LMR24220TLX

LMR24220 SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage Regulator in micro SMD October 5, 2011 LMR24220 SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage Regulator in micro SMD Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Input voltage range of 4.5V to 42V Output voltage range of 0.8V to 24V Output current up to 2.0A Integrated low RDS(ON) synchronous MOSFETs for high efficiency Up to 1 MHz switching frequency Low shutdown Iq, 25 µA typical Programmable soft-start No loop compensation required COT architecture with ERM 28 bump micro SMD (2.45 x 3.64 x 0.60 mm) packaging Fully enabled for WEBENCH®™ Power Designer System Performance Efficiency vs Load Current (VOUT = 3.3V) 100 90 EFFICIENCY (%) 80 70 60 50 40 0.0 VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V 0.4 0.8 1.2 1.6 LOAD CURRENT (A) 2.0 30167670 Performance Benefits ■ Tiny overall solution reduces system cost ■ Integrated synchronous MOSFETs provides high efficiency at low output voltages ■ COT with ERM architecture requires no loop compensation, reduces component count, and provides ultra fast transient response ■ Stable with low ESR capacitors VOUT Regulation vs Load Current (VOUT = 3.3V) 0.8 0.6 0.4 ΔVOUT (%) 0.2 0.0 -0.2 -0.4 -0.6 -0.8 0.0 0.4 0.8 1.2 1.6 LOAD CURRENT (A) 2.0 30167671 Applications ■ ■ ■ ■ Point-of-Load Conversions from 5V, 12V and 24V Rails Space Constrained Applications Industrial Distributed Power Applications Power Meters VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation © 2011 National Semiconductor Corporation 301676 www.national.com LMR24220 Typical Application 30167601 Connection Diagram 30167669 28–ball micro SMD — Balls Facing Down NS Package Number TLC28VFA Ordering Information Order Number LMR24220TL LMR24220TLX Package Type 28–ball micro SMD 28–ball micro SMD NSC Package Drawing TLC28VFA TLC28VFA Supplied As 250 Units on Tape and Reel 1000 Units on Tape and Reel www.national.com 2 LMR24220 Pin Descriptions Ball A2, A3, B2, B3, C2, C3, D2, D3, D4 A4, B4 C4 Name SW Description Switching Node Application Information Internally connected to the source of the main MOSFET and the drain of the Synchronous MOSFET. Connect to the inductor. Supply pin to the device. Nominal input range is 4.5V to 42V. Connect a 33 nF capacitor from the SW pin to this pin. An internal diode charges the capacitor during the main MOSFET off-time. Ground for all internal circuitry other than the PGND pin. An 8 µA internal current source charges an external capacitor to provide the soft- start function. Internally connected to the regulation and over-voltage comparators. The regulation setting is 0.8V at this pin. Connect to feedback resistors. Connect a voltage higher than 1.26V to enable the regulator. Leaving this input open circuit will enable the device at internal UVLO level. An external resistor from the VIN pin to this pin sets the main MOSFET on-time. Nominally regulated to 6V. Connect a capacitor of not less than 680 nF between the VCC and AGND pins for stable operation. Synchronous MOSFET source connection. Tie to a ground plane. VIN BST Input supply voltage Connection for bootstrap capacitor E3, E4, F1, F2, F3, G3 G2 G1 AGND SS FB Analog Ground Soft-start Feedback G4 EN Enable F4 E1, E2 RON VCC On-time Control Start-up regulator Output A1, B1, C1, D1 GND Power Ground 3 www.national.com LMR24220 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, RON to AGND SW to AGND SW to AGND (Transient) VIN to SW BST to SW All Other Inputs to AGND ESD Rating (Note 2) Human Body Model -0.3V to 43.5V -0.3V to 43.5V -2V (< 100ns) -0.3V to 43.5V -0.3V to 7V -0.3V to 7V ±2kV Storage Temperature Range Junction Temperature (TJ) -65°C to +150°C 150°C (Note 1) 4.5V to 42V −40°C to +125°C Operating Ratings Supply Voltage Range (VIN) Junction Temperature Range (TJ) Thermal Resistance (θJA) 28 ball μSMD(Note 5) 50°C/W For soldering specifications: see product folder at www.national.com and www.national.com/ms/MS/ MSSOLDERING. pdf Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated the following conditions apply: VIN = 18V, VOUT = 3.3V.