LM2738YMY

LM2738 550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator April 10, 2008 LM2738 550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator General Description The LM2738 regulator is a monolithic, high frequency, PWM step-down DC/DC converter in an 8-pin LLP or 8-pin eMSOP package. It provides all the active functions for local DC/DC conversion with fast transient response and accurate regulation in the smallest possible PCB area. With a minimum of external components, the LM2738 is easy to use. The ability to drive 1.5A loads with an internal 250mΩ NMOS switch using state-of-the-art 0.5µm BiCMOS technology results in the best power density available. Switching frequency is internally set to 550kHz (LM2738Y) or 1.6MHz (LM2738X), allowing the use of extremely small surface mount inductors and chip capacitors. Even though the operating frequencies are very high, efficiencies up to 90% are easy to achieve. External enable is included, featuring an ultra-low stand-by current of 400nA. The LM2738 utilizes current-mode control and internal compensation to provide highperformance regulation over a wide range of operating conditions. Additional features include internal soft-start circuitry to reduce in-rush current, cycle-by-cycle current limit, thermal shutdown, and output over-voltage protection. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Space Saving LLP-8 and eMSOP-8 package 3.0V to 20V input voltage range 0.8V to 18V output voltage range 1.5A output current 550kHz (LM2738Y) and 1.6MHz (LM2738X) switching frequencies 250mΩ NMOS switch 400nA shutdown current 0.8V, 2% internal voltage reference Internal soft-start Current-Mode, PWM operation Thermal shutdown Applications ■ ■ ■ ■ ■ ■ Local Point of Load Regulation Core Power in HDDs Set-Top Boxes Battery Powered Devices USB Powered Devices DSL Modems Typical Application Circuit Efficiency vs Load Current VIN = 12V, VOUT = 3.3V 30049101 30049145 © 2008 National Semiconductor Corporation 300491 www.national.com LM2738 Connection Diagrams 30049161 30049163 8-Pin LLP - TOP VIEW NS Package Number SDA08A 8-Pin eMSOP - TOP VIEW NS Package Number MUY08A Ordering Information Order Number LM2738XSD LM2738XSDX LM2738YSD LM2738YSDX LM2738XMY LM2738XMYX LM2738YMY LM2738YMYX Frequency Option 1.6MHz 8-Lead LLP 0.55MHz 1.6MHz 8-Lead eMSOP 0.55MHz MUY08A SJBB SDA08A L174B STDB Package Type NSC Package Drawing Package Marking Supplied As 1000 Tape and Reel 4500 Tape and Reel 1000 Tape and Reel 4500 Tape and Reel 1000 Tape and Reel 3500 Tape and Reel 1000 Tape and Reel 3500 Tape and Reel L237B * Contact the local sales office for the lead-free package. Pin Descriptions Pin 1 2 3 4 5, 7 6 8 DAP Name BOOST VIN VCC EN GND FB Function Boost voltage that drives the internal NMOS control switch. A bootstrap capacitor is connected between the BOOST and SW pins. Supply voltage for output power stage. Connect a bypass capacitor to this pin. Must tie pins 2 and 3 together at package. Input supply voltage of the IC. Connect a bypass capacitor to this pin. Must tie pin 2 and 3 together at the package. Enable control input. Logic high enables operation. Do not allow this pin to float or be greater than VIN + 0.3V. Signal and power ground pins. Place the bottom resistor of the feedback network as close as possible to these pins. Feedback pin. Connect FB to the external resistor divider to set output voltage. Output switch. Connects to the inductor, catch diode, and bootstrap capacitor. Signal and power ground. Must be connected to GND on the PCB. SW GND www.national.com 2 LM2738 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN, VCC SW Voltage Boost Voltage Boost to SW Voltage FB Voltage EN Voltage Junction Temperature ESD Susceptibility (Note 2) Storage Temp. Range -0.5V to 24V -0.5V to 24V -0.5V to 30V -0.5V to 6.0V -0.5V to 3.0V -0.5V to (VIN + 0.3V) 150°C 2kV -65°C to 150°C Soldering Information Infrared/Convection Reflow (15sec) Wave Soldering Lead Temp. (10sec) 220°C 260°C Operating Ratings VIN, VCC SW Voltage Boost Voltage Boost to SW Voltage Junction Temperature Range (Note 1) 3V to 20V -0.5V to 20V -0.5V to 25.5V 2.5V to 5.5V −40°C to +125°C 60°C/W 165°C Thermal Resistance θJA for LLP/eMSOP(Note 3) Thermal Shutdown (Note 3) Electrical Characteristics Specifications with standard typeface are for TJ = 25°C, and those in boldface type apply over the full Operating Temperature Range (TJ = -40°C to 125°C). VIN = 12V, VBOOST - VSW = 5V unless otherwise specified. