LM2727/LM2737 N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages General Description Features The LM2727 and LM2737 are high-speed, synchronous, switching regulator controllers. They are intended to control currents of 0.7A to 20A with up to 95% conversion efficiencies. The LM2727 employs output over-voltage and undervoltage latch-off. For applications where latch-off is not desired, the LM2737 can be used. Power up and down sequencing is achieved with the power-good flag, adjustable soft-start and output enable features. The LM2737 and LM2737 operate from a low-current 5V bias and can convert from a 2.2V to 16V power rail. Both parts utilize a fixedfrequency, voltage-mode, PWM control architecture and the switching frequency is adjustable from 50kHz to 2MHz by adjusting the value of an external resistor. Current limit is achieved by monitoring the voltage drop across the onresistance of the low-side MOSFET, which enhances low duty-cycle operation. The wide range of operating frequencies gives the power supply designer the flexibility to finetune component size, cost, noise and efficiency. The adaptive, non-overlapping MOSFET gate-drivers and high-side bootstrap structure helps to further maximize efficiency. The high-side power FET drain voltage can be from 2.2V to 16V and the output voltage is adjustable down to 0.6V. n Input power from 2.2V to 16V n Output voltage adjustable down to 0.6V n Power Good flag, adjustable soft-start and output enable for easy power sequencing n Output over-voltage and under-voltage latch-off (LM2727) n Output over-voltage and under-voltage flag (LM2737) n Reference Accuracy: 1.5% (0˚C - 125˚C) n Current limit without sense resistor n Soft start n Switching frequency from 50 kHz to 2 MHz n TSSOP-14 package Applications n n n n n Cable Modems Set-Top Boxes/ Home Gateways DDR Core Power High-Efficiency Distributed Power Local Regulation of Core Power Typical Application 20049410 © 2003 National Semiconductor Corporation DS200494 www.national.com LM2727/LM2737 N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages June 2003 LM2727/LM2737 Connection Diagram 20049411 14-Lead Plastic TSSOP θJA = 155˚C/W NS Package Number MTC14 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. Pin Description BOOT (Pin 1) - Supply rail for the N-channel MOSFET gate drive. The voltage should be at least one gate threshold above the regulator input voltage to properly turn on the high-side N-FET. LG (Pin 2) - Gate drive for the low-side N-channel MOSFET. This signal is interlocked with HG to avoid shoot-through problems. PGND (Pins 3, 13) - Ground for FET drive circuitry. It should be connected to system ground. SGND (Pin 4) - Ground for signal level circuitry. It should be connected to system ground. VCC (Pin 5) - Supply rail for the controller. PWGD (Pin 6) - Power Good. This is an open drain output. The pin is pulled low when the chip is in UVP, OVP, or UVLO mode. During normal operation, this pin is connected to VCC or other voltage source through a pull-up resistor. ISEN (Pin 7) - Current limit threshold setting. This sources a fixed 50µA current. A resistor of appropriate value should be connected between this pin and the drain of the low-side FET. www.national.com SS (Pin 9) - Soft start pin. A capacitor connected between this pin and ground sets the speed at which the output voltage ramps up. Larger capacitor value results in slower output voltage ramp but also lower inrush current. FB (Pin 10) - 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) - The switching frequency is set by connecting a resistor between this pin and ground. SD (Pin 12) - IC Logic Shutdown. When this pin is pulled low the chip turns off the high side switch and turns on the low side switch. While this pin is low, the IC will not start up. An internal 20µA pull-up connects this pin to VCC. HG (Pin 14) - Gate drive for the high-side N-channel MOSFET. This signal is interlocked with LG to avoid shootthrough problems. 2 Infrared or Convection (20sec) (Note 1) ESD Rating If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. BOOTV Supply Voltage (VCC) 21V Junction Temperature 150˚C Storage Temperature −65˚C to 150˚C 2 kV Operating Ratings 7V VCC 235˚C 4.5V to 5.5V Junction Temperature Range −40˚C to +125˚C Thermal Resistance (θJA) 155˚C/W Soldering Information Lead Temperature (soldering, 10sec) 260˚C Electrical Characteristics VCC = 5V 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 VFB_ADJ VON IQ-V5 Parameter FB Pin Voltage UVLO Thresholds Operating VCC Current Min Typ Max VCC = 4.5V, 0˚C to +125˚C Conditions 0.591 0.6 0.609 VCC = 5V, 0˚C to +125˚C 0.591 0.6 0.609 VCC = 5.5V, 0˚C to +125˚C 0.591 0.6 0.609 VCC = 4.5V, −40˚C to +125˚C 0.589 0.6 0.609 VCC = 5V, −40˚C to +125˚C 0.589 0.6 0.609 VCC = 5.5V, −40˚C to +125˚C 0.589 0.6 0.609 Rising Falling 4.2 3.6 Units V V SD = 5V, FB = 0.55V Fsw = 600kHz 1 1.5 2 SD = 5V, FB = 0.65V Fsw = 600kHz 