(Note 3) Symbol Start-Up Regulator, VCC VCC VIN - VCC IVCCL VCC-UVLO VCC-UVLO-HYS tVCC-UVLO-D IIN IIN-SD RDS-UP-ON RDS- DN-ON VG-UVLO Soft-start ISS Current Limit ICL ON/OFF Timer ton ton-MIN toff Enable Input VEN VEN-HYS VFB VFB-OV IFB EN Pin input threshold Enable threshold hysteresis In-regulation feedback voltage Feedback over-voltage threshold FB pin current VEN rising VEN falling VSS ≥ 0.8V TJ = −40°C to +125°C 0.784 0.888 1.13 1.18 90 0.8 0.920 5 0.816 0.945 1.23 V mV V V nA ON timer pulse width VIN = 10V, RON = 100 kΩ VIN = 30V, RON = 100 kΩ ON timer minimum pulse width OFF timer pulse width 1.38 0.47 150 260 ns ns µs Syn. MOSFET current limit threshold LMR24220 2.156 2.8 3.4 A SS pin source current VSS = 0.5V 11 µA VCC output voltage VIN - VCC dropout voltage VCC current limit (Note 4) VCC under-voltage lockout threshold (UVLO) VCC UVLO hysteresis VCC UVLO filter delay IIN operating current No switching, VFB = 1V IIN operating current, Device shutdown VEN = 0V Main MOSFET RDS(on) Syn. MOSFET RDS(on) Gate drive voltage UVLO VBST - VSW increasing CCC = 680nF, no load ICC = 20mA VCC = 0V VIN increasing VIN decreasing – μSMD package 40 3.55 5.0 6.0 350 65 3.75 150 3 0.7 25 0.18 0.11 3.3 1 40 0.375 0.225 4.2 3.95 7.2 V mV mA V mV µs mA µA Ω Ω V Parameter Conditions Min Typ Max Units Switching Characteristics Regulation and Over-Voltage Comparator www.national.com 4 LMR24220 Symbol Thermal Shutdown TSD TSD-HYS Parameter Thermal shutdown temperature Thermal shutdown temperature hysteresis Conditions TJ rising TJ falling Min Typ 165 20 Max Units °C °C Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National's Average Outgoing Quality Level (AOQL). Note 4: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading. Note 5: θJA calculations were performed in general accordance with JEDEC standards JESD51–1 to JESD51–11. 5 www.national.com LMR24220 Typical Performance Characteristics Unless otherwise speficified all curves are taken at VIN = 18V with the configuration in the typical application circuit for VOUT = 3.3V (Figure 8) TA = 25°C. VCC vs ICC VCC vs VIN 30167604 30167605 ton vs VIN Switching Frequency, fSW vs VIN, VOUT=0.8V, 700 SWITCHING FREQENCY (kHZ) 600 500 400 300 200 100 0 0 30167606 Ron = 12.4kΩ; L = 2.2μH, Io = 0.5A Ron = 12.4kΩ; L = 2.2μH, Io = 2A Ron = 49.9kΩ; L = 3.3μH, Io = 0.5A Ron = 49.9kΩ; L = 3.3μH, Io = 2A 10 20 30 VIN (v) 40 50 30167675 VFB vs Temperature RDS(on) vs Temperature 30167608 30167609 www.national.com 6 LMR24220 Efficiency vs Load Current (VOUT = 3.3V) 100 90 EFFICIENCY (%) 80 70 60 50 40 0.0 VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V 0.4 0.8 1.2 1.6 LOAD CURRENT (A) 2.0 30167670 VOUT Regulation vs Load Current (VOUT = 3.3V) 0.8 0.6 0.4 ΔVOUT (%) 0.2 0.0 -0.2 -0.4 -0.6 -0.8 0.0 0.4 0.8 1.2 1.6 LOAD CURRENT (A) 2.0 30167671 VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V Efficiency vs Load Current (VOUT = 0.8V) 100 90 EFFICIENCY (%) 80 70 60 50 40 0.0 VIN = 4.5V VIN = 9V VIN = 12v VIN = 24V VIN = 42v 0.4 0.8 1.2 1.6 LOAD CURRENT (A) 2.0 30167672 VOUT Regulation vs Load Current (VOUT = 0.8V) 0.6 0.5 0.4 ΔVOUT (%) 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.0 0.4 0.8 1.2 1.6 LOAD CURRENT (A) 2.0 30167673 VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V Power Up (VOUT = 3.3V, 2A Loaded) Startup with Enable (VOUT = 3.3V, 2A Loaded) 30167676 30167614 7 www.national.com LMR24220 Shutdown Transient (VOUT = 3.3V, 2A Loaded) Continuous Mode Operation (VOUT = 3.3V, 2A Loaded) 30167615 30167616 Discontinuous Mode Operation (VOUT = 3.3V, 0.050A Loaded) DCM to CCM Transition (VOUT = 3.3V, 0.50A - 2A Load) 30167617 30167618 Load Transient (VOUT = 3.3V, 0.20A - 2A Load,) 30167619 www.national.com 8 LMR24220 Simplified Functional Block Diagram 30167620 9 www.national.com LMR24220 Functional Description The LMR24220 Step Down Switching Regulator features all required functions to implement a cost effective, efficient buck power converter capable of supplying 2A to a load. It contains Dual N-Channel main and synchronous MOSFETs. The Constant ON-Time (COT) regulation scheme requires no loop compensation, results in fast load transient response and simple circuit implementation. The regulator can function properly even with an all ceramic output capacitor network, and does not rely on the output capacitor’s ESR for stability. The