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol VFB IFB UVLO Parameter Feedback Voltage Feedback Input Bias Current Undervoltage Lockout Undervoltage Lockout UVLO Hysteresis FSW DMAX DMIN RDS(ON) ICL IQ Switching Frequency Maximum Duty Cycle Minimum Duty Cycle Switch ON Resistance Switch Current Limit Quiescent Current Quiescent Current (shutdown) IBOOST VEN_TH IEN ISW Boost Pin Current Shutdown Threshold Voltage Enable Threshold Voltage Enable Pin Current Switch Leakage LM2738X LM2738Y LM2738X , Load=150mA LM2738Y, Load=150mA LM2738X LM2738Y VBOOST - VSW = 3V, Load=400mA VBOOST - VSW = 3V, VIN = 3V Switching Non-Switching VEN = 0V LM2738X (27% Duty Cycle) LM2738Y (27% Duty Cycle) VEN Falling VEN Rising Sink/Source VIN = 20V 1.4 2.0 1.28 0.364 Sink/Source VIN Rising VIN Falling 2.0 Conditions Min (Note 4) 0.784 Typ (Note 5) 0.800 0.02 0.1 2.7 2.3 0.4 1.6 0.55 92 95 7.5 2 250 2.9 1.9 1.9 400 4.5 2.5 10 100 0.4 3 500 1.92 0.676 MHz % % mΩ A mA mA nA mA V nA nA 100 2.90 V Max (Note 4) 0.816 Units V %/V nA ΔVFB/ΔVIN Feedback Voltage Line Regulation VIN = 3V to 20V Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see Electrical Characteristics. Note 2: Human body model, 1.5kΩ in series with 100pF. Note 3: Typical thermal shutdown will occur if the junction temperature exceeds 165°C. The maximum power dissipation is a function of TJ(MAX) , θJA and TA . The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA)/θJA . All numbers apply for packages soldered directly onto a 3” x 3” PC board with 2 oz. copper on 4 layers in still air in accordance to JEDEC standards. Thermal resistance varies greatly with layout, copper thicknes, number of layers in PCB, power distribution, number of thermal vias, board size, ambient temperature, and air flow. Note 4: Guaranteed to National’s Average Outgoing Quality Level (AOQL). Note 5: Typicals represent the most likely parametric norm. 3 www.national.com LM2738 Typical Performance Characteristics unless specified otherwise. Efficiency vs Load Current - "X" VOUT = 5V All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C, Efficiency vs Load Current - "Y" VOUT = 5V 30049197 30049198 Efficiency vs Load Current - "X" VOUT = 3.3V Efficiency vs Load Current - "Y" VOUT = 3.3V 30049151 30049152 Efficiency vs Load Current - "X" VOUT = 1.5V Efficiency vs Load Current - "Y" VOUT = 1.5V 30049199 30049131 www.national.com 4 LM2738 Typical Performance Characteristics unless specified otherwise. Oscillator Frequency vs Temperature - "X" All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C, Oscillator Frequency vs Temperature - "Y" 30049127 30049128 Current Limit vs Temperature VIN = 5V IQ Non-Switching vs Temperature 30049147 30049129 VFB vs Temperature RDSON vs Temperature 30049133 30049130 5 www.national.com LM2738 Typical Performance Characteristics unless specified otherwise. Line Regulation - "X" (VOUT = 1.5V, IOUT = 750mA) All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C, Line Regulation - "Y" (VOUT = 1.5V, IOUT = 750mA) 30049156 30049154 Line Regulation - "X" (VOUT = 3.3V, IOUT = 750mA) Line Regulation - "Y" (VOUT = 3.3V, IOUT = 750mA) 30049155 30049153 Load Regulation - "X" (VOUT = 1.5V) Load Regulation - "Y" (VOUT = 1.5V) 30049176 30049175 www.national.com 6 LM2738 Typical Performance Characteristics unless specified otherwise. Load Regulation - "X" (VOUT = 3.3V) All curves taken at VIN = 12V, VBOOST - VSW = 5V, and TA = 25°C, Load Regulation - "Y" (VOUT = 3.3V) 30049177 30049178 IQ Switching vs Temperature Load Transient - "X" (VOUT = 3.3V, VIN = 12V) 30049194 30049146 Startup - "X" (VOUT = 3.3V, VIN = 12, IOUT=1.5A (Resistive Load)) In-Rush Current - "X" (VOUT = 3.3V, VIN = 12V, IOUT=1.5A (Resistive Load) ) 30049190 30049191 7 www.national.com LM2738 Block Diagram 30049106 FIGURE 1. Simplified Internal Block Diagram Application Information THEORY OF OPERATION The LM2738 is a constant frequency PWM buck regulator IC that delivers a 1.5A load current. The regulator has a preset switching frequency of either 550kHz (LM2738Y) or 1.6MHz (LM2738X). These high frequencies allow the LM2738 to operate with small surface mount capacitors and inductors, resulting in DC/DC converters that require a minimum amount of board space. The LM2738 is internally compensated, so it is simple to use, and requires few external components. The LM2738 uses current-mode control to regulate the output voltage. The following operating description of the LM2738 will