0.8 1.7 2.2 0.15 0.4 0.7 mA Shutdown VCC Current SD = 0V tPWGD1 PWGD Pin Response Time FB Voltage Going Up 6 µs tPWGD2 PWGD Pin Response Time FB Voltage Going Down 6 µs 20 µA ISD ISS-ON ISS-OC ISEN-TH SD Pin Internal Pull-up Current SS Pin Source Current SS Voltage = 2.5V 0˚C to +125˚C -40˚C to +125˚C 8 5 SS Pin Sink Current During Over SS Voltage = 2.5V Current ISEN Pin Source Current Trip Point 0˚C to +125˚C -40˚C to +125˚C 11 11 15 15 95 35 28 50 50 mA µA µA 65 65 µA ERROR AMPLIFIER GBW G Error Amplifier Unity Gain Bandwidth 5 MHz Error Amplifier DC Gain 60 dB SR Error Amplifier Slew Rate IFB FB Pin Bias Current FB = 0.55V FB = 0.65V 6 IEAO EAO Pin Current Sourcing and Sinking VEAO = 2.5, FB = 0.55V VEAO = 2.5, FB = 0.65V 2.8 0.8 mA VEA Error Amplifier Maximum Swing Minimum Maximum 1.2 3.2 V 0 0 3 15 30 V/µA 100 155 nA www.national.com LM2727/LM2737 Absolute Maximum Ratings LM2727/LM2737 Electrical Characteristics (Continued) VCC = 5V 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 BOOTV = 12V, EN = 0 0˚C to +125˚C -40˚C to +125˚C 95 95 160 215 µA GATE DRIVE IQ-BOOT BOOT Pin Quiescent Current RDS1 Top FET Driver Pull-Up ON resistance BOOT-SW = 5V@350mA 3 Ω RDS2 Top FET Driver Pull-Down ON resistance BOOT-SW = 5V@350mA 2 Ω RDS3 Bottom FET Driver Pull-Up ON resistance BOOT-SW = 5V@350mA 3 Ω RDS4 Bottom FET Driver Pull-Down ON resistance BOOT-SW = 5V@350mA 2 Ω OSCILLATOR fOSC D PWM Frequency Max Duty Cycle RFADJ = 590kΩ 50 RFADJ = 88.7kΩ 300 RFADJ = 42.2kΩ, 0˚C to +125˚C 500 600 700 RFADJ = 42.2kΩ, -40˚C to +125˚C 490 600 700 RFADJ = 17.4kΩ 1400 RFADJ = 11.3kΩ 2000 fPWM = 300kHz fPWM = 600kHz 90 88 kHz % LOGIC INPUTS AND OUTPUTS VSD-IH SD Pin Logic High Trip Point VSD-IL SD Pin Logic Low Trip Point 0˚C to +125˚C -40˚C to +125˚C 1.3 1.25 1.6 1.6 PWGD Pin Trip Points FB Voltage Going Down 0˚C to +125˚C -40˚C to +125˚C 0.413 0.410 0.430 0.430 0.446 0.446 V FB Voltage Going Up 0˚C to +125˚C -40˚C to +125˚C 0.691 0.688 0.710 0.710 0.734 0.734 V VPWGD-TH-LO VPWGD-TH-HI VPWGD-HYS PWGD Pin Trip Points 2.6 PWGD Hysteresis (LM2737 only) FB Voltage Going Down FB Voltage Going Up 35 110 3.5 V V 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 human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin. www.national.com 4 LM2727/LM2737 Typical Performance Characteristics Efficiency (VO = 3.3V) FSW = 300kHz, TA = 25˚C Efficiency (VO = 1.5V) FSW = 300kHz, TA = 25˚C 20049412 20049413 Bootpin Current vs Temperature for BOOTV = 12V FSW = 600kHz, Si4826DY FET, No-Load VCC Operating Current vs Temperature FSW = 600kHz, No-Load 20049415 20049414 PWM Frequency vs Temperature for RFADJ = 43.2kΩ Bootpin Current vs Temperature with 5V Bootstrap FSW = 600kHz, Si4826DY FET, No-Load 20049416 20049417 5 www.national.com LM2727/LM2737 Typical Performance Characteristics (Continued) RFADJ vs PWM Frequency (in 100 to 800kHz range), TA = 25˚C RFADJ vs PWM Frequency (in 900 to 2000kHz range), TA = 25˚C 20049418 20049419 Switch Waveforms (HG Falling) VIN = 5V, VO = 1.8V IO = 3A, CSS = 10nF FSW = 600kHz VCC Operating Current Plus Boot Current vs PWM Frequency (Si4826DY FET, TA = 25˚C) 20049423 20049420 Start-Up (No-Load) VIN = 10V, VO = 1.2V CSS = 10nF, FSW = 300kHz Switch Waveforms (HG Rising) VIN = 5V, VO = 1.8V IO = 3A, FSW = 600kHz 20049424 www.national.com 20049421 6 LM2727/LM2737 Typical Performance Characteristics (Continued) Start-Up (Full-Load) VIN = 10V, VO = 1.2V IO = 10A, CSS = 10nF FSW = 300kHz Start Up (No-Load, 10x CSS) VIN = 10V, VO = 1.2V CSS = 100nF, FSW = 300kHz 20049422 20049426 Shutdown VIN = 10V, VO = 1.2V IO = 10A, CSS = 10nF FSW = 300kHz Start Up (Full Load, 10x CSS) VIN = 10V, VO = 1.2V IO = 10A, CSS = 100nF FSW = 300kHz 20049425 20049427 Start Up (Full Load, 10x CSS) VIN = 10V, VO = 1.2V IO = 10A, CSS = 100nF FSW = 300kHz Load Transient Response (IO = 0 to 4A) VIN = 12V, VO = 1.2V FSW = 300kHz 20049433 20049428 7 www.national.com LM2727/LM2737 Typical Performance Characteristics (Continued) Line Transient Response (VIN =5V to 12V) VO = 1.2V, IO = 5A FSW = 300kHz Load Transient Response (IO = 4 to 0A) VIN = 12V, VO = 1.2V FSW = 300kHz 20049429 20049430 Line Transient Response VO = 1.2V, IO = 5A FSW = 300kHz Line Transient Response (VIN =12V to 5V) VO = 1.2V, IO = 5A FSW = 300kHz 20049431 www.national.com 20049432 8 LM2727/LM2737 Block Diagram 20049401 Application Information THEORY OF OPERATION The LM2727 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 power good (PWRGD), output shutdown (SD), over voltage protection (OVP) and under voltage protection (UVP). The over-voltage and under-voltage signals are OR gated to drive the Power Good signal and a shutdown latch, which turns off the high side gate and turns on the low side gate if pulled low. Current limit is achieved by sensing the voltage VDS across the low side FET. During current limit the high side gate is turned off and the low side gate turned on. The soft start capacitor is discharged