LM2833 1.5MHz/3MHz 3.0A Step-Down DC-DC Switching Regulator General Description Features The LM2833 regulator is a monolithic, high frequency, PWM step-down DC/DC converter available in a 10-pin LLP or eMSOP package. It contains all the active functions to provide local DC/DC conversion with fast transient response and accurate regulation in the smallest possible PCB area. With a minimum of external components, the LM2833 is easy to use. The ability to drive 3.0A loads with an internal 56 mΩ PMOS switch using state-of-the-art 0.5µm BiCMOS technology results in the best power density available. The world-class control circuitry allows on-times as low as 30ns, thus supporting exceptionally high frequency conversion over the entire 3V to 5.5V input operating range down to the minimum output voltage of 0.6V. Switching frequency is internally set to 1.5MHz or 3.0MHz, allowing the use of extremely small surface mount inductors and capacitors. Even though the operating frequency is high, efficiencies up to 93% are easy to achieve. External shutdown is included, featuring an ultra-low stand-by current of 300nA. The LM2833 utilizes peak currentmode control and internal compensation to provide highperformance regulation over a wide range of operating conditions. Additional features include internal soft-start circuitry to reduce inrush current, cycle-by-cycle current limit, frequency foldback, thermal shutdown, and output over-voltage protection. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Input voltage range of 3.0V to 5.5V Output voltage range of 0.6V to 4.5V Tiny eMSOP-10 or LLP-10 package 3.0A steady-state output current High switching frequencies 1.5MHz (LM2833X) 3.0MHz (LM2833Z) Enable pin 56mΩ PMOS switch 0.6V, 2% internal voltage reference over line and temperature Internal soft-start Internally compensated peak current-mode control Cycle-by-cycle current limit and thermal shutdown Frequency foldback protection Input voltage UVLO (Under-voltage lockout) Output over-voltage protection Applications ■ ■ ■ ■ ■ ■ Multimedia Set Top Box Broadband Communications Core Power in HDDs Data Acquisition/Telemetry USB Powered Devices DSL Modems Typical Application Circuit 30013201 30013212 © 2009 National Semiconductor Corporation 300132 www.national.com LM2833 1.5MHz/3MHz 3.0A Step-Down DC-DC Switching Regulator January 13, 2009 LM2833 Connection Diagrams 30013203 30013202 10-pin LLP 10-pin eMSOP Ordering Information Order Number Frequency Option LM2833XMY LM2833XMYX LM2833XSD LM2833ZMY LM2833ZSD NSC Package Drawing Top Mark eMSOP-10 MUC10A SPYB LLP-10 SDA10A 2833X eMSOP-10 MUC10A SPZB LLP-10 SDA10A 2833Z 1.5MHz LM2833XSDX LM2833ZMYX Package Type 3MHz LM2833ZSDX Supplied As 1000 units Tape and Reel 3500 units Tape and Reel 1000 units Tape and Reel 4500 units Tape and Reel 1000 units Tape and Reel 3500 units Tape and Reel 1000 units Tape and Reel 4500 units Tape and Reel Pin Descriptions Pin(s) Name Description 1 VINC Input supply for internal bias and control circuitry. Need to locally bypass this pin to GND. 2 EN Enable control input. Logic high enables operation. Do not allow this pin to float or subject to voltages greater than VIN + 0.3V. 3 SGND 4 NC No user function, connect this pin to GND. 5 FB Feedback pin. Connect this pin to the external resistor divider to set output voltage. 6 PGND 7, 8 SW 9, 10 VIND DAP Die Attach Pad www.national.com Signal (analog) ground. Place the bottom resistor of the feedback network as close as possible to this pin for good load regulation. Power ground pin. Provides ground return path for the internal driver. Switch pins. Connect these pins to the inductor and catch diode. Input supply voltage. Connect a bypass capacitor locally from these pins to PGND. Connect to system ground for low thermal impedance, but it cannot be used as a primary GND connection. 2 If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VINC, VIND FB Voltage EN Voltage SW Voltage ESD Susceptibility (Note 4) LM2833 Junction Temperature (Note 2) 150°C Storage Temperature −65°C to +150°C Soldering Information Infrared/Convection Reflow (15sec) 220°C Absolute Maximum Ratings (Note 1) -0.5V to 7V -0.5V to 3V -0.5V to 7V -0.5V to 7V 2kV Operating Ratings VINC, VIND Junction Temperature 3V to 5.5V −40°C to +125°C Electrical Characteristics Unless otherwise specified under the Conditions column, VIN = 5V. Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of -40°C to +125°C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm, and are provided for reference purposes only. Symbol VFB ΔVFB/(ΔVINxVFB) IB UVLO Parameter Feedback Voltage Feedback Voltage Line Regulation Min Typ Max LLP-10 Package Conditions 0.588 0.600 0.612 eMSOP-10 Package 0.584 0.600 0.616 VIN = 3V to 5.5V 0.08 Feedback Input Bias Current Undervoltage Lockout VIN Rising fSW Switching Frequency DMAX Maximum Duty Cycle DMIN Minimum Duty Cycle RDS(ON) ICL VEN_TH Switch On Resistance 1.1 2.35 1.5 1.95 LM2833Z 2.25 3.0 3.75 LM2833X 86 95 LM2833Z 80 90 0.35 5 LM2833Z 7 LLP-10 Package 58 eMSOP-10 Package 56 3.4 1.8 Enable Pin Current Quiescent Current (switching) nA V V MHz % % 90 4.4 Shutdown Threshold Voltage Switch Leakage fFB 2.90 LM2833X Enable Threshold Voltage IEN VFB_F 2.70 1.85 Switch Current Limit ISW IQ 100 LM2833X V %/V 0.1 VIN Falling UVLO Hysteresis Units mΩ A 0.4 V 100 nA Sink/Source 100 nA LM2833X, VFB = 0.55 3.2 5 LM2833Z, VFB = 0.55 4.3 6.5 mA Quiescent Current (shutdown) All Options VEN = 0V 300 nA FB Frequency Foldback Threshold All Options 0.32 V LM2833X, VFB = 0V 400 LM2833Z, VFB = 0V 800 Foldback Frequency 3 kHz www.national.com LM2833 Symbol Parameter Conditions Min Typ Max Units LLP-10 Package 53 eMSOP-10 Package 50 LLP-10 Package 12 eMSOP-10 Package 11 Thermal Shutdown Threshold Junction Temperature Rising 165 °C Thermal Shutdown Hysteresis Junction Temperature Falling 15 °C θJA Junction to Ambient 0 LFPM Air Flow (Note 3) θJC Junction to Case (Note 3) TSD TSD_HYS °C/W °C/W Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in the recommended Operating Ratings is not implied. The recommended Operating Ratings indicate conditions at which the device is functional and should not be operated beyond such conditions. Note 2: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device. Note 3: Applies for packages soldered directly onto a 4” x 3” 4-layer standard JEDEC board in still air. Note 4: Human body model, 1.5kΩ in series with 100pF. www.national.com 4 LM2833 Typical Performance Characteristics Unless otherwise specified, VIN = 5V and TA = 25°C. Efficiency vs Load Current - "LM2833X" and "LM2833Z" Efficiency vs Load Current - "LM2833X" 30013213 30013214 Efficiency vs Load Current - "LM2833Z" Oscillator Frequency vs Temperature - "LM2833X" 30013215 30013225 Oscillator Frequency vs Temperature - "LM2833Z" Current Limit vs Temperature 30013226 30013227 5 www.national.com LM2833 RDS(ON) vs Temperature (LLP-10 Package) RDS(ON) vs Temperature (eMSOP-10 Package) 30013229 30013230 LM2833X IQ (Quiescent Current) LM2833Z IQ (Quiescent Current) 30013231 30013232 VFB vs Temperature Frequency Foldback 30013228 30013233 www.national.com 6 LM2833 Loop Gain and Phase - "LM2833X" Loop Gain and Phase - "LM2833Z" 30013235 30013236 Load Step Response - "LM2833X" Line Transient Response - "LM2833X" 30013247 30013248 Startup by EN - "LM2833X" Shutdown by EN - "LM2833X" 30013252 30013253 7 www.national.com LM2833 Startup with EN tied to VIN - "LM2833X" Short-circuit Triggering - "LM2833X" 30013250 30013254 Short-circuit Release - "LM2833X" Recovery from Thermal Shutdown - "LM2833X" 30013251 www.national.com 30013240 8 LM2833 Block Diagram 30013269 FIGURE 1. Simplified Block Diagram 9 www.national.com LM2833 required to initiate switching. Do not allow this pin to float or rise to 0.3V above VIN. It should be noted that when the EN pin voltage rises above 1.8V while the input voltage is greater than UVLO, there is 15µs delay before switching starts. During this delay the LM2833 will go through a power on reset state after which the internal soft-start process commences. During soft-start, the error amplifier’s reference voltage ramps from 0V to its nominal value of 0.6V in approximately 600µs. This forces the regulator output to ramp up in a controlled fashion, which helps reduce inrush current seen at the input and minimizes output voltage overshoot. The simplest way to enable the operation of the LM2833 is to connect the EN pin to VIN which allows self start-up of the LM2833 whenever the input voltage is applied. However, when an input voltage of slow rise time is used to power the application and if both the input voltage and the output voltage are not fully established before the soft-start time elapses, the control circuit will command maximum duty cycle operation of the internal power switch to bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.6V, the duty cycle will have to reduce from the maximum value accordingly, to maintain regulation. It takes a finite amount of time for this reduction of duty cycle and this can result in a transient in output voltage for a short duration, as shown in Figure 3. In applications where this output voltage overshoot is undesirable, one