refer to the Simplified Block Diagram (Figure 1) and to the waveforms in Figure 2. The LM2738 supplies a regulated output voltage by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current (IL) increases with a linear slope. IL is measured by the current-sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through Schottky diode D1, which forces the SW pin to swing below ground by the forward voltage (VD) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage. 30049107 FIGURE 2. LM2738 Waveforms of SW Pin Voltage and Inductor Current BOOST FUNCTION Capacitor CBOOST and diode D2 in Figure 3 are used to generate a voltage VBOOST. VBOOST - VSW is the gate drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, VBOOST needs to be at least 2.5V greater than VSW. It is recommended that VBOOST be greater than 2.5V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum operating limit of 5.5V. 8 www.national.com LM2738 5.5V > VBOOST – VSW > 2.5V for best performance. When the LM2738 starts up, internal circuitry from the BOOST pin supplies a maximum of 20mA to CBOOST. This current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin will continue to source current to CBOOST until the voltage at the feedback pin is greater than 0.76V. There are various methods to derive VBOOST: 1. From the input voltage (3.0V < VIN < 5.5V) 2. From the output voltage (2.5V < VOUT < 5.5V) 3. From an external distributed voltage rail (2.5V < VEXT < 5.5V) 4. From a shunt or series zener diode In the Simplifed Block Diagram of Figure 1, capacitor CBOOST and diode D2 supply the gate-drive voltage for the NMOS switch. Capacitor CBOOST is charged via diode D2 by VIN. During a normal switching cycle, when the internal NMOS control switch is off (TOFF) (refer to Figure 2), VBOOST equals VIN minus the forward voltage of D2 (VFD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (VFD1). Therefore the voltage stored across CBOOST is VBOOST - VSW = VIN - VFD2 + VFD1 When the NMOS switch turns on (TON), the switch pin rises to VSW = VIN – (RDSON x IL), forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1 which is approximately 2VIN - 0.4V for many applications. Thus the gate-drive voltage of the NMOS switch is approximately VIN - 0.2V An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 3. The output voltage should be between 2.5V and 5.5V, so that proper gate voltage will be applied to the internal switch. In this circuit, CBOOST provides a gate drive voltage that is slightly less than VOUT. (VINMIN – VD3) > 2.5V 30049109 FIGURE 4. Zener Reduces Boost Voltage from VIN An alternative method is to place the zener diode D3 in a shunt configuration as shown in Figure 5. A small 350mW to 500mW 5.1V zener in a SOT-23 or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3V, 0.1µF capacitor (C4) should be placed in parallel with the zener diode. When the internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The 0.1 µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time. 30049148 FIGURE 5. Boost Voltage Supplied from the Shunt Zener on VIN Resistor R3 should be chosen to provide enough RMS current to the zener diode (D3) and to the BOOST pin. A recommended choice for the zener current (IZENER) is 1 mA. The current I BOOST into the BOOST pin supplies the gate current of the NMOS control switch and varies typically according to the following formula for the X version: IBOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA IBOOST can be calculated for the Y version using the following: 30049108 IBOOST = 0.22 x (D + 0.54) x (VZENER - VD2) µA where D is the duty cycle, VZENER and VD2 are in volts, and IBOOST is in milliamps. VZENER is the voltage applied to the anode of the boost diode (D2), and VD2 is the average forward voltage across D2. Note that this formula for IBOOST gives typical current. For the worst case IBOOST, increase the current by 40%. In that case, the worst case boost current will be IBOOST-MAX = 1.4 x IBOOST R3 will then be given by R3 = (VIN - VZENER) / (1.4 x IBOOST + IZENER) FIGURE 3. VOUT Charges CBOOST In applications where both VIN and VOUT are greater than 5.5V, or less than 3V, CBOOST cannot be charged directly from these voltages. If VIN and VOUT are greater than 5.5V, CBOOST can be charged from VIN or VOUT minus a zener voltage by placing a zener diode D3 in series with D2, as shown in Figure 4. When using a series zener diode from the input, ensure that the regulation of the input supply doesn’t