by a 95µA source (reducing the maximum duty cycle) until the current is under control. The LM2737 does not latch off during UVP or OVP, and uses the HIGH and LOW comparators for the powergood function only. An application for a microprocessor might need a delay of 3ms, in which case CSS would be 12nF. For a different device, a 100ms delay might be more appropriate, in which case CSS would be 400nF. (390 10%) During soft start the PWRGD flag is forced low and is released when the voltage reaches a set value. At this point this chip enters normal operation mode, the Power Good flag is released, and the OVP and UVP functions begin to monitor Vo. NORMAL OPERATION While in normal operation mode, the LM2727/37 regulates the output voltage by controlling the duty cycle of the high side and low side FETs. The equation governing output voltage is: START UP When VCC exceeds 4.2V and the enable pin EN sees a logic high the soft start capacitor begins charging through an internal fixed 10µA source. During this time the output of the error amplifier is allowed to rise with the voltage of the soft start capacitor. This capacitor, Css, determines soft start time, and can be determined approximately by: The PWM frequency is adjustable between 50kHz and 2MHz and is set by an external resistor, RFADJ, between the FREQ pin and ground. The resistance needed for a desired frequency is approximately: 9 www.national.com LM2727/LM2737 Application Information until VCC rises above 4.2V. As with shutdown, the soft start capacitor is discharged through a FET, ensuring that the next start-up will be smooth. (Continued) CURRENT LIMIT Current limit is realized by sensing the voltage across the low side FET while it is on. The RDSON of the FET is a known value, hence the current through the FET can be determined as: VDS = I * RDSON MOSFET GATE DRIVERS The LM2727/37 has two gate drivers designed for driving N-channel MOSFETs in a synchronous mode. Power for the drivers is supplied through the BOOTV pin. For the high side gate (HG) to fully turn on the top FET, the BOOTV voltage must be at least one VGS(th) greater than Vin. (BOOTV ≥ 2*Vin) This voltage can be supplied by a separate, higher voltage source, or supplied from a local charge pump structure. In a system such as a desktop computer, both 5V and 12V are usually available. Hence if Vin was 5V, the 12V supply could be used for BOOTV. 12V is more than 2*Vin, so the HG would operate correctly. For a BOOTV of 12V, the initial gate charging current is 2A, and the initial gate discharging current is typically 6A. The current limit is determined by an external resistor, RCS, connected between the switch node and the ISEN pin. A constant current of 50µA 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: RCS = RDSON(LOW) * ILIM/50µA For example, a conservative 15A current limit in a 10A design with a minimum RDSON of 10mΩ would require a 3.3kΩ resistor. Because current sensing is done across the low side FET, no minimum high side on-time is necessary. In the current limit mode the LM2727/37 will turn the high side off and the keep low side on for as long as necessary. The chip also discharges the soft start capacitor through a fixed 95µA source. In this way, smooth ramping up of the output voltage as with a normal soft start is ensured. The output of the LM2727/37 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 lasts for an extended time. During the first few nanoseconds after the low side gate turns on, the low side FET body diode conducts. This causes an additional 0.7V drop in VDS. The range of VDS is normally much lower. For example, if RDSON were 10mΩ and the current through the FET was 10A, VDS would be 0.1V. The current limit would see 0.7V as a 70A current and enter current limit immediately. Hence current limit is masked during the time it takes for the high side switch to turn off and the low side switch to turn on. 20049402 FIGURE 1. BOOTV Supplied by Charge Pump In a system without a separate, higher voltage, a charge pump (bootstrap) can be built using a diode and small capacitor, Figure 1. The capacitor serves to maintain enough voltage between the top FET gate and source to control the device even when the top FET is on and its source has risen up to the input voltage level. The LM2727/37 gate drives use a BiCMOS design. Unlike some other bipolar control ICs, the gate drivers have rail-torail swing, ensuring no spurious turn-on due to capacitive coupling. UVP/OVP The output undervoltage protection and overvoltage protection mechanisms engage at 70% and 118% of the target output voltage, respectively. In either case, the LM2727 will turn off the high side switch and turn on the low side switch, and discharge the soft start capacitor through a MOSFET switch. The chip remains in this state until the shutdown pin has been pulled to a logic low and then released. The UVP function is masked only during the first