LP38502TJ-ADJ

LP38500/2-ADJ, LP38500A/2A-ADJ 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs May 19, 2008 LP38500/2-ADJ, LP38500A/2A-ADJ 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs General Description National's FlexCap LDO's feature unique compensation that allows the use of any type of output capacitor with no limits on minimum or maximum ESR. The LP38500/2 series of lowdropout linear regulators operates from a +2.7V to +5.5V input supply. These ultra low dropout linear regulators respond very quickly to step changes in load, which makes them suitable for low voltage microprocessor applications. Developed on a CMOS process, (utilizing a PMOS pass transistor), the LP38500/2 has low quiescent current that changes little with load current. Ground Pin Current: Typically 2 mA at 1.5A load current. Disable Mode: Typically 25 nA quiescent current when the Enable pin is pulled low. Simplified Compensation: Stable with any type of output capacitor, regardless of ESR. Precision Output: "A" grade versions available with 1.5% VADJ tolerance (25°C) and 3% over line, load and temperature. Features ■ FlexCap: Stable with ceramic, tantalum, or aluminum ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ capacitors Stable with 10 µF input/output capacitor Adjustable output voltage from 0.6V to 5V Low ground pin current 25 nA quiescent current in shutdown mode Guaranteed output current of 1.5A Available in TO-263, TO-263 THIN, and LLP-8 packages Guaranteed VADJ accuracy of ±1.5% @ 25°C (A Grade) Guaranteed accuracy of ±3.5% @ 25°C (STD) Over-Temperature and Over-Current protection −40°C to +125°C operating TJ range Enable pin (LP38502) Applications ■ ASIC Power Supplies In: Printers, Graphics Cards, DVD Players Set Top Boxes, Copiers, Routers ■ DSP and FPGA Power Supplies ■ SMPS Regulator ■ Conversion from 3.3V or 5V Rail Typical Application Circuit 30036119 © 2008 National Semiconductor Corporation 300361 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ Connection Diagrams for TO-263 (TS) Package 30036121 30036122 Top View (LP38500TS-ADJ) TO-263 Package Top View (LP38502TS-ADJ) TO-263 Package Connection Diagrams for TO-263 THIN (TJ) Package 30036163 30036162 Top View (LP38500TJ-ADJ, LP38500ATJ-ADJ) TO-263 THIN Package Top View (LP38502TJ-ADJ, LP38502ATJ-ADJ) TO-263 THIN Package Pin Descriptions for TO-263 (TS), TO-263 THIN (TJ) Packages Pin # Designation EN 1 N/C 2 3 4 5 IN GND OUT ADJ Function Enable (LP38502 only). Pull high to enable the output, low to disable the output. This pin has no internal bias and must be either tied to the input voltage, or actively driven. In the LP38500, this pin has no internal connections. It can be left floating or used for trace routing. Input Supply Ground Regulated Output Voltage Sets output voltage The DAP is used to remove heat from the device by conducting it to the copper clad area on the PCB which acts as the heatsink. The DAP is electrically connected to the backside of the die. The DAP must be connected to ground potential, but can not be used as the only ground connection. DAP DAP www.national.com 2 LP38500/2-ADJ, LP38500A/2A-ADJ Connection Diagrams for LLP-8 (SD) Package 30036159 30036160 Top View (LP38500SD-ADJ, LP38500ASD-ADJ) LLP-8 Package Top View (LP38502SD-ADJ, LP38502ASD-ADJ) LLP-8 Package Pin Descriptions for LLP-8 (SD) Package Pin # 1 Designation GND IN 2 EN 3, 4 5, 6, 7 8 IN OUT ADJ Function Ground Input Supply (LP38500 only). Input Supply pins share current and must be connected together on the PC Board. Enable (LP38502 only). Pull high to enable the output, low to disable the output. This pin has no internal bias and must be either tied to the input voltage, or actively driven. Input Supply. Input Supply pins share current and must be connected together on the PC Board. Regulated Output Voltage. Output pins share current and must be connected together on the PC Board. Sets output voltage The DAP is used to remove heat from the device by conducting it to a copper clad area on the PCB which acts as a heatsink. The DAP is electrically connected to the backside of the die. The DAP must be connected to ground potential, but can not be used as the only ground connection. DAP DAP Ordering Information TABLE 1. Package Marking and Ordering Information Output Voltage Order Number LP38500SDX-ADJ LP38500SD-ADJ LP38502SDX-ADJ ADJ LP38502SD-ADJ LP38500ASDX-ADJ LP38500ASD-ADJ LP38502ASDX-ADJ LP38502ASD-ADJ LP38500TSX-ADJ ADJ LP38500TS-ADJ LP38502TSX-ADJ LP38502TS-ADJ LP38500TJ-ADJ ADJ LP38502TJ-ADJ LP38500ATJ-ADJ LP38502ATJ-ADJ TO-263 THIN TO-263 LLP-8 Package Type Package Marking LP38500SD-ADJ LP38500SD-ADJ LP38502SD-ADJ LP38502SD-ADJ LP38500ASD-ADJ LP38500ASD-ADJ LP38502ASD-ADJ LP38502ASD-ADJ LP38500TS-ADJ LP38500TS-ADJ LP38502TS-ADJ LP38502TS-ADJ LP38500TJ-ADJ LP38502TJ-ADJ LP38500ATJ-ADJ LP38502ATJ-ADJ Supplied