LMR24210 LMR24210 SIMPLE SWITCHER® 42Vin, 1.0A Step-Down Voltage Regulator in micro SMD Literature Number: SNVS738B LMR24210 SIMPLE SWITCHER® 42Vin, 1.0A Step-Down Voltage Regulator in micro SMD ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ System Performance Input voltage range of 4.5V to 42V Output voltage range of 0.8V to 24V Output current up to 1.0A 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 Efficiency vs Load Current (VOUT = 3.3V) 100 90 EFFICIENCY (%) Features 80 70 60 50 Performance Benefits 40 ■ Tiny overall solution reduces system cost ■ Integrated synchronous MOSFETs provides high 0.0 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 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 1.0 30173870 VOUT Regulation vs Load Current (VOUT = 3.3V) 0.20 Applications 0.15 Point-of-Load Conversions from 5V, 12V and 24V Rails Space Constrained Applications Industrial Distributed Power Applications Power Meters 0.10 ΔVOUT (%) ■ ■ ■ ■ VIN = 4.5V VIN = 9V VIN = 12v VIN = 24V VIN = 42v 0.05 0.00 -0.05 -0.10 -0.15 -0.20 0.0 VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 1.0 30173871 SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation © 2011 National Semiconductor Corporation 301738 www.national.com LMR24210 SIMPLE SWITCHER® 42Vin, 1.0A Step-Down Voltage Regulator in micro SMD October 17, 2011 LMR24210 Typical Application 30173801 Connection Diagram 30173869 28–ball micro SMD — Balls Facing Down NS Package Number TLC28VFA Ordering Information Order Number Package Type NSC Package Drawing Supplied As LMR24210TL 28–ball micro SMD TLC28VFA 250 Units on Tape and Reel LMR24210TLX 28–ball micro SMD TLC28VFA 1000 Units on Tape and Reel www.national.com 2 Ball Name Description Application Information A2, A3, B2, B3, C2, C3, D2, D3, D4 SW Switching Node Internally connected to the source of the main MOSFET and the drain of the Synchronous MOSFET. Connect to the inductor. A4, B4 VIN Input supply voltage Supply pin to the device. Nominal input range is 4.5V to 42V. C4 BST Connection for bootstrap capacitor Connect a 33 nF capacitor from the SW pin to this pin. An internal diode charges the capacitor during the main MOSFET off-time. E3, E4, F1, F2, F3, G3 AGND Analog Ground Ground for all internal circuitry other than the PGND pin. G2 SS Soft-start An 8 µA internal current source charges an external capacitor to provide the soft- start function. G1 FB Feedback Internally connected to the regulation and over-voltage comparators. The regulation setting is 0.8V at this pin. Connect to feedback resistors. G4 EN Enable Connect a voltage higher than 1.26V to enable the regulator. Leaving this input open circuit will enable the device at internal UVLO level. F4 RON On-time Control An external resistor from the VIN pin to this pin sets the main MOSFET on-time. E1, E2 VCC Start-up regulator Output Nominally regulated to 6V. Connect a capacitor of not less than 680 nF between the VCC and AGND pins for stable operation. A1, B1, C1, D1 GND Power Ground Synchronous MOSFET source connection. Tie to a ground plane. 3 www.national.com LMR24210 Pin Descriptions LMR24210 Storage Temperature Range Junction Temperature (TJ) 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 Operating Ratings -65°C to +150°C 150°C (Note 1) Supply Voltage Range (VIN) Junction Temperature Range (TJ) -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 4.5V to 42V −40°C to +125°C 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 ±2kV 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 Parameter Conditions Min Typ Max Units 5.0 6.0 7.2 V Start-Up Regulator, VCC VCC VCC output voltage CCC = 680nF, no load VIN - VCC VIN - VCC dropout voltage ICC = 20mA IVCCL VCC current limit (Note 4) VCC = 0V VCC under-voltage lockout threshold (UVLO) VIN increasing VCC-UVLO-HYS VCC UVLO hysteresis VIN decreasing – μSMD package 150 tVCC-UVLO-D