charging of the soft start capacitor, when voltage is first applied to the VCC pin. In contrast, the LM2737 is designed to continue operating during UVP or OVP conditions, and to resume normal operation once the fault condition is cleared. As with the LM2727, the powergood flag goes low during this time, giving a logic-level warning signal. POWER GOOD SIGNAL The power good signal is the or-gated flag representing over-voltage and under-voltage protection. If the output voltage is 18% over it’s nominal value, VFB = 0.7V, or falls 30% below that value, VFB = 0.41V, the power good flag goes low. The converter then turns off the high side gate, and turns on the low side gate. Unlike the output (LM2727 only) the power good flag is not latched off. It will return to a logic high whenever the feedback pin voltage is between 70% and 118% of 0.6V. SHUT DOWN If the shutdown pin SD is pulled low, the LM2727/37 discharges the soft start capacitor through a MOSFET switch. The high side switch is turned off and the low side switch is turned on. The LM2727/37 remains in this state until SD is released. UVLO The 4.2V turn-on threshold on VCC has a built in hysteresis of 0.6V. Therefore, if VCC drops below 3.6V, the chip enters UVLO mode. UVLO consists of turning off the top FET, turning on the bottom FET, and remaining in that condition www.national.com 10 In the case of a desktop computer system, the input current slew rate is the system power supply or "silver box" output current slew rate, which is typically about 0.1A/µs. Total input capacitor ESR is 9mΩ, hence ∆V is 10*0.009 = 90 mV, and the minimum inductance required is 0.9µH. The input inductor should be rated to handle the DC input current, which is approximated by: (Continued) DESIGN CONSIDERATIONS The following is a design procedure for all the components needed to create the circuit shown in Figure 3 in the Example Circuits section, a 5V in to 1.2V out converter, capable of delivering 10A with an efficiency of 85%. The switching frequency is 300kHz. The same procedures can be followed to create the circuit shown in Figure 3, Figure 4, and to create many other designs with varying input voltages, output voltages, and output currents. In this case IIN-DC is about 2.8A. One possible choice is the TDK SLF12575T-1R2N8R2, a 1.2µH device that can handle 8.2Arms, and has a DCR of 7mΩ. INPUT CAPACITOR The input capacitors in a Buck switching converter are subjected to high stress due to the input current waveform, which is a square wave. Hence input caps are selected for their ripple current capability and their ability to withstand the heat generated as that ripple current runs through their ESR. Input rms ripple current is approximately: 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. (∆Io) 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. As shown in the efficiency calculations, however, 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 FETs 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: The power dissipated by each input capacitor is: Here, n is the number of capacitors, and indicates that power loss in each cap decreases rapidly as the number of input caps increase. The worst-case ripple for a Buck converter occurs during full load, when the duty cycle D = 50%. In the 5V to 1.2V case, D = 1.2/5 = 0.24. With a 10A maximum load the ripple current is 4.3A. The Sanyo 10MV5600AX aluminum electrolytic capacitor has a ripple current rating of 2.35A, up to 105˚C. Two such capacitors make a conservative design that allows for unequal current sharing between individual caps. Each capacitor has a maximum ESR of 18mΩ at 100 kHz. Power loss in each device is then 0.05W, and total loss is 0.1W. Other possibilities for input and output capacitors include MLCC, tantalum, OSCON, SP, and POSCAPS. Plugging in the values for output current ripple, input voltage, output voltage, switching frequency, and assuming a 40% peak-to-peak output current ripple yields an inductance of 1.5µH. The output inductor must be rated to handle the peak current (also equal to the peak switch current), which is (Io + 0.5*∆Io). This is 12A for a 10A design. The Coilcraft D05022152HC is 1.5µH, is rated to 15Arms, and has a DCR of 4mΩ. 