simple solution is to add a feed-forward capacitor (CFF) across the top feedback resistor R1 to speed Gm Amplifier recovery. In practice, a 27nF to 100nF ceramic capacitor is usually a good choice to remove the overshoot completely or limit the overshoot to an insignificant level during startup, as shown in Figure 4. Another more effective solution is to control EN pin voltage by a separate logic signal, and pull the signal high only after VIN is fully established. In this way, the chip can execute a normal, complete soft start process, minimizing any output voltage overshoot. Under some circumstances at cold temperature, this approach may also be required to minimize any unwanted output voltage transients that may occur when the input voltage rises slowly. For a fast rising input voltage (100µs for example), there is no need to control EN separately or add a feed-forward capacitor since the soft-start can bring up output voltage smoothly as shown in Figure 5. During startup, the LM2833 gradually increases the switching frequency from 400kHz (LM2833X) or 800kHz (LM2833Z) to the nominal fixed value, as the feedback voltage increases (see Frequency Foldback section for more information). Since the internal corrective ramp signal adjusts its slope dynamically, and is proportional to the switching frequency during startup, a larger output capacitance may be required to insure a smooth output voltage rise, at low programmed output voltage and high output load current. Application Information THEORY OF OPERATION The LM2833 is a constant frequency PWM buck regulator IC that delivers a 3.0A load current. The regulator is available in preset switching frequencies of 1.5MHz or 3.0MHz. This high frequency allows the LM2833 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. The LM2833 is internally compensated, therefore it is simple to use and requires few external components. The LM2833 uses peak current-mode control to regulate the output voltage. The following description of operation of the LM2833 will refer to the Typical Application Circuit, to the waveforms in Figure 2 and simplified block diagram in Figure 1. The LM2833 supplies a regulated output voltage by switching the internal PMOS power switch at a 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 PMOS power 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 internal power switch turns off until the next switching cycle begins. During the switch offtime, the inductor current discharges through the catch 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. 30013266 FIGURE 2. SW Pin Voltage and Inductor Current Waveforms SOFT-START/SHUTDOWN The LM2833 has both enable and shutdown modes that are controlled by the EN pin. Connecting a voltage source greater than 1.8V to the EN pin enables the operation of the LM2833, while reducing this voltage below 0.4V places the part in a low quiescent current (300nA typical) shutdown mode. There is no internal pull-up on EN pin, therefore an external signal is www.national.com 10 LOAD STEP RESPONSE The LM2833 has a fixed internal loop compensation, which results in a small-signal loop bandwidth highly related to the output voltage level. In general, the loop bandwidth at low voltage is larger than at high voltage due to the increased overall loop gain. The limited bandwidth at high output voltage may pose a challenge when loop step response is concerned. In this case, one effective approach to improving loop step response is to add a feed-forward capacitor (CFF) in the range of 27nF to 100nF in parallel with the upper feedback resistor (assuming the lower feedback resistor is 2kΩ), as shown in Figure 6. The feed-forward capacitor introduces a zero-pole pair which helps compensate the loop. The position of the zero-pole pair is a function of the feedback resistors and capacitor: 30013260 FIGURE 3. Startup Response to VIN Note the factor in parenthesis is the ratio of the output voltage to the feedback voltage. As the output voltage gets close to 0.6V, the pole moves towards the zero, tending to cancel it out. Consequently, adding CFF will have less effect on the step response at lower output voltages. As an example, Figure 8 shows that at the output voltage of 3.3V, a 47nF of CFF can boost the loop bandwidth to 117kHz, from the original 23kHz as shown in Figure 7. Correspondingly, the responses to a load step between 0.3A and 3A without and with CFF are shown in Figure 9 and Figure 10 respectively. The higher loop bandwidth as a result of CFF reduces the total output excursion by more than half. Aside from the above approach, increasing the output capacitance is generally also effective to reduce the excursion in output voltage caused by a load step. This approach remains valid for applications where the desired output voltages are close to the feedback voltage. 