create a voltage that falls outside the recommended VBOOST voltage. (VINMAX – VD3) < 5.5V 9 www.national.com LM2738 For example, using the X-version let VIN = 10V, VZENER = 5V, VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then IBOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11kΩ ENABLE PIN / SHUTDOWN MODE The LM2738 has a shutdown mode that is controlled by the enable pin (EN). When a logic low voltage is applied to EN, the part is in shutdown mode and its quiescent current drops to typically 400nA. The voltage at this pin should never exceed VIN + 0.3V. SOFT-START This function forces VOUT to increase at a controlled rate during start up. During soft-start, the error amplifier’s reference voltage ramps from 0V to its nominal value of 0.8V in approximately 600µs. This forces the regulator output to ramp up in a more linear and controlled fashion, which helps reduce in rush current. OUTPUT OVERVOLTAGE PROTECTION The overvoltage comparator compares the FB pin voltage to a voltage that is 16% higher than the internal reference Vref. Once the FB pin voltage goes 16% above the internal reference, the internal NMOS control switch is turned off, which allows the output voltage to decrease toward regulation. UNDERVOLTAGE LOCKOUT Undervoltage lockout (UVLO) prevents the LM2738 from operating until the input voltage exceeds 2.7V (typ). The UVLO threshold has approximately 400mV of hysteresis, so the part will operate until VIN drops below 2.3V (typ). Hysteresis prevents the part from turning off during power up if the VIN ramp-up is non-monotonic. CURRENT LIMIT The LM2738 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a current limit comparator detects if the output switch current exceeds 2.9A (typ), and turns off the switch until the next switching cycle begins. THERMAL SHUTDOWN Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150°C. VSW can be approximated by: VSW = IOUT x RDSON The diode forward drop (VD) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the VD, the higher the operating efficiency of the converter. The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the inductor value will decrease the output ripple current. One must ensure that the minimum current limit (2.0A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (ILPK) in the inductor is calculated by: ILPK = IOUT + ΔiL 30049180 FIGURE 6. Inductor Current In general, ΔiL = 0.1 x (IOUT) → 0.2 x (IOUT) If ΔiL = 33.3% of 1.50A, the peak current in the inductor will be 2.0A. The minimum guaranteed current limit over all operating conditions is 2.0A. One can either reduce ΔiL, or make the engineering judgment that zero margin will be safe enough. The typical current limit is 2.9A. The LM2738 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the output capacitor section for more details on calculating output voltage ripple. Now that the ripple current is determined, the inductance is calculated by: Design Guide INDUCTOR SELECTION The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN): Where The catch diode (D1) forward voltage drop and the voltage drop across the internal NMOS switch must be included to calculate a more accurate duty cycle. Calculate D by using the following formula: When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. Because 10 www.national.com LM2738 of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maximum output current. For example, if the designed maximum output current is 1.0A and the peak current is 1.25A, then the inductor should be specified with a saturation current limit of > 1.25A. There is no need to specify the saturation or peak current of the inductor at the 2.9A typical switch current limit. Because of the operating frequency of the LM2738, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety of ferrite-based inductors is huge. Lastly, inductors with lower series resistance (RDCR) will provide better operating efficiency. For recommended inductors see Example Circuits. INPUT CAPACITOR An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent Series Inductance). The recommended input capacitance is 10 µF.The input voltage rating is specifically stated by the capacitor manufacturer. Make sure to check any recommended deratings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The input capacitor maximum RMS input current rating (IRMS-IN) must be greater than: ripple will be approximately sinusoidal and 90° phase shifted from the switching action. Given the availability and quality of MLCCs and the expected output voltage of designs using the LM2738, there is really no need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not. Since the output capacitor is one of the two external components that control the stability of the regulator control loop, most applications will require a minimum of 22 µF of output capacitance. Capacitance, in general, is often increased when operating at lower duty cycles. Refer to the circuit examples at the end of the datasheet for suggested output capacitances of common applications. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R types. CATCH DIODE The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than: ID1 = IOUT x (1-D) The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward voltage drop. OUTPUT VOLTAGE The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and R1 is connected between VO and the FB pin. A good value for R2 is 10k. When designing a unity gain converter (Vo = 0.8V), R1 should be between 0Ω and 100Ω, and R2 should not be loaded. Neglecting inductor ripple simplifies the above equation to: It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always calculate the RMS at the point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. A large leaded capacitor will have high ESL and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LM2738, leaded capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount capacitors are strongly recommended. Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and multilayer ceramic capacitors (MLCC) are all good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R type capacitors due to their tolerance and temperature characteristics. Consult capacitor manufacturer datasheets to see how rated capacitance varies over operating conditions. OUTPUT CAPACITOR The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output ripple of the converter is: VREF = 0.80V PCB LAYOUT CONSIDERATIONS When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration is the close coupling of the GND connections of the input capacitor and the catch diode D1. These ground ends should be close to one another and be connected to the GND plane with at least two through-holes. Place these components as close to the IC as possible. Next in importance is the location of the GND connection of the output capacitor, which should be near the GND connections of CIN and D1. There should be a continuous ground plane on the bottom layer of a two-layer board except under the switching node island. The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup and inaccurate regulation. The feedback resistors should be placed as close as possible to the IC, with the GND of R1 placed as close as possible to the GND of the IC. The VOUT trace to R2 should be routed away from the inductor and any other traces that are switching. High AC currents flow through the VIN, SW and VOUT traces, so they should be as short and wide as possible. However, making the traces wide increases radiated noise, so the designer must make this trade-off. Radiated noise can be decreased by choosing a 11 www.national.com When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output LM2738 shielded inductor. The remaining components should also be placed as close as possible to the IC. Please see Application Note AN-1229 for further considerations and the LM2738 demo board as an example of a four-layer layout. RECOMMENED OPERATING AREA DUE TO MINIMUM ON TIME The LM2738 operates over a wide range of conditions, which is limited by the ON time of the device. A graph is provided to show the recommended operating area for the "X" at the full load (1.5A) and at 25°C ambient. The "Y" version of the LM2738 operates at a lower frequency and therefore operates over the entire range of operating voltages. 