operating frequency remains constant with line variations due to the inverse relationship between the input voltage and the on-time. The valley current limit detection circuit, with the limit set internally at 2.8A, inhibits the main MOSFET until the inductor current level subsides. The LMR24220 can be applied in numerous applications and can operate efficiently for inputs as high as 42V. Protection features include output over-voltage protection, thermal shutdown, VCC under-voltage lock-out and gate drive under-voltage lock-out. The LMR24220 is available in a small micro SMD chip scale package. VOUT = 0.8V x (RFB1 + RFB2)/RFB2 (3) Startup Regulator (VCC) A startup regulator is integrated within the LMR24220. The input pin VIN can be connected directly to a line voltage up to 42V. The VCC output regulates at 6V, and is current limited to 65 mA. Upon power up, the regulator sources current into an external capacitor CVCC, which is connected to the VCC pin. For stability, CVCC must be at least 680 nF. When the voltage on the VCC pin is higher than the under-voltage lock-out (UVLO) threshold of 3.75V, the main MOSFET is enabled and the SS pin is released to allow the soft-start capacitor CSS to charge. The minimum input voltage is determined by the dropout voltage of the regulator and the VCC UVLO falling threshold (≊3.7V). If VIN is less than ≊4.0V, the regulator shuts off and VCC goes to zero. Regulation Comparator The feedback voltage at the FB pin is compared to a 0.8V internal reference. In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at the FB pin falls below 0.8V. The main MOSFET stays on for the on-time, causing the output voltage and consequently the voltage of the FB pin to rise above 0.8V. After the on-time period, the main MOSFET stays off until the voltage of the FB pin falls below 0.8V again. Bias current at the FB pin is nominally 5 nA. COT Control Circuit Overview COT control is based on a comparator and a one-shot ontimer, with the output voltage feedback (feeding to the FB pin) compared with an internal reference of 0.8V. If the voltage of the FB pin is below the reference, the main MOSFET is turned on for a fixed on-time determined by a programming resistor RON and the input voltage VIN, upon which the on-time varies inversely. Following the on-time, the main MOSFET remains off for a minimum of 260 ns. Then, if the voltage of the FB pin is below the reference, the main MOSFET is turned on again for another on-time period. The switching will continue to achieve regulation. The regulator will operate in the discontinuous conduction mode (DCM) at a light load, and the continuous conduction mode (CCM) with a heavy load. In the DCM, the current through the inductor starts at zero and ramps up to a peak during the on-time, and then ramps back to zero before the end of the off-time. It remains zero and the load current is supplied entirely by the output capacitor. The next on-time period starts when the voltage at the FB pin falls below the internal reference. The operating frequency in the DCM is lower and varies larger with the load current as compared with the CCM. Conversion efficiency is maintained since conduction loss and switching loss are reduced with the reduction in the load and the switching frequency respectively. The operating frequency in the DCM can be calculated approximately as follows: Zero Coil Current Detect The current of the synchronous MOSFET is monitored by a zero coil current detection circuit which inhibits the synchronous MOSFET when its current reaches zero until the next on-time. This circuit enables the DCM operation, which improves the efficiency at a light load. Over-Voltage Comparator The voltage at the FB pin is compared to a 0.92V internal reference. If it rises above 0.92V, the on-time is immediately terminated. This condition is known as over-voltage protection (OVP). It can occur if the input voltage or the output load changes suddenly. Once the OVP is activated, the main MOSFET remains off until the voltage at the FB pin falls below 0.92V. The synchronous MOSFET will stay on to discharge the inductor until the inductor current reduces to zero, and then switches off. ON-Time Timer, Shutdown (1) In the