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 (∆Vo) and to supply load current during fast load transients. In this example the output current is 10A 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 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, ∆Vo and the designed output current ripple, ∆Io, is: INPUT INDUCTOR The input inductor serves two basic purposes. First, in high power applications, the input inductor helps insulate the input power supply from switching noise. This is especially important if other switching converters draw current from the same supply. Noise at high frequency, such as that developed by the LM2727 at 1MHz operation, could pass through the input stage of a slower converter, contaminating and possibly interfering with its operation. An input inductor also helps shield the LM2727 from high frequency noise generated by other switching converters. The second purpose of the input inductor is to limit the input current slew rate. During a change from no-load to full-load, the input inductor sees the highest voltage change across it, equal to the full load current times the input capacitor ESR. This value divided by the maximum allowable input current slew rate gives the minimum input inductance: 11 www.national.com LM2727/LM2737 Application Information LM2727/LM2737 Application Information Rbypass and Cbypass are standard filter components designed to ensure smooth DC voltage for the chip supply and for the bootstrap structure, if it is used. Use 10Ω for the resistor and a 2.2µF ceramic for the cap. Cb is the bootstrap capacitor, and should be 0.1µF. (In the case of a separate, higher supply to the BOOTV pin, this 0.1µF cap can be used to bypass the supply.) Using a Schottky device for the bootstrap diode allows the minimum drop for both high and low side drivers. The On Semiconductor BAT54 or MBR0520 work well. (Continued) 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 6mΩ. Three Sanyo 10MV5600AX capacitors in parallel will give an equivalent ESR of 6mΩ. The total bulk capacitance of 16.8mF is enough to supply even severe load transients. Using the same capacitors for both input and output also keeps the bill of materials simple. Rp is a standard pull-up resistor for the open-drain power good signal, and should be 10kΩ. If this feature is not necessary, it can be omitted. RCS is the resistor used to set the current limit. Since the design calls for a peak current magnitude (Io + 0.5 * ∆Io) of 12A, a safe setting would be 15A. (This is well below the saturation current of the output inductor, which is 25A.) Following the equation from the Current Limit section, use a 3.3kΩ resistor. RFADJ is used to set the switching frequency of the chip. Following the equation in the Theory of Operation section, the closest 1% tolerance resistor to obtain fSW = 300kHz is 88.7kΩ. CSS depends on the users requirements. Based on the equation for CSS in the Theory of Operation section, for a 3ms delay, a 12nF capacitor will suffice. MOSFETS MOSFETS are a critical part of any switching controller and have a direct impact on the system efficiency. In this case the target efficiency is 85% and this is the variable that will determine which devices are acceptable. Loss from the capacitors, inductors, and the LM2727 itself are detailed in the Efficiency section, and come to about 0.54W. To meet the target efficiency, this leaves 1.45W for the FET conduction loss, gate charging loss, and switching loss. Switching loss is particularly difficult to estimate because it depends on many factors. When the load current is more than about 1 or 2 amps, conduction losses outweigh the switching and gate charging losses. This allows FET selection based on the RDSON of the FET. Adding the FET switching and gatecharging losses to the equation leaves 1.2W for conduction losses. The equation for conduction loss is: PCnd = D(I2o * RDSON *k) + (1-D)(I2o * RDSON *k) The factor k is a constant which is added to account for the increasing RDSON of a FET due to heating. Here, k = 1.3. The Si4442DY has a typical RDSON of 4.1mΩ. When plugged into the equation for PCND the result is a loss of 0.533W. If this design were for a 5V to 2.5V circuit, an equal number of FETs on the high and low sides would be the best solution. With the duty cycle D = 0.24, it becomes apparent that the low side FET carries the load current 76% of the time. Adding a second FET in parallel to the bottom FET could improve the efficiency by lowering the effective RDSON. The lower the duty cycle, the more effective a second or even third FET can be. For a minimal increase in gate charging loss (0.054W) the decrease in conduction loss is 0.15W. What was an 85% design improves to 86% for the added cost of one SO-8 MOSFET. EFFICIENCY CALCULATIONS A reasonable estimation of the efficiency of a switching controller can be obtained by adding together the loss is each current carrying element and using the equation: The following shows an efficiency calculation to complement the Circuit of Figure 3. Output power for this circuit is 1.2V x 10A = 12W. Chip Operating Loss PIQ = IQ-VCC *VCC 2mA x 5V = 0.01W FET Gate Charging Loss PGC = n * VCC * QGS * fOSC The value n is the total number of FETs used. The Si4442DY has a typical total gate charge, QGS, of 36nC and an rds-on of 4.1mΩ. For a single FET on top and bottom: 2*5*36E-9*300,000 = 0.108W FET Switching Loss PSW = 0.5 * Vin * IO * (tr + tf)* fOSC CONTROL LOOP COMPONENTS The circuit is this design example and the others shown in the Example Circuits section have been compensated to improve their DC gain and bandwidth. The result of this compensation is better line and load transient responses. For the LM2727, the top feedback divider resistor, Rfb2, is also a part of the compensation. For the 10A, 5V to 1.2V design, the values are: Cc1 = 4.7pF 10%, Cc2 = 1nF 10%, Rc = 229kΩ 1%. These values give a phase margin of 63˚ and a bandwidth of 29.3kHz. The Si4442DY has a typical rise time tr and fall time tf of 11 and 47ns, respectively. 