30049187 FIGURE 7. LM2738X - 1.6MHz (25°C, LOAD=1.5A) www.national.com 12 LM2738 Calculating Efficiency, and Junction Temperature The complete LM2738 DC/DC converter efficiency can be calculated in the following manner. If the inductor ripple current is fairly small, the conduction losses can be simplified to: PCOND = IOUT2 x RDSON x D Switching losses are also associated with the internal NFET switch. They occur during the switch on and off transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine this loss is to empirically measure the rise and fall times (10% to 90%) of the switch at the switch node. Switching Power Loss is calculated as follows: PSWR = 1/2(VIN x IOUT x FSW x TRISE) PSWF = 1/2(VIN x IOUT x FSW x TFALL) PSW = PSWR + PSWF Another loss is the power required for operation of the internal circuitry: PQ = IQ x VIN IQ is the quiescent operating current, and is typically around 1.9mA for the 0.55MHz frequency option. Typical Application power losses are: Power Loss Tabulation VIN VOUT IOUT VD FSW 12.0V 3.3V 1.25A 0.34V 550kHz 1.9mA 8nS 8nS 275mΩ 70mΩ 0.275 86.7% PQ PSWR PSWF PCOND PIND PLOSS PINTERNAL 22.8mW 33mW 33mW 118mW 110mW 634mW 207mW PDIODE 317mW POUT 4.125W Or Calculations for determining the most significant power losses are shown below. Other losses totaling less than 2% are not discussed. Power loss (PLOSS) is the sum of two basic types of losses in the converter: switching and conduction. Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D): VSW is the voltage drop across the internal NFET when it is on, and is equal to: VSW = IOUT x RDSON VD is the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufactures Electrical Characteristics section. If the voltage drop across the inductor (VDCR) is accounted for, the equation becomes: IQ TRISE TFALL RDS(ON) INDDCR D η ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS ΣPCOND + PSWF + PSWR + PQ = PINTERNAL PINTERNAL = 207mW The conduction losses in the free-wheeling Schottky diode are calculated as follows: PDIODE = VD x IOUT x (1-D) Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky diode that has a low forward voltage drop. Another significant external power loss is the conduction loss in the output inductor. The equation can be simplified to: PIND = IOUT2 x RDCR The LM2738 conduction loss is mainly associated with the internal NFET switch: Thermal Definitions TJ = Chip junction temperature TA = Ambient temperature RθJC = Thermal resistance from chip junction to device case RθJA = Thermal resistance from chip junction to ambient air Heat in the LM2738 due to internal power dissipation is removed through conduction and/or convection. Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor). Heat Transfer goes as: Silicon → package → lead frame → PCB Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural convection occurs when air currents rise from the hot device to cooler air. 13 www.national.com LM2738 Thermal impedance is defined as: enters thermal shutdown. If the SW-pin is monitored, it will be obvious when the internal NFET stops switching, indicating a junction temperature of 165°C. Knowing the internal power dissipation from the above methods, the junction temperature, and the ambient temperature RθJA can be determined. Thermal impedance from the silicon junction to the ambient air is defined as: The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can greatly effect RθJA. The type and number of thermal vias can also make a large difference in the thermal impedance. Thermal vias are necessary in most applications. They conduct heat from the surface of the PCB to the ground plane. Four to six thermal vias should be placed under the exposed pad to the ground plane if the LLP package is used. Thermal impedance also depends on the thermal properties due to the application's operating conditions (Vin, Vo, Io etc), and the surrounding circuitry. Silicon Junction Temperature Determination Method 1: To accurately measure the silicon temperature for a given application, two methods can be used. The first method requires the user to know the thermal impedance of the silicon junction to top case temperature. Some clarification needs to be made before we go any further. RθJC is the thermal impedance from all six sides of an IC package to silicon junction. RΦJC is the thermal impedance from top case to the silicon junction. In this data sheet we will use RΦJC so that it allows the user to measure top case temperature with a small thermocouple attached to the top case. RΦJC is approximately 30°C/Watt for the 8-pin LLP package with the exposed pad. Knowing the internal dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench we have: Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be found. An example of calculating RθJA for an application using the National Semiconductor LM2738 LLP demonstration board is shown below. The four layer PCB is constructed using FR4 with ½ oz copper traces. The copper ground plane is on the bottom layer. The ground plane is accessed by two vias. The board measures 3.0cm x 3.0cm. It was placed in an oven with no forced