continuous conduction mode (CCM), the current flows through the inductor in the entire switching cycle, and never reaches zero during the off-time. The operating frequency remains relatively constant with load and line variations. The CCM operating frequency can be calculated approximately as follows: The on-time of the LMR24220 main MOSFET is determined by the resistor RON and the input voltage VIN. It is calculated as follows: (4) The inverse relationship of ton and VIN gives a nearly constant frequency as VIN is varied. RON should be selected such that the on-time at maximum VIN is greater than 150 ns. The ontimer has a limiter to ensure a minimum of 150 ns for ton. This limits the maximum operating frequency, which is governed by the following equation: (2) The output voltage is set by two external resistors RFB1 and RFB2. The regulated output voltage is www.national.com 10 LMR24220 (5) The LMR24220 can be remotely shutdown by pulling the voltage of the EN pin below 1V. In this shutdown mode, the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the EN pin allows normal operation to resume because the EN pin is internally pulled up. the main MOSFET is turned off, the inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current exceeds 2.8A, the current limit comparator toggles, and as a result disabling the start of the next on-time period. The next switching cycle starts when the recirculating current falls back below 2.8A (and the voltage at the FB pin is below 0.8V). The inductor current is monitored during the on-time of the synchronous MOSFET. As long as the inductor current exceeds 2.8A, the main MOSFET will remain inhibited to achieve current limit. The operating frequency is lower during current limit due to a longer off-time. Figure 2 illustrates an inductor current waveform. On average, the output current IOUT is the same as the inductor current IL, which is the average of the rippled inductor current. In case of current limit (the current limit portion of Figure 2), the next on-time will not initiate until the current drops below 2.8 (assume the voltage at the FB pin is lower than 0.8V). During each on-time the current ramps up an amount equal to: 30167625 FIGURE 1. Shutdown Implementation (6) During current limit, the LMR24220 operates in a constant current mode with an average output current IOUT(CL) equal to 2.8A + ILR / 2. However, due to thermal limitations, the device may not support load currents greater than 2A for extended periods. Current Limit Current limit detection is carried out during the off-time by monitoring the re-circulating current through the synchronous MOSFET. Referring to the Functional Block Diagram, when 30167626 FIGURE 2. Inductor Current - Current Limit Operation 11 www.national.com LMR24220 N-Channel MOSFET and Driver The LMR24220 integrates an N-Channel main MOSFET and an associated floating high voltage main MOSFET gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor CBST and an internal high voltage diode. CBST connecting between the BST and SW pins powers the main MOSFET gate driver during the main MOSFET on-time. During each off-time, the voltage of the SW pin falls to approximately -1V, and CBST charges from VCC through the internal diode. The minimum off-time of 260 ns provides enough time for charging CBST in each cycle. Thermal Protection The junction temperature of the LMR24220 should not exceed the maximum limit. Thermal protection is implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin. Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation resumes. Soft-Start The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an 8 µA internal current source charges up an external capacitor CSS connecting to the SS pin. The ramping voltage at the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage VOUT in a controlled manner. The soft start time duration to reach steady state operation is given by the formula: tSS=VREFx CSS / 8µA = 0.8V x CSS / 8µA This equation can be rearranged as follows: CSS= tSSx 8µA / 0.8V Use of a 4.7nF capacitor results in a 0.5ms soft-start duration. This is a recommended value. Note that high values of CSS capacitance will cause more output voltage drop when a load transient