30013261 FIGURE 4. Startup Response to VIN with CFF 30013262 FIGURE 5. Startup Response to VIN with 100µs rise time 11 www.national.com LM2833 FREQUENCY FOLDBACK The LM2833 uses frequency foldback to help limit switch current and power dissipation during start-up, short-circuit and over load conditions by sensing if the feedback voltage is below 0.32V (typical). The LM2833 will reduce the switching frequency from the nominal fixed value (1.5MHz or 3.0MHz) down to 400kHz (LM2833X) or 800kHz (LM2833Z) when the feedback voltage drops to 0V. See the Frequency Foldback plot in the Typical Performance Characteristics section. LM2833 30013255 FIGURE 6. Adding a CFF Capacitor 30013245 FIGURE 9. Load Step Response without CFF 30013237 FIGURE 7. Loop Gain and Phase without CFF 30013246 FIGURE 10. Load Step Response with CFF OUTPUT OVER-VOLTAGE PROTECTION The LM2833 has a built in output over-voltage comparator that compares the FB pin voltage to a threshold voltage that is 15% higher than the internal reference VREF. Once the FB pin voltage exceeds this threshold level (typically 0.69V), the internal PMOS power switch is turned off, which allows the output voltage to decrease towards regulation. UNDER-VOLTAGE LOCKOUT Under-voltage lockout (UVLO) prevents the LM2833 from operating until the input voltage exceeds 2.70V (typical). The UVLO threshold has approximately 350mV of hysteresis, so the part will operate until VIN drops below 2.35V (typical). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic. 30013238 CURRENT LIMIT The LM2833 uses cycle-by-cycle current limiting to protect the internal power switch. During each switching cycle, a current limit comparator detects if the power switch current exceeds 4.4A (typical), and turns off the switch until the next switching cycle begins. FIGURE 8. Loop Gain and Phase with CFF www.national.com 12 Design Guide INDUCTOR SELECTION The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VOUT) to input voltage (VIN): The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to calculate a more accurate duty cycle. Calculate D by using the following formula: where fSW is the switching frequency. 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 properly. Because of the operating frequency of the LM2833, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety and availability of ferrite-based inductors is large. Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended inductor selection, refer to Design Examples. VSW can be approximated by: VSW = IOUT x RDS(ON) Where IOUT is output load current. 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 (ΔiL, as defined in Figure 2). 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. In general, the ratio of ripple current to the output current is optimized when it is set between 0.2 and 0.4 for output currents above 2A. This ratio r is defined as: 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 rating, RMS current rating, and ESL (Equivalent Series Inductance). 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: One must ensure that the minimum current limit (3.4A) 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/2 Neglecting inductor ripple simplifies the above equation to: When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1A, r can be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for the maximum ripple ratio at any current below 2A is: 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. As a rule of thumb, a large leaded capacitor will have high ESL and a 1206 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LM2833, leaded capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than that required to provide stable operation. It is strongly recommended to use ceramic capacitors due to their low ESR and low ESL. A 22µF multilayer ceramic capacitor r = 0.387 x IOUT-0.3667 Note that this is just a guideline, and it needs to be combined with two important factors for proper selection of inductance values at any operating condition. The first consideration is at output voltage above 2.5V, one needs to ensure that the inductance given by the above guideline should not be less than 1µH for the LM2833X or 0.5µH for the LM2833Z. Since the LM2833 has a fixed internal corrective ramp signal, a very low inductance value at high output voltage will generate a very 13 