0.5*5*10*58E-9*300,000 = 0.435W SUPPORT CAPACITORS AND RESISTORS The Cinx capacitors are high frequency bypass devices, designed to filter harmonics of the switching frequency and input noise. Two 1µF ceramic capacitors with a sufficient voltage rating (10V for the Circuit of Figure 3) will work well in almost any case. www.national.com 12 LM2727/LM2737 Application Information Input Inductor Loss (Continued) PLin = I2in * DCRinput-L FET Conduction Loss PCn = 0.533W Input Capacitor Loss 2.822*0.007 = 0.055W Output Inductor Loss PLout = I2o * DCRoutput-L 2 10 *0.004 = 0.4W System Efficiency 2 4.28 *0.018/2 = 0.084W Example Circuits 20049403 FIGURE 2. 5V-16V to 3.3V, 10A, 300kHz This circuit and the one featured on the front page have been designed to deliver high current and high efficiency in a small package, both in area and in height The tallest component in this circuit is the inductor L1, which is 6mm tall. The compensation has been designed to tolerate input voltages from 5 to 16V. 13 www.national.com LM2727/LM2737 Example Circuits (Continued) 20049404 FIGURE 3. 5V to 1.2V, 10A, 300kHz This circuit design, detailed in the Design Considerations section, uses inexpensive aluminum capacitors and off-theshelf inductors. It can deliver 10A at better than 85% efficiency. Large bulk capacitance on input and output ensure stable operation. 20049405 FIGURE 4. 5V to 1.8V, 3A, 600kHz The example circuit of Figure 4 has been designed for minimum component count and overall solution size. A switching frequency of 600kHz allows the use of small input/ output capacitors and a small inductor. The availability of separate 5V and 12V supplies (such as those available from desk-top computer supplies) and the low current further www.national.com reduce component count. Using the 12V supply to power the MOSFET drivers eliminates the bootstrap diode, D1. At low currents, smaller FETs or dual FETs are often the most efficient solutions. Here, the Si4826DY, an asymmetric dual FET in an SO-8 package, yields 92% efficiency at a load of 2A. 14 LM2727/LM2737 Example Circuits (Continued) 20049406 FIGURE 5. 3.3V to 0.8V, 5A, 500kHz The circuit of Figure 5 demonstrates the LM2727 delivering a low output voltage at high efficiency (87%) A separate 5V supply is required to run the chip, however the input voltage can be as low as 2.2 15 www.national.com LM2727/LM2737 Example Circuits (Continued) 20049407 FIGURE 6. 1.8V and 3.3V, 1A, 1.4MHz, Simultaneous The circuits in Figure 6 are intended for ADSL applications, where the high switching frequency keeps noise out of the data transmission range. In this design, the 1.8 and 3.3V outputs come up simultaneously by using the same softstart capacitor. Because two current sources now charge the same capacitor, the capacitance must be doubled to achieve the same softstart time. (Here, 40nF is used to achieve a www.national.com 5ms softstart time.) A common softstart capacitor means that, should one circuit enter current limit, the other circuit will also enter current limit. In addition, if both circuits are built with the LM2727, a UVP or OVP fault on one circuit will cause both circuits to latch off. The additional compensation components Rc2 and Cc3 are needed for the low ESR, all ceramic output capacitors, and the wide (3x) range of Vin. 16 LM2727/LM2737 Example Circuits (Continued) 20049408 FIGURE 7. 12V Unregulated to 3.3V, 3A, 750kHz This circuit shows the LM27x7 paired with a cost effective solution to provide the 5V chip power supply, using no extra components other than the LM78L05 regulator itself. The input voltage comes from a ’brick’ power supply which does not regulate the 12V line tightly. Additional, inexpensive 10uF ceramic capacitors (Cinx and Cox) help isolate devices with sensitive databands, such as DSL and cable modems, from switching noise and harmonics. 20049409 FIGURE 8. 