goes across the DCM-CCM boundary. If a fast load transient response is desired for steps between DCM and CCM mode the softstart capacitor value should be less than 18nF (which corresponds to a soft-start time of 1.8ms). An internal switch grounds the SS pin if any of the following three cases happens: (i) VCC is below the under-voltage lockout threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in Figure 3. Thermal Derating Temperature rise increases with frequency, load current, input voltage and smaller board dimensions. On a typical board, the LMR24220 is capable of supplying 2A below an ambient temperature of 50°C under worst case operation with input voltage of 42V. Figure 4 shows a thermal derating curve for the output current without thermal shutdown against ambient temperature up to 125°C. Obtaining 2A output current is possible at higher temperature by increasing the PCB ground plane area, adding air flow or reducing the input voltage or operating frequency 2.4 2.0 MAXIMUM IOUT (A) 1.6 1.2 0.8 0.4 0.0 0 25 50 75 100 AMBIENT TEMPERATURE (°C) 125 30167674 FIGURE 4. Thermal Derating Curve, θJA=40°C/W, Vo = 3.3V, fs = 500kHz (tested on the evaluation board) 30167627 FIGURE 3. Alternate Shutdown Implementation www.national.com 12 LMR24220 Applications Information EXTERNAL COMPONENTS The following guidelines can be used to select external components. RFB1 and RFB2 : These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ, satisfying the following ratio: RFB1/RFB2 = (VOUT/0.8V) - 1 (7) the saturation current of the inductor and current limits of the main and synchronous MOSFETs. Also, the lower peak of ILR must be positive if CCM operation is required. For VOUT = 0.8V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than 20 µA. This is needed because the converter operation needs a minimum inductor current ripple to maintain good regulation when no load is connected. RON: Equation (2) can be used to select RON if a desired operating frequency is selected. But the minimum value of RON is determined by the minimum on-time. It can be calculated as follows: 30167631 (8) If RON calculated from (2) is smaller than the minimum value determined in (8), a lower frequency should be selected to recalculate RON by (2). Alternatively, VIN(MAX) can also be limited in order to keep the frequency unchanged. The relationship of VIN(MAX) and RON is shown in Figure 5. On the other hand, the minimum off-time of 260 ns can limit the maximum duty ratio. FIGURE 6. Inductor selection for VOUT = 3.3V 30167632 FIGURE 7. Inductor selection for VOUT = 0.8V Figure 6 and Figure 7 show curves on inductor selection for various VOUT and RON. For small RON, according to (8), VIN is limited. Some curves are therefore limited as shown in the figures. CVCC: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false triggering of the VCC UVLO at the main MOSFET on/off transitions. CVCC should be no smaller than 680 nF for stability, and should be a good quality, low ESR, ceramic capacitor. COUT and COUT3: COUT should generally be no smaller than 10 µF. Experimentation is usually necessary to determine the minimum value for COUT, as the nature of the load may require a larger value. A load which creates significant transients requires a larger COUT than a fixed load. COUT3 is a small value ceramic capacitor located close to the LMR24220 to further suppress high frequency noise at VOUT. A 100 nF capacitor is recommended. CIN and CIN3: The function of CIN is to supply most of the main MOSFET current during the on-time, and limit the voltage rip- 30167629 FIGURE 5. Maximum VIN for selected RON L: The main parameter affected by the inductor is the amplitude of inductor current ripple (ILR). Once ILR is selected, L can be determined by: (9) where VIN is the maximum input voltage and fSW is determined from (2). If the output current IOUT is determined, by assuming that IOUT = IL, the higher and lower peak of ILR can be determined. Beware that the higher peak of ILR should not be larger than 13 www.national.com LMR24220 ple at the VIN pin, assuming that the voltage source connecting to the VIN pin has finite output impedance. If the voltage source’s dynamic impedance is high (effectively a