VCC UVLO filter delay No switching, VFB = 1V 0.7 1 mA 25 40 µA VCC-UVLO IIN IIN-SD IIN operating current 350 mV 40 65 mA 3.55 3.75 3.95 mV 3 IIN operating current, Device shutdown VEN = 0V V µs Switching Characteristics RDS-UP-ON Main MOSFET RDS(on) 0.18 0.375 Ω RDS- DN-ON Syn. MOSFET RDS(on) 0.11 0.225 Ω 4 V VG-UVLO Gate drive voltage UVLO VBST - VSW increasing 3.3 SS pin source current VSS = 0.5V 11 Syn. MOSFET current limit threshold LMR24210 ON timer pulse width VIN = 10V, RON = 100 kΩ 1.38 VIN = 30V, RON = 100 kΩ 0.47 Soft-start ISS µA Current Limit ICL 1.2 1.8 2.6 A ON/OFF Timer ton ton-MIN toff µs ON timer minimum pulse width 150 ns OFF timer pulse width 260 ns Enable Input VEN VEN-HYS EN Pin input threshold VEN rising Enable threshold hysteresis VEN falling 1.13 1.18 1.23 90 V mV Regulation and Over-Voltage Comparator VFB VFB-OV IFB In-regulation feedback voltage VSS ≥ 0.8V TJ = −40°C to +125°C Feedback over-voltage threshold FB pin current 0.8 0.816 V 0.888 0.920 0.945 V 5 Thermal Shutdown www.national.com 0.784 4 nA Parameter Conditions Min Typ Max Units TSD Thermal shutdown temperature TJ rising 165 °C TSD-HYS Thermal shutdown temperature hysteresis TJ falling 20 °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 LMR24210 Symbol Unless otherwise specified 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 30173804 30173805 ton vs VIN Switching Frequency, fSW vs VIN, VOUT=0.8V, 1000 SWITCHING FREQENCY (kHZ) LMR24210 Typical Performance Characteristics Ron = 12.4kΩ; L = 3.3μH, Io = 0.4A Ron = 12.4kΩ; L = 3.3μH, Io = 1A Ron = 12.4kΩ; L = 8.2μH, Io = 0.4A Ron = 12.4kΩ; L = 8.2μH, Io = 1A Ron = 7.68kΩ; L = 3.3μH, Io = 0.4A Ron = 7.68kΩ; L = 3.3μH, Io = 1A Ron = 7.68kΩ; L = 8.2μH, Io = 0.4A Ron = 7.68kΩ; L = 8.2μH, Io = 1A 900 800 700 600 500 400 300 200 100 0 0 10 20 30 VIN (v) 30173806 50 30173875 VFB vs Temperature RDS(on) vs Temperature 30173808 www.national.com 40 30173809 6 VOUT Regulation vs Load Current (VOUT = 3.3V) 100 0.20 0.15 90 0.10 80 ΔVOUT (%) EFFICIENCY (%) LMR24210 Efficiency vs Load Current (VOUT = 3.3V) 70 0.05 0.00 -0.05 60 VIN = 4.5V VIN = 9V VIN = 12v VIN = 24V VIN = 42v 50 40 0.0 VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V -0.10 -0.15 -0.20 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 1.0 0.0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 1.0 30173870 30173871 Efficiency vs Load Current (VOUT = 0.8V) VOUT Regulation vs Load Current (VOUT = 0.8V) 100 1.0 90 0.6 0.4 80 ΔVOUT (%) EFFICIENCY (%) VIN = 4.5V VIN = 9V VIN = 12V VIN = 24V VIN = 42V 0.8 70 60 40 0.0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 0.0 -0.2 -0.4 VIN = 4.5V VIN = 9V VIN = 12v VIN = 24V VIN = 42v 50 0.2 -0.6 -0.8 -1.0 1.0 0.0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 30173872 1.0 30173873 Power Up (VOUT = 3.3V, 1A Loaded) Enable Transient (VOUT = 3.3V, 1A Loaded) 30173814 30173876 7 www.national.com LMR24210 Shutdown Transient (VOUT = 3.3V, 1A Loaded) Continuous Mode Operation (VOUT = 3.3V, 1A Loaded) 30173816 30173815 Discontinuous Mode Operation (VOUT = 3.3V, 0.050A Loaded) DCM to CCM Transition (VOUT = 3.3V, 0.50A - 1A Load) 30173817 30173818 Load Transient (VOUT = 3.3V, 0.20A - 1A Load,) 30173819 www.national.com 8 LMR24210 Simplified Functional Block Diagram 30173820 9 www.national.com LMR24210 Functional Description VOUT = 0.8V x (RFB1 + RFB2)/RFB2 The LMR24210 Step Down Switching Regulator features all required functions to implement a cost effective, efficient buck power converter capable of supplying 1A 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 1.8A, inhibits the main MOSFET until the inductor current level subsides. The LMR24210 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 LMR24210 is available in a small micro SMD chip scale package. (3) Startup Regulator (VCC) A startup regulator is integrated within the LMR24210. 