIN to minimum and recheck overall bandwidth and phase margin 4. Change IO to maximum and recheck overall bandwidth and phase margin The process ends when the both bandwidth and the phase margin are sufficiently high. For this example input voltage can vary from 3.0 to 3.6V and output current can vary from 0 to 4A, and after a few iterations a moderate gain factor of 110,000 is used. The error amplifier of the LM2744 has a unity-gain bandwidth of 9MHz. In order to model the effect of this limitation, the open-loop gain can be calculated as: 20106075 FIGURE 13. Error Amp Gain and Phase In VM regulators, the top feedback resistor RFB2 forms a part of the compensation. Setting RFB2 to 10kΩ, ± 1% usually gives values for the other compensation resistors and capacitors that fall within a reasonable range. (Capacitances > 1pF, resistances < 1MΩ) CC1, CC2, CC3, RC1, and RC2 are selected to provide the poles and zeroes at the desired frequencies, using the following equations: The new error amplifier transfer function that takes into account unity-gain bandwidth is: The gain and phase of the error amplifier are shown in Figure 13. 17 www.national.com LM2744 Application Information (Continued) In practice, a good trade off between phase margin and bandwidth can be obtained by selecting the closest ± 10% capacitor values above what are suggested for CC1 and CC2, the closest ± 10% capacitor value below the suggestion for CC3, and the closest ± 1% resistor values below the suggestions for RC1, RC2. Note that if the suggested value for RC2 is less than 100Ω, it should be replaced by a short circuit. Following this guideline, the compensation components will be: CC1 = 27pF ± 10%, CC2 = 820pF ± 10% CC3 = 2.7nF ± 10%, RC1 = 39.2kΩ ± 1% RC2 = 2.55kΩ ± 1% The transfer function of the compensation block can be derived by considering the compensation components as impedance blocks ZF and ZI around an inverting op-amp: 20106085 20106086 FIGURE 14. Overall Loop Gain and Phase The bandwidth of this example circuit is 59kHz, with a phase margin of 60˚. EFFICIENCY CALCULATIONS The following is a sample calculation. A reasonable estimation of the efficiency of a switching buck controller can be obtained by adding together the Output Power (POUT) loss and the Total Power (PTOTAL) loss: As with the generic equation, GEA-ACTUAL must be modified to take into account the limited bandwidth of the error amplifier. The result is: The Output Power (POUT) for theTypical Application Circuit design is (1.2V * 4A) = 4.8W. The Total Power (PTOTAL), with an efficiency calculation to complement the design, is shown below. The majority of the power losses are due to low and highside of MOSFET’s losses. The losses in any MOSFET are group of switching (PSW) and conduction losses(PCND). PFET = PSW + PCND = 61.38mW + 270.42mW PFET = 331.8mW FET Switching Loss (PSW) PSW = PSW(ON) + PSW(OFF) The total control loop transfer function H is equal to the power stage transfer function multiplied by the error amplifier transfer function. H = GPS x HEA The bandwidth and phase margin can be read graphically from Bode plots of HEA are shown in Figure 14. www.national.com 18 LM2744 Application Information (Continued) PSW = 0.5 * VIN * IOUT * (tr + tf)* FOSC PSW = 0.5 x 3.3V x 4A x 300kHz x 31ns PSW = 61.38mW The FDS6898A has a typical turn-on rise time tr and turn-off fall time tf of 15ns and 16ns, respectively. The switching losses for this type of dual N-Channel MOSFETs are 0.061W. FET Conduction Loss (PCND) PCND = PCND1 + PCND2 PCND1 = I2OUT x RDS(ON) x k x D PCND2 = I2OUT x RDS(ON) x k x (1-D) RDS(ON) = 13mΩ and the factor is a constant value (k = 1.3) to account for the increasing RDS(ON) of a FET due to heating. PCND1 = (4A)2 x 13mΩ x 1.3 x 0.364 where, Here n is the number of paralleled capacitors, ESR is the equivalent series resistance of each, and PCAP is the dissipation in each. So for example if we use only one input capacitor of 24 mΩ. PCAP = 88.8mW Output Inductor Loss (PIND) PIND = I2OUT * DCR where DCR is the DC resistance. Therefore, for example PIND = (4A)2 x 11mΩ PIND = 176mW 2 PCND2 = (4A) x 13mΩ x 1.3 x (1 - 0.364) PCND = 98.42mW + 172mW = 270.42mW There are few additional losses that are taken into account: IC Operating Loss (PIC) PIC = IQ_VCC x VCC, where IQ-VCC is the typical operating VCC current PIC= 1.5mA *3.3V = 4.95mW FET Gate Charging Loss (PGATE) PGATE = n * VCC * QGS * FOSC Total System Efficiency PGATE = 2 x 3.3V x 3nC x 300kHz PGATE = 5.94mW The value n is the total number of FETs used and QGS is the typical gate-source charge value, which is 3nC. For the FDS6898A the gate charging loss is 5.94mW. Input Capacitor Loss (PCAP) 19 www.national.com LM2744 Example Circuits 20106032 FIGURE 15. 5V to 3.3V @ 2A, VREF = 1.2V, FSW = 300kHz PART PART NUMBER TYPE PACKAGE U1 LM2744 Synchronous Controller TSSOP-14 Q1 FDS6898A Dual N-MOSFET SO-8 DESCRIPTION VENDOR NSC 20V, 10mΩ@ 4.5V, 16nC Fairchild D1 MBR0520LTI Schottky Diode SOD-123 L1 DO3316P-472 Inductor 12.95 x 9.4 x 5.21mm 4.7µH, 4.8Arms 18mΩ Coilcraft CIN1 16SP100M Aluminum Electrolytic 10.0mm x 5.0mm 100µF, 16V, 2.89Arms Sanyo CO1 6SP220M Aluminum Electrolytic 10.0mm x 5.0mm 220µF, 6.3V 3.1Arms Sanyo CBOOT, CIN2, CO2 VJ0805Y104KXXA Capacitor 0805 0.1µF, 10% Vishay CCC VJ0805M104MXQ Capacitor 0805 1µF, 20% Vishay CC3 VJ0805Y332KXXA Capacitor 0805 3300pF, 10% Vishay CSS VJ0805A123KXAA Capacitor 0805 12nF, 10% Vishay CC2 VJ0805Y122KXXA Capacitor 0805 1200pF 10% Vishay CC1 VJ0805A270KXAA Capacitor 0805 27pF, 10% Vishay RFB2 CRCW08051002F Resistor 0805 10.0kΩ 1% Vishay RFB1 CRCW08055761F Resistor 0805 5.76kΩ1% Vishay RFADJ CRCW08051103F Resistor 0805 110kΩ 1% Vishay RC2 CRCW08052101F Resistor 0805 2.1kΩ 1% Vishay RCS CRCW08057870F Resistor 0805 787Ω 1% Vishay RCC CRCW080510R0F Resistor 0805 10.0Ω 1% Vishay RC1 CRCW08053832F Resistor 0805 38.3kΩ 1% Vishay RPULL-UP CRCW08051003J Resistor 0805 100kΩ 5% Vishay www.national.com 20 LM2744 Example Circuits (Continued) 20106034 FIGURE 16. 3.3V to 2.5V @ 2A, VREF = 0.8V, FSW = 300kHz PART PART NUMBER TYPE PACKAGE U1 LM2744 Synchronous Controller TSSOP-14 DESCRIPTION VENDOR Q1 FDS6898A Dual N-MOSFET SO-8 D1 MBR0520LTI Schottky Diode SOD-123 L1 DO3316P-332 Inductor 12.95 x 9.4 x 5.21mm CIN1 16SP100M Aluminum Electrolytic 10.0mm x 5.0mm 100µF, 16V 2.89Arms Sanyo CO1 6SP220M Aluminum Electrolytic 10.0mm x 5.0mm 220µF, 6.3V 3.1Arms Sanyo CBOOT, CIN2, CO 2 VJ0805Y104KXXA Capacitor 0805 0.1µF, 10% Vishay NSC 20V, 10mΩ@ 4.5V, 16nC Fairchild 3.3µH, 5.4Arms 15mΩ Coilcraft CCC VJ0805M104MXQ Capacitor 0805 1µF, 20% Vishay CC3 VJ0805Y222KXXA Capacitor 0805 2200pF, 10% Vishay CSS VJ0805A123KXAA Capacitor 0805 12nF, 10% Vishay CC2 VJ0805Y272KXAA Capacitor 0805 2700pF 10% Vishay CC1 VJ0805A820KXAA Capacitor 0805 82pF, 10% Vishay RFB2 CRCW08051002F Resistor 0805 10.0kΩ 1% Vishay RFB1 CRCW08054641F Resistor 0805 4.64kΩ 1% Vishay RFADJ CRCW08051103F Resistor 0805 110kΩ 1% Vishay RC2 CRCW08052551F Resistor 0805 2.55kΩ 1% Vishay RCS CRCW08058450F Resistor 0805 845Ω 1% Vishay RCC CRCW080510R0F Resistor 0805 10.0Ω 1% Vishay RC1 CRCW08051372F Resistor 0805 13.7kΩ 1% Vishay RPULL-UP CRCW08051003J Resistor 0805 100kΩ 5% Vishay 21 www.national.com LM2744 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference Physical Dimensions inches (millimeters) unless otherwise noted TSSOP-14 Pin Package NS Package Number MTC14 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. 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