current source), CIN supplies the average input current, but not the ripple current. At the maximum load current, when the main MOSFET turns on, the current to the VIN pin suddenly increases from zero to the lower peak of the inductor’s ripple current and ramps up to the higher peak value. It then drops to zero at turn-off. The average current during the on-time is the load current. For a worst case calculation, CIN must be capable of supplying this average load current during the maximum on-time. CIN is calculated from: (10) where IOUT is the load current, ton is the maximum on-time, and ΔVIN is the allowable ripple voltage at VIN. CIN3’s purpose is to help avoid transients and ringing due to long lead inductance at the VIN pin. A low ESR 0.1 µF ceramic chip capacitor located close to the LMR24220 is recommended. CBST: A 33 nF high quality ceramic capacitor with low ESR is recommended for CBST since it supplies a surge current to charge the main MOSFET gate driver at turn-on. Low ESR also helps ensure a complete recharge during each off-time. CSS: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the regulation comparator and the output voltage to reach their final value. The time is determined from the following equation: PC BOARD LAYOUT The LMR24220 regulation, over-voltage, and current limit comparators are very fast and may respond to short duration noise pulses. Layout is therefore critical for optimum performance. It must be as neat and compact as possible, and all external components must be as close to their associated pins of the LMR24220 as possible. Refer to the functional block diagram, the loop formed by CIN, the main and synchronous MOSFET internal to the LMR24220, and the PGND pin should be as small as possible. The connection from the PGND pin to CIN should be as short and direct as possible. Vias should be added to connect the ground of CIN to a ground plane, located as close to the capacitor as possible. The bootstrap capacitor CBST should be connected as close to the SW and BST pins as possible, and the connecting traces should be thick. The feedback resistors and capacitor RFB1, RFB2, and CFB should be close to the FB pin. A long trace running from VOUT to RFB1 is generally acceptable since this is a low impedance node. Ground RFB2 directly to the AGND pin. The output capacitor COUT should be connected close to the load and tied directly to the ground plane. The inductor L should be connected close to the SW pin with as short a trace as possible to reduce the potential for EMI (electromagnetic interference) generation. If it is expected that the internal dissipation of the LMR24220 will produce excessive junction temperature during normal operation, making good use of the PC board’s ground plane can help considerably to dissipate heat. Additionally the use of thick traces, where possible, can help conduct heat away from the LMR24220. Judicious positioning of the PC board within the end product, along with the use of any available air flow (forced or natural convection) can help reduce the junction temperature. Package Considerations The die has exposed edges and can be sensitive to ambient light. For applications with direct high intensitiy ambient red, infrared, LED or natural light it is recommended to have the device shielded from the light source to avoid abnormal behavior. (11) CFB: If the output voltage is higher than 1.6V, CFB is needed in the Discontinuous Conduction Mode to reduce the output ripple. The recommended value for CFB is 10 nF. 30167635 FIGURE 8. Typical Application Schematic for VOUT = 3.3V www.national.com 14 LMR24220 30167636 FIGURE 9. Typical Application Schematic for VOUT = 0.8V 15 www.national.com LMR24220 Physical Dimensions inches (millimeters) unless otherwise noted 28–Ball μSMD NS Package Number TLC28VFA X1 = 2449 +/- 30 µm X2 = 3643 +/- 30 µm X3 = 600 +/- 75µm www.national.com 16 LMR24220 Notes 17 www.national.com LMR24220 SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage Regulator in micro SMD Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc 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/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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