airflow. The ambient temperature was raised to 144°C, and at that temperature, the device went into thermal shutdown. From the previous example: PINTERNAL = 207mW If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above 109°C Tj - (RθJA x PLOSS) = TA 125°C - (102°C/W x 207mW) = 104°C LLP Package Therefore: Tj = (RΦJC x PLOSS) + TC From the previous example: Tj = (RΦJC x PINTERNAL) + TC Tj = 30°C/W x 0.207W + TC The second method can give a very accurate silicon junction temperature. The first step is to determine RθJA of the application. The LM2738 has over-temperature protection circuitry. When the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a hysteresis of about 15°C. Once the silicon temperature has decreased to approximately 150°C, the device will start to switch again. Knowing this, the RθJA for any application can be characterized during the early stages of the design one may calculate the RθJA by placing the PCB circuit into a thermal chamber. Raise the ambient temperature in the given working application until the circuit www.national.com 14 30049174 FIGURE 8. Internal LLP Connection For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 9). By increasing the size of ground plane, and adding thermal vias, the RθJA for the application can be reduced. LM2738 30049179 FIGURE 9. 8-Lead LLP PCB Dog Bone Layout 15 www.national.com LM2738 LM2738X Circuit Example 1 30049142 FIGURE 10. LM2738X (1.6MHz) VBOOST Derived from VIN 5V to 1.5V/1.5A Bill of Materials for Figure 10 Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 6.3V, X5R 22µF, 6.3V, X5R 0.1uF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 2.2µH, 1.9A, 8.87kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% Part Number LM2738X C3216X5ROJ106M C3216X5ROJ226M C1005X7R1C104K CRS08 BAT54WS MSS5131-222ML CRCW06038871F CRCW06031022F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Diodes, Inc. Coilcraft Vishay Vishay Vishay www.national.com 16 LM2738 LM2738X Circuit Example 2 30049193 FIGURE 11. LM2738X (1.6MHz) VBOOST Derived from VOUT 12V to 3.3V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 25V, X7R 33µF, 6.3V, X5R 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 5µH, 2.9A 31.6kΩ, 1% 10kΩ, 1% 100kΩ, 1% Part Number NSC C3225X7R1E106M C3216X5ROJ336M C1005X7R1C104K CRS08 BAT54WS MSS7341- 502NL CRCW06033162F CRCW06031002F CRCW06031003F Manufacturer LM2738X TDK TDK TDK Toshiba Diodes, Inc. Coilcraft Vishay Vishay Vishay 17 www.national.com LM2738 LM2738X Circuit Example 3 30049144 FIGURE 12. LM2738X (1.6MHz) VBOOST Derived from VSHUNT 18V to 1.5V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap C4, Shunt Cap D1, Catch Diode D2, Boost Diode D3, Zener Diode L1 R1 R2 R3 R4 Part Value 1.5A Buck Regulator 10µF, 25V, X7R 47µF, 6.3V, X5R 0.1µF, 16V, X7R 0.1µF, 6.3V, X5R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 5.1V 250Mw SOT-23 2.7µH, 1.76A 8.87kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% 4.12kΩ, 1% Part Number LM2738X C3225X7R1E106M C3216X5ROJ476M C1005X7R1C104K C1005X5R0J104K CRS08 BAT54WS BZX84C5V1 VLCF5020T-2R7N1R7 CRCW06038871F CRCW06031022F CRCW06031003F CRCW06034121F Manufacturer National Semiconductor TDK TDK TDK TDK Toshiba Diodes, Inc. Vishay TDK Vishay Vishay Vishay Vishay www.national.com 18 LM2738 LM2738X Circuit Example 4 30049149 FIGURE 13. LM2738X (1.6MHz) VBOOST Derived from Series Zener Diode (VIN) 15V to 1.5V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode D3, Zener Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 25V, X7R 47µF, 6.3V, X5R 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 11V 350Mw SOT-23 3.3µH, 3.5A 8.87kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% Part Number LM2738X C3225X7R1E106M C3216X5ROJ476M C1005X7R1C104K CRS08 BAT54WS BZX84C11T MSS7341-332NL CRCW06038871F CRCW06031022F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Diodes, Inc. Diodes, Inc. Coilcraft Vishay Vishay Vishay 19 www.national.com LM2738 LM2738X Circuit Example 5 30049150 FIGURE 14. LM2738X (1.6MHz) VBOOST Derived from Series Zener Diode (VOUT) 15V to 9V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode D3, Zener Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 25V, X7R 22µF, 16V, X5R 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 4.3V 350mw SOT-23 6.2µH, 2.5A 102kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% Part Number LM2738X C3225X7R1E106M C3216X5R1C226M C1005X7R1C104K CRS08 BAT54WS BZX84C4V3 MSS7341-622NL CRCW06031023F