www.national.com LM2833 steep down slope of inductor current, which will result in an insufficient slope compensation, and cause instability known as sub-harmonic oscillation. Another consideration is at low load current, one needs to ensure that the inductance value given by the guideline should not exceed 10µH for the LM2833X and 4.7µH for the LM2833Z, since too much inductance effectively flattens the down slope of the inductor current, and may significantly limit the system bandwidth and phase margin resulting in instability. The LM2833 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: THERMAL SHUTDOWN Thermal shutdown limits total power dissipation by turning off the internal power switch when the IC junction temperature typically exceeds 165°C. After thermal shutdown occurs, the power switch does not turn on again until the junction temperature drops below approximately 150°C. LM2833 (MLCC) is a good choice for most applications. In cases where large capacitance is required, use surface mount capacitors such as Tantalum capacitors and place at least a 4.7µF ceramic capacitor close to the VIN pin. For MLCCs it is recommended to use X7R or X5R dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating conditions. VREF = 0.60V EFFICIENCY ESTIMATION The complete LM2833 DC/DC converter efficiency can be calculated in the following manner: 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: Or When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output 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 LM2833, 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 22µF output capacitance. In the case of low output voltage, a larger output capacitance is required to ensure sufficient phase margin. Capacitance can often, but not always, be increased significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R types. Again, verify actual capacitance at the desired operating voltage and temperature. Check the RMS current rating of the capacitor. The maximum RMS current rating of the capacitor is: Calculations for determining the most significant power losses are shown below. Other losses totaling less than 2% are not discussed. The main power loss (PLOSS) in the converter includes two basic types of losses: switching loss and conduction loss. In addition, there is loss associated with the power required for the internal circuitry of IC. Conduction losses usually dominate at higher output loads, whereas switching losses 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 power switch when it is on, and is equal to: VSW = IOUT x RDS(ON) VD is the forward voltage drop across the catch diode. It can be obtained from the diode manufactures Electrical Characteristics section. If the DC voltage drop across the inductor (VDCR) is accounted for, the equation becomes: One may select a 1206 size MLCC for output capacitor, since its current rating is typically above 1A, more than enough for the requirement. CATCH DIODE The catch diode conducts during the switch off-time. A Schottky diode is recommended for its fast switching time and low forward voltage drop. The catch diode should be chosen such that its current rating is greater than: The conduction losses in the catch diode are calculated as follows: ID = IOUT x (1-D) PDIODE = VD x 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. Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky diode with a low forward voltage drop. Another significant external power loss is the conduction loss in the output inductor. The equation can be simplified to: 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 VOUT and the FB pin. A good value for R2 is 2kΩ. www.national.com PIND = IOUT2 x RDCR The LM2833 conduction loss is mainly associated with the internal power switch: 14 When planning layout there are a few things to consider to achieve a clean, regulated output. The most important consideration is the close coupling of the GND connections of the input capacitor C1 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. The next consideration is the location of the GND connection of the output capacitor C2, which should be near the GND connections of C1 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 signal ground SGND (pin 3) and power ground PGND (pin 6) should be tied together and connected to ground plane through vias. The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup that causes inaccurate regulation. The feedback resistors should be placed as close as possible to the IC, with the GND of R2 placed as close as possible to the SGND of the IC. The VOUT trace to