12V to 5V, 1.8A, 100kHz In situations where low cost is very important, the LM27x7 can also be used as an asynchronous controller, as shown in the above circuit. Although a a schottky diode in place of the bottom FET will not be as efficient, it will cost much less than the FET. The 5V at low current needed to run the LM27x7 could come from a zener diode or inexpensive regulator, such as the one shown in Figure 7. Because the LM27x7 senses current in the low side MOSFET, the current limit feature will not function in an asynchronous design. The ISEN pin should be left open in this case. 17 www.national.com LM2727/LM2737 TABLE 1. Bill of Materials for Typical Application Circuit ID U1 Part Number LM2727 Q1, Q2 Si4884DY L1 RLF7030T-1R5N6R1 Type Synchronous Controller Size Parameters Qty. Vendor TSSOP-14 TSSOP-14 1 NSC N-MOSFET Inductor SO-8 30V, 4.1mΩ, 36nC 1 Vishay 7.1x7.1x3.2mm 1.5µH, 6.1A 9.6mΩ 1 TDK TDK Cin1, Cin2 C2012X5R1J106M MLCC 0805 10µF 6.3V 2 Cinx C3216X7R1E105K Capacitor 1206 1µF, 25V 1 TDK Co1, Co2 6MV2200WG 10mm D 20mm H 2200µF 6.3V125mΩ 2 Sanyo Vishay AL-E Cboot VJ1206X104XXA Capacitor 1206 0.1µF, 25V 1 Cin C3216X7R1E225K Capacitor 1206 0.1µF, 25V 1 TDK Css VJ1206X123KXX Capacitor 1206 12nF, 25V 1 Vishay Cc1 VJ1206A2R2KXX Capacitor 1206 2.2pF 10% 1 Vishay Cc2 VJ1206A181KXX Capacitor 1206 180pF 10% 1 Vishay Rin CRCW1206100J Resistor 1206 10Ω 5% 1 Vishay Rfadj CRCW12066342F Resistor 1206 63.4kΩ 1% 1 Vishay Rc1 CRCW12063923F Resistor 1206 392kΩ 1% 1 Vishay Rfb1 CRCW12061002F Resistor 1206 10kΩ 1% 1 Vishay Rfb2 CRCW12061002F Resistor 1206 10kΩ 1% 1 Vishay Rcs CRCW1206222J Resistor 1206 2.2kΩ 5% 1 Vishay TABLE 2. Bill of Materials for Circuit of Figure 2 (Identical to BOM for 1.5V except as noted below) ID Part Number Size Parameters Qty. Vendor L1 RLF12560T-2R7N110 Inductor Type 12.5x12.8x6mm 2.7µH, 14.4A 4.5mΩ 1 TDK Co1, Co2, Co3, Co4 10TPB100M POSCAP 7.3x4.3x2.8mm 100µF 10V 1.9Arms 4 Sanyo Cc1 VJ1206A6R8KXX Capacitor 1206 6.8pF 10% 1 Vishay Cc2 VJ1206A271KXX Capacitor 1206 270pF 10% 1 Vishay Cc3 VJ1206A471KXX Capacitor 1206 470pF 10% 1 Vishay Rc2 CRCW12068451F Resistor 1206 8.45kΩ 1% 1 Vishay Rfb1 CRCW12061102F Resistor 1206 11kΩ 1% 1 Vishay Qty. Vendor 1 NSC TABLE 3. Bill of Materials for Circuit of Figure 3 ID Part Number Type Synchronous Controller Size Parameters U1 LM2727 Q1 Si4442DY N-MOSFET SO-8 30V, 4.1mΩ, @ 4.5V, 36nC 1 Vishay Q2 Si4442DY N-MOSFET SO-8 30V, 4.1mΩ, @ 4.5V, 36nC 1 Vishay Schottky Diode TSSOP-14 D1 BAT-54 SOT-23 30V 1 Vishay Lin SLF12575T-1R2N8R2 Inductor 12.5x12.5x7.5mm 12µH, 8.2A, 6.9mΩ 1 Coilcraft L1 D05022-152HC Inductor 22.35x16.26x8mm 1.5µH, 15A,4mΩ 1 Coilcraft 16mm D 25mm H 5600µF10V 2.35Arms 2 Sanyo Cin1, Cin2 10MV5600AX Aluminum Electrolytic Cinx C3216X7R1E105K Capacitor 1206 1µF, 25V 1 TDK Co1, Co2, Co3 10MV5600AX Aluminum Electrolytic 16mm D 25mm H 5600µF10V 2.35Arms 2 Sanyo Vishay Cboot VJ1206X104XXA Capacitor 1206 0.1µF, 25V 1 Cin C3216X7R1E225K Capacitor 1206 2.2µF, 25V 1 TDK Css VJ1206X123KXX Capacitor 1206 12nF, 25V 1 Vishay www.national.com 18 ID Part Number Size Parameters Qty. Vendor Cc1 VJ1206A4R7KXX Capacitor Type 1206 4.7pF 10% 1 Vishay Cc2 VJ1206A102KXX Capacitor 1206 1nF 10% 1 Vishay Vishay Rin CRCW1206100J Resistor 1206 10Ω 5% 1 Rfadj CRCW12068872F Resistor 1206 88.7kΩ 1% 1 Vishay Rc1 CRCW12062293F Resistor 1206 229kΩ 1% 1 Vishay Rfb1 CRCW12064991F Resistor 1206 4.99kΩ 1% 1 Vishay Rfb2 CRCW12064991F Resistor 1206 4.99kΩ 1% 1 Vishay Rcs CRCW1206152J Resistor 1206 1.5kΩ 5% 1 Vishay ID Part Number Qty. Vendor U1 LM2727 1 NSC Q1/Q2 Si4826DY L1 DO3316P-222 Cin1 Co1 TABLE 4. Bill of Materials for Circuit of Figure 4 Type Synchronous Controller Size Parameters TSSOP-14 Asymetric Dual N-MOSFET SO-8 30V, 24mΩ/ 8nC Top 16.5mΩ/ 15nC 1 Vishay Inductor 12.95x9.4x 5.21mm 2.2µH, 6.1A, 12mΩ 1 Coilcraft 10TPB100ML POSCAP 7.3x4.3x3.1mm 100µF 10V 1.9Arms 1 Sanyo 4TPB220ML POSCAP 7.3x4.3x3.1mm 220µF 4V 1.9Arms 1 Sanyo Cc C3216X7R1E105K Capacitor 1206 1µF, 25V 1 TDK Cin C3216X7R1E225K Capacitor 1206 2.2µF, 25V 1 TDK Css VJ1206X123KXX Capacitor 1206 12nF, 25V 1 Vishay Cc1 VJ1206A100KXX Capacitor 1206 10pF 10% 1 Vishay Cc2 VJ1206A561KXX Capacitor 1206 560pF 10% 1 Vishay Rin CRCW1206100J Resistor 1206 10Ω 5% 1 Vishay Rfadj CRCW12064222F Resistor 1206 42.2kΩ 1% 1 Vishay Rc1 CRCW12065112F Resistor 1206 51.1kΩ 1% 1 Vishay Rfb1 CRCW12062491F Resistor 1206 2.49kΩ 1% 1 Vishay Rfb2 CRCW12064991F Resistor 1206 4.99kΩ 1% 1 Vishay Rcs CRCW1206272J Resistor 1206 2.7kΩ 5% 1 Vishay Qty. Vendor 1 NSC TABLE 5. Bill of Materials for Circuit of Figure 5 ID Part Number U1 LM2727 Type Q1 Si4884DY N-MOSFET SO-8 30V, 13.5mΩ, @ 4.5V 15.3nC 1 Vishay Q2 Si4884DY N-MOSFET SO-8 30V, 13.5mΩ, @ 4.5V 15.3nC 1 Vishay Synchronous Controller Schottky Diode Size Parameters TSSOP-14 D1 BAT-54 SOT-23 30V 1 Vishay Lin P1166.102T Inductor 7.29x7.29 3.51mm 1µH, 11A 3.7mΩ 1 Pulse L1 P1168.102T Inductor 12x12x4.5 mm 1µH, 11A, 3.7mΩ 1 Pulse Cin1 10MV5600AX Aluminum Electrolytic 16mm D 25mm H 5600µF 10V 2.35Arms 1 Sanyo Cinx C3216X7R1E105K Capacitor 1206 1µF, 25V 1 TDK Co1, Co2, Co3 16MV4700WX Aluminum Electrolytic 12.5mm D 30mm H 4700µF 16V 2.8Arms 2 Sanyo Cboot VJ1206X104XXA Capacitor 1206 0.1µF, 25V 1 Vishay Cin C3216X7R1E225K Capacitor 1206 2.2µF, 25V 1 TDK Css VJ1206X123KXX Capacitor 1206 12nF, 25V 1 Vishay 19 www.national.com LM2727/LM2737 TABLE 3. Bill of Materials for Circuit of Figure 3 (Continued) LM2727/LM2737 TABLE 5. Bill of Materials for Circuit of Figure 5 (Continued) ID Part Number Size Parameters Qty. Cc1 VJ1206A4R7KXX Capacitor Type 1206 4.7pF 10% 1 Vendor Vishay Cc2 