CRCW06031022F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Diodes, Inc. Diodes, Inc. Coilcraft Vishay Vishay Vishay www.national.com 20 LM2738 LM2738Y Circuit Example 6 30049142 FIGURE 15. LM2738Y (550kHz) VBOOST Derived from VIN 5V to 1.5V/1.5A Bill of Materials for Figure 15 Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 6.3V, X5R 47µF, 6.3V, X5R 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 6.2µH, 2.5A, 8.87kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% Part Number LM2738Y C3216X5ROJ106M C3216X5ROJ476M C1005X7R1C104K CRS08 BAT54WS MSS7341-622NL CRCW06038871F CRCW06031022F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Diodes, Inc. Coilcraft Vishay Vishay Vishay 21 www.national.com LM2738 LM2738Y Circuit Example 7 30049193 FIGURE 16. LM2738Y (550kHz) VBOOST Derived from VOUT 12V to 3.3V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 25V, X7R 47µF, 6.3V, X5R 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 12µH, 1.7A, 31.6kΩ, 1% 10.0 kΩ, 1% 100kΩ, 1% Part Number LM2738Y C3225X7R1E106M C3216X5ROJ476M C1005X7R1C104K CRS08 BAT54WS MSS7341-123NL CRCW06033162F CRCW06031002F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Vishay Coilcraft Vishay Vishay Vishay www.national.com 22 LM2738 LM2738Y Circuit Example 8 30049144 FIGURE 17. LM2738Y (550kHz) VBOOST Derived from VSHUNT 18V to 1.5V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap C4, Shunt Cap D1, Catch Diode D2, Boost Diode D3, Zener Diode L1 R1 R2 R3 R4 Part Value 1.5A Buck Regulator 10µF, 25V, X7R (47µF, 6.3V, X5R) x 2 = 94µF 0.1µF, 16V, X7R 0.1µF, 6.3V, X5R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 5.1V 250Mw SOT-23 8.7µH, 2.2A 8.87kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% 4.12kΩ, 1% Part Number LM2738Y C3225X7R1E106M C3216X5ROJ476M C1005X7R1C104K C1005X5R0J104K CRS08 BAT54WS BZX84C5V1 MSS7341-872NL CRCW06038871F CRCW06031022F CRCW06031003F CRCW06034121F Manufacturer National Semiconductor TDK TDK TDK TDK Toshiba Diodes, Inc. Vishay Coilcraft Vishay Vishay Vishay Vishay 23 www.national.com LM2738 LM2738Y Circuit Example 9 30049149 FIGURE 18. LM2738Y (550kHz) VBOOST Derived from Series Zener Diode (VIN) 15V to 1.5V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode D3, Zener Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 25V, X7R (47µF, 6.3V, X5R) x 2 = 94µF 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 11V 350Mw SOT-23 8.7µH, 2.2A 8.87kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% Part Number LM2738Y C3225X7R1E106M C3216X5ROJ476M C1005X7R1C104K CRS08 BAT54WS BZX84C11T MSS7341-872NL CRCW06038871F CRCW06031022F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Diodes, Inc. Diodes, Inc. Coilcraft Vishay Vishay Vishay www.national.com 24 LM2738 LM2738Y Circuit Example 10 30049150 FIGURE 19. LM2738Y (550kHz) VBOOST Derived from Series Zener Diode (VOUT) 15V to 9V/1.5A Bill of Materials for Part ID U1 C1, Input Cap C2, Output Cap C3, Boost Cap D1, Catch Diode D2, Boost Diode D3, Zener Diode L1 R1 R2 R3 Part Value 1.5A Buck Regulator 10µF, 25V, X7R 22µF, 16V, X5R 0.1µF, 16V, X7R 0.34VF Schottky 1.5A, 30V 1VF @ 100mA Diode 4.3V 350mw SOT-23 15µH, 2.1A 102kΩ, 1% 10.2kΩ, 1% 100kΩ, 1% Part Number LM2738Y C3225X7R1E106M C3216X5R1C226M C1005X7R1C104K CRS08 BAT54WS BZX84C4V3 SLF7055T150M2R1-3PF CRCW06031023F CRCW06031022F CRCW06031003F Manufacturer National Semiconductor TDK TDK TDK Toshiba Diodes, Inc. Diodes, Inc. TDK Vishay Vishay Vishay 25 www.national.com LM2738 Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead LLP Package NS Package Number SDA08A 8-Lead eMSOP Package NS Package Number MUY08A www.national.com 26 LM2738 Notes 27 www.national.com LM2738 550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock Conditioners Data Converters Displays Ethernet Interface LVDS Power Management Switching Regulators LDOs LED Lighting PowerWise Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/displays www.national.com/ethernet www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH Analog University App Notes Distributors Green Compliance Packaging Design Support www.national.com/webench www.national.com/AU www.national.com/appnotes www.national.com/contacts www.national.com/quality/green www.national.com/packaging www.national.com/quality www.national.com/refdesigns www.national.com/feedback Quality and Reliability Reference Designs Feedback THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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