R1 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. Radiated noise can be decreased by choosing a 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 LM2833 demo board as an example of a four-layer layout. If the inductor ripple current is fairly small, the conduction losses can be simplified to: PCOND = IOUT2 x RDS(ON) x D Switching losses are also associated with the internal power 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 measuring the rise and fall times (10% to 90%) of the switch at the switch node. Switching Power Loss is calculated as follows: PSWR = 0.5 x (VIN x IOUT x fSW x TRISE) PSWF = 0.5 x (VIN x IOUT x fSW x TFALL) PSW = PSWR + PSWF The power loss required for operation of the internal circuitry is given by: PQ = IQ x VIN IQ is the quiescent operating current, and is typically around 3.2mA for the LM2833X, and 4.3mA for the LM2833Z. An example of efficiency calculation for a typical application is shown in Table 1: TABLE 1. Power Loss Tabulation Conditions Power loss VIN 5V VOUT 3.3V IOUT 3.0A POUT 9.9W VD 0.33V PDIODE 277mW RDS(ON) 56mΩ PCOND 363mW fSW 1.5MHz TRISE 10ns TFALL 10ns PSW 225mW INDDCR 28mΩ PIND 252mW IQ 3.2mA PQ 16mW 89.7% η D is calculated to be 0.72 PLOSS = Σ ( PCOND + PSW + PQ + PIND + PDIODE ) PLOSS = 1.133W 15 www.national.com LM2833 PCB LAYOUT CONSIDERATIONS LM2833 LM2833X Design Example 1 30013207 FIGURE 11. LM2833X (1.5MHz): VIN = 3.3V, Output = 1.2V/3.0A Bill of Materials Part ID Part Value Manufacturer U1 3.0A Buck Regulator NSC Part Number LM2833X C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5R0J226M C2, Output Cap 47µF, 6.3V, X5R TDK C3216X5R0J476M C3, Bypass Cap 0.22µF, 10V, X7R Murata GRM216R71A224KC01D D1, Catch Diode Schottky, 0.33V at 3A, VR=30V Toshiba CMS01 www.national.com L1 1.8µH, 3.6A TDK LTF5022T-1R8N3R6 R1 2.0kΩ, 1% Vishay CRCW08052K00FKEA R2 2.0kΩ, 1% Vishay CRCW08052K00FKEA R3 10Ω, 1% Vishay CRCW080510R0FKEA 16 LM2833 LM2833X Design Example 2 30013267 FIGURE 12. LM2833X (1.5MHz): VIN = 5V, Output = 3.3V/3.0A Bill of Materials Part ID Part Value Manufacturer U1 3.0A Buck Regulator NSC Part Number LM2833X C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5R0J226M C2, Output Cap 47µF, 6.3V, X5R TDK C3216X5R0J476M C3, Bypass Cap 0.22µF, 10V, X7R Murata GRM216R71A224KC01D CFF, Feed-forward Cap 47nF, 10V, X7R AVX 0805ZC473JAZ2A D1, Catch Diode Schottky, 0.43V at 3A, VR=30V Vishay SSA33L-E3/61T L1 1.2µH, 4.2A TDK LTF5022T-1R2N4R2 R1 10.2kΩ, 1% Vishay CRCW080510K2FKEA R2 2.26kΩ, 1% Vishay CRCW08052K26FKEA R3 10Ω, 1% Vishay CRCW080510R0FKEA 17 www.national.com LM2833 LM2833Z Design Example 3 30013208 FIGURE 13. LM2833Z (3MHz): VIN = 3.3V, Output = 1.2V/3.0A Bill of Materials Part ID Part Value Manufacturer U1 3.0A Buck Regulator NSC Part Number LM2833Z C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5R0J226M C2, Output Cap 47µF, 6.3V, X5R TDK C3216X5R0J476M C3, Bypass Cap 0.22µF, 10V, X7R Murata GRM216R71A224KC01D D1, Catch Diode Schottky, 0.33V at 3A, VR=30V Toshiba CMS01 www.national.com L1 1.0µH, 4.0A Taiyo Yuden NP04SZB1R0N R1 2.0kΩ, 1% Vishay CRCW08052K00FKEA R2 2.0kΩ, 1% Vishay CRCW08052K00FKEA R3 10Ω, 1% Vishay CRCW080510R0FKEA 18 LM2833 LM2833Z Design Example 4 30013268 FIGURE 14. LM2833Z (3MHz): VIN = 5V, Output = 3.3V/3.0A Bill of Materials Part ID Part Value Manufacturer U1 3.0A Buck Regulator NSC Part Number LM2833Z C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5R0J226M C2, Output Cap 47µF, 6.3V, X5R TDK C3216X5R0J476M C3, Bypass Cap 0.22µF, 10V, X7R Murata GRM216R71A224KC01D CFF, Feed-forward Cap 47nF, 10V, X7R AVX 0805ZC473JAZ2A D1, Catch Diode Schottky, 0.43V at 3A, VR=30V Vishay SSA33L-E3/61T L1 1.0µH, 4.0A Taiyo Yuden NP04SZB1R0N R1 10.2kΩ, 1% Vishay CRCW080510K2FKEA R2 2.26kΩ, 1% Vishay CRCW08052K26FKEA R3 10Ω, 1% Vishay CRCW080510R0FKEA 19 www.national.com LM2833 Physical Dimensions inches (millimeters) unless otherwise noted 10-Lead eMSOP Package NS Package Number MUC10A 10-Lead LLP Package NS Package Number SDA10A www.national.com 20 LM2833 Notes 21 www.national.com LM2833 1.5MHz/3MHz 3.0A Step-Down DC-DC Switching Regulator Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy PowerWise® Solutions www.national.com/powerwise Solutions www.national.com/solutions Serial Digital Interface (SDI) www.national.com/sdi Mil/Aero www.national.com/milaero Temperature Sensors www.national.com/tempsensors Solar Magic® www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless Analog University® www.national.com/AU THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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