VJ1206A681KXX Capacitor 1206 680pF 10% 1 Vishay Vishay Rin CRCW1206100J Resistor 1206 10Ω 5% 1 Rfadj CRCW12064992F Resistor 1206 49.9kΩ 1% 1 Vishay Rc1 CRCW12061473F Resistor 1206 147kΩ 1% 1 Vishay Rfb1 CRCW12061492F Resistor 1206 14.9kΩ 1% 1 Vishay Rfb2 CRCW12064991F Resistor 1206 4.99kΩ 1% 1 Vishay Rcs CRCW1206332J Resistor 1206 3.3kΩ 5% 1 Vishay ID Part Number Qty. Vendor U1 LM2727 1 NSC Q1/Q2 Si4826DY Assymetric Dual N-MOSFET SO-8 30V, 24mΩ/ 8nC Top 16.5mΩ/ 15nC 1 Vishay D1 BAT-54 Schottky Diode SOT-23 30V 1 Vishay TDK TABLE 6. Bill of Materials for Circuit of Figure 6 Type Synchronous Controller Size Parameters TSSOP-14 Lin RLF7030T-1R0N64 Inductor 6.8x7.1x3.2mm 1µH, 6.4A, 7.3mΩ 1 L1 RLF7030T-3R3M4R1 Inductor 6.8x7.1x3.2mm 3.3µH, 4.1A, 17.4mΩ 1 TDK Cin1 C4532X5R1E156M MLCC 1812 15µF 25V 3.3Arms 1 Sanyo Co1 C4532X5R1E156M MLCC 1812 15µF 25V 3.3Arms 1 Sanyo Cboot VJ1206X104XXA Capacitor 1206 0.1µF, 25V 1 TDK Cin C3216X7R1E225K Capacitor 1206 2.2µF, 25V 1 TDK Css VJ1206X393KXX Capacitor 1206 39nF, 25V 1 Vishay Cc1 VJ1206A220KXX Capacitor 1206 22pF 10% 1 Vishay Cc2 VJ1206A681KXX Capacitor 1206 680pF 10% 1 Vishay Cc3 VJ1206A681KXX Capacitor 1206 680pF 10% 1 Vishay Rin CRCW1206100J Resistor 1206 10Ω 5% 1 Vishay Rfadj CRCW12061742F Resistor 1206 17.4kΩ 1% 1 Vishay Rc1 CRCW12061072F Resistor 1206 10.7kΩ 1% 1 Vishay Rc2 CRCW120666R5F Resistor 1206 66.5Ω 1% 1 Vishay Rfb1 CRCW12064991F Resistor 1206 4.99kΩ 1% 1 Vishay Rfb2 CRCW12061002F Resistor 1206 10kΩ 1% 1 Vishay Rcs CRCW1206152J Resistor 1206 1.5kΩ 5% 1 Vishay Qty. Vendor TABLE 7. Bill of Materials for 3.3V Circuit of Figure 6 (Identical to BOM for 1.8V except as noted below) ID Part Number Type Inductor Size Parameters L1 RLF7030T-4R7M3R4 6.8x7.1x 3.2mm 4.7µH, 3.4A, 26mΩ 1 TDK Cc1 VJ1206A270KXX Capacitor 1206 27pF 10% 1 Vishay Cc2 VJ1206X102KXX Capacitor 1206 1nF 10% 1 Vishay Cc3 VJ1206A821KXX Capacitor 1206 820pF 10% 1 Vishay Rc1 CRCW12061212F Resistor 1206 12.1kΩ 1% 1 Vishay Rc2 CRCW12054R9F Resistor 1206 54.9Ω 1% 1 Vishay Rfb1 CRCW12062211F Resistor 1206 2.21kΩ 1% 1 Vishay Rfb2 CRCW12061002F Resistor 1206 10kΩ 1% 1 Vishay Qty. Vendor 1 NSC TABLE 8. Bill of Materials for Circuit of Figure 7 ID Part Number U1 LM2727 www.national.com Type Synchronous Controller Size TSSOP-14 20 Parameters ID Part Number U2 LM78L05 Voltage Regulator Type SO-8 Q1/Q2 Si4826DY Assymetric Dual N-MOSFET SO-8 Schottky Diode Size Parameters Qty. Vendor 1 NSC 30V, 24mΩ/ 8nC Top 16.5mΩ/ 15nC 1 Vishay D1 BAT-54 SOT-23 30V 1 Vishay Lin RLF7030T-1R0N64 Inductor 6.8x7.1x3.2mm 1µH, 6.4A, 7.3mΩ 1 TDK L1 SLF12565T-4R2N5R5 Inductor 12.5x12.5x6.5mm 4.2µH, 5.5A, 15mΩ 1 TDK Cin1 16MV680WG D: 10mm L: 12.5mm 680µF 16V 3.4Arms 1 Sanyo Al-E Cinx C3216X5R1C106M MLCC 1210 10µF 16V 3.4Arms 1 TDK Co1 Co2 16MV680WG MLCC 1812 15µF 25V 3.3Arms 1 Sanyo Cox C3216X5R10J06M MLCC 1206 10µF 6.3V 2.7A TDK Cboot VJ1206X104XXA Capacitor 1206 0.1µF, 25V 1 Cin C3216X7R1E225K Capacitor 1206 2.2µF, 25V 1 Vishay TDK Css VJ1206X123KXX Capacitor 1206 12nF, 25V 1 Vishay Cc1 VJ1206A8R2KXX Capacitor 1206 8.2pF 10% 1 Vishay Cc2 VJ1206X102KXX Capacitor 1206 1nF 10% 1 Vishay Cc3 VJ1206X472KXX Capacitor 1206 4.7nF 10% 1 Vishay Rfadj CRCW12063252F Resistor 1206 32.5kΩ 1% 1 Vishay Rc1 CRCW12065232F Resistor 1206 52.3kΩ 1% 1 Vishay Rc2 CRCW120662371F Resistor 1206 2.37Ω 1% 1 Vishay Rfb1 CRCW12062211F Resistor 1206 2.21kΩ 1% 1 Vishay Rfb2 CRCW12061002F Resistor 1206 10kΩ 1% 1 Vishay Rcs CRCW1206202J Resistor 1206 2kΩ 5% 1 Vishay Qty. Vendor 1 NSC Vishay TABLE 9. Bill of Materials for Circuit of Figure 8 ID Part Number U1 LM2727 Q1 Si4894DY D2 MBRS330T3 L1 SLF12565T-470M2R4 D1 MBR0520 Cin1 16MV680WG Cinx C3216X5R1C106M Co1, Co2 16MV680WG Type Synchronous Controller Size Parameters TSSOP-14 N-MOSFET SO-8 30V, 15mΩ, 11.5nC 1 Schottky Diode SO-8 30V, 3A 1 ON 12.5x12.8x 4.7mm 47µH, 2.7A 53mΩ 1 TDK Inductor Schottky Diode 1812 20V 0.5A 1 ON Al-E 1206 680µF, 16V, 1.54Arms 1 Sanyo MLCC 1206 10µF, 16V, 3.4Arms 1 TDK D: 10mm L: 12.5mm 680µF 16V 26mΩ 2 Sanyo Al-E Cox C3216X5R10J06M MLCC 1206 10µF, 6.3V 2.7A 1 TDK Cboot VJ1206X104XXA Capacitor 1206 0.1µF, 25V 1 Vishay Cin C3216X7R1E225K Capacitor 1206 2.2µF, 25V 1 TDK Css VJ1206X123KXX Capacitor 1206 12nF, 25V 1 Vishay Cc1 VJ1206A561KXX Capacitor 1206 56pF 10% 1 Vishay Cc2 VJ1206X392KXX Capacitor 1206 3.9nF 10% 1 Vishay Cc3 VJ1206X223KXX Capacitor 1206 22nF 10% 1 Vishay Rfadj CRCW12062673F Resistor 1206 267kΩ 1% 1 Vishay Rc1 CRCW12066192F Resistor 1206 61.9kΩ 1% 1 Vishay Vishay Rc2 CRCW12067503F Resistor 1206 750kΩ 1% 1 Rfb1 CRCW12061371F Resistor 1206 1.37kΩ 1% 1 Vishay Rfb2 CRCW12061002F Resistor 1206 10kΩ 1% 1 Vishay Rcs CRCW1206122F Resistor 1206 1.2kΩ 5% 1 Vishay 21 www.national.com LM2727/LM2737 TABLE 8. Bill of Materials for Circuit of Figure 7 (Continued) LM2727/LM2737 N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages 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. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.