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 The on-time of the LMR24210 main MOSFET is determined by the resistor RON and the input voltage VIN. It is calculated as follows: (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: (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 (5) The LMR24210 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. 30173825 FIGURE 1. Shutdown Implementation (6) Current Limit During current limit, the LMR24210 operates in a constant current mode with an average output current IOUT(CL) equal to 1.8A + ILR / 2. Current limit detection is carried out during the off-time by monitoring the re-circulating current through the synchronous 30173826 FIGURE 2. Inductor Current - Current Limit Operation 11 www.national.com LMR24210 MOSFET. Referring to the Functional Block Diagram, when 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 1.8A, 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 1.8A (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 1.8A, 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 1.8A (assume the voltage at the FB pin is lower than 0.8V). During each on-time the current ramps up an amount equal to: Thermal Protection The LMR24210 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. The junction temperature of the LMR24210 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 Thermal Derating 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= 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. Temperature rise increases with frequency, load current, input voltage and smaller board dimensions. On a typical board, the LMR24210 is capable of supplying 1A below an ambient temperature of 90°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 1A output current is possible at higher temperature by increasing the PCB ground plane area, adding airflow or reducing the input voltage or operating frequency. 1.2 1.0 MAXIMUM IOUT (A) LMR24210 N-Channel MOSFET and Driver 0.8 0.6 0.4 0.2 0.0 0 25 50 75 100 AMBIENT TEMPERATURE (°C) 125 30173874 FIGURE 4. Thermal Derating Curve, θJA=40°C/W, Vo = 3.3V, fs = 500kHz (tested on the evaluation board) 30173827 FIGURE 3. Alternate Shutdown Implementation www.national.com 12 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) 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: 30173831 FIGURE 6. Inductor selection for VOUT = 3.3V (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. 30173832 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 LMR24210 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- 30173829 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 LMR24210 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. Applications Information LMR24210 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: PC BOARD LAYOUT The LMR24210 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 LMR24210 as possible. Refer to the functional block diagram, the loop formed by CIN, the main and synchronous MOSFET internal to the LMR24210, 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 LMR24210 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 LMR24210. 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. (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 LMR24210 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: 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. 30173835 FIGURE 8. Typical Application Schematic for VOUT = 3.3V www.national.com 14 LMR24210 30173836 FIGURE 9. Typical Application Schematic for VOUT = 0.8V 15 www.national.com LMR24210 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 LMR24210 Notes 17 www.national.com LMR24210 SIMPLE SWITCHER® 42Vin, 1.0A 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 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 References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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