As: Tape and Reel of 4500 Units Tape and Reel of 1000 Units Tape and Reel of 4500 Units Tape and Reel of 1000 Units Tape and Reel of 4500 Units Tape and Reel of 1000 Units Tape and Reel of 4500 Units Tape and Reel of 1000 Units Tape and Reel of 500 Units Rail of 45 Units Tape and Reel of 500 Units Rail of 45 Units Tape and Reel of 1000 Units Tape and Reel of 1000 Units Tape and Reel of 1000 Units Tape and Reel of 1000 Units 3 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Storage Temperature Range Lead Temperature (Soldering, 5 sec.) ESD Rating (Note 2) Power Dissipation(Note 3) Input Pin Voltage (Survival) Enable Pin Voltage (Survival) Output Pin Voltage (Survival) IOUT (Survival) −65°C to +150°C 260°C ±2 kV Internally Limited −0.3V to +6.0V −0.3V to +6.0V −0.3V to +6.0V Internally Limited Operating Ratings Input Supply Voltage Enable Input Voltage Output Current (DC) Junction Temperature(Note 3) VOUT (Note 1) 2.7V to 5.5V 0.0V to 5.5V 0 to 1.5A −40°C to +125°C 0.6V to 5V Electrical Characteristics LP38500/2–ADJ Unless otherwise specified: VIN = 3.3V, IOUT = 10 mA, CIN = 10 µF, COUT = 10 µF, VEN = VIN, VOUT = 1.8V. 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 at TJ = 25°C, and are provided for reference purposes only. Symbol VADJ VADJ IADJ VDO ΔVOUT/ΔVIN Parameter Adjust Pin Voltage (Note 6) Adjust Pin Voltage (Note 6) "A" GRADE Adjust Pin Bias Current Dropout Voltage (Note 7) Output Voltage Line Regulation (Notes 4, 6) Output Voltage Load Regulation (Notes 5, 6) Conditions 2.7V ≤ VIN ≤ 5.5V 2.7V ≤ VIN ≤ 5.5V 2.7V ≤ VIN ≤ 5.5V IOUT = 1.5A 2.7V ≤ VIN ≤ 5.5V — 10 mA ≤ IOUT ≤ 1.5A 10 mA ≤ IOUT ≤ 1.5A Min 0.584 0.575 0.596 0.587 Typ 0.605 Max 0.626 0.635 0.614 0.623 750 275 375 — Units V 0.605 50 220 0.04 0.05 0.18 0.33 2 0.025 3.6 V nA mV %/V ΔVOUT/ΔIOUT IGND IDISABLED IOUT(PK) ISC VIH(EN) VIL(EN) td(off) 10 mA ≤ IOUT ≤ 1.5A — — — — 3.5 4.5 0.125 15 %/A Ground Pin Current In Normal 10 mA ≤ IOUT ≤ 1.5A Operation Mode Ground Pin Current Peak Output Current Short Circuit Current Enable Logic High Enable Logic Low Turn-off delay VEN < VIL(EN) VOUT ≥ VOUT(NOM) - 5% VOUT = 0V VOUT = ON VOUT = OFF Time from VEN < VIL(EN) to VOUT = OFF ILOAD = 1.5A Time from VEN >VIH(EN) to VOUT = ON ILOAD = 1.5A VEN = VIN VEN = 0V mA µA A A 2 1.4 — — 3.7 — — 25 — 0.65 — Enable Input (LP38502 Only) V µs — — — 25 1 0.1 — — — td(on) IIH(EN) IIL(EN) Turn-on delay Enable Pin High Current Enable Pin Low Current nA www.national.com 4 LP38500/2-ADJ, LP38500A/2A-ADJ Symbol AC Parameters Parameter Conditions VIN = 3.0V, IOUT = 1.5A f = 120Hz VIN = 3.0V, IOUT = 1.5A f = 1 kHz f = 120Hz, COUT = 10 µF CER BW = 100Hz – 100kHz COUT = 10 µF CER TJ rising TO-263, TO-263 THIN(Note 8) 1 sq. in. copper LLP-8 (Note 9) TO-263, TO-263 THIN LLP-8 Min Typ Max Units — — — — 58 56 1.0 100 — dB — — — µV/√Hz µV (rms) PSRR Ripple Rejection ρn(l/f) en Output Noise Density Output Noise Voltage Thermal Characteristics TSD ΔTSD θJ-A Thermal Shutdown Thermal Resistance Junction to Ambient Thermal Resistance Junction to Ambient Thermal Resistance Junction to Case — — — — — — 170 10 37 80 5 16 — — — °C/W — — — °C/W Thermal Shutdown Hysteresis TJ falling from TSD °C θJ-C 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 does not guarantee specific performance limits. For guaranteed specifications and 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: Operating junction temperature must be evaluated, and derated as needed, based on ambient temperature (TA), power dissipation (PD), maximum allowable operating junction temperature (TJ(MAX)), and package thermal resistance (θJA). See Application Information. Note 4: Output voltage line regulation is defined as the change in output voltage from the nominal value due to change in the voltage at the input. Note 5: Output voltage load regulation is defined as the change in output voltage from the nominal value due to change in the load current. Note 6: The line and load regulation specification contains only the typical number. However, the limits for line and load regulation are included in the adjust voltage tolerance specification. Note 7: Dropout voltage is defined as the minimum input to output differential voltage at which the output drops 2% below the nominal value. For any output voltage less than 2.5V, the minimum VIN operating voltage is the limiting factor. Note 8: The value of θJA for the TO-263 (TS) package and TO-263 THIN (TJ) package can range from approximately 30 to 60°C/W depending on the amount of PCB copper dedicated to heat transfer (See Application Information). Note 9: θJA for the LLP-8 package was measured using the LP38502SD-ADJ evaluation board (See Application Information). 5 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ Typical Performance Characteristics CIN = 10 µF, COUT = 10 µF, IOUT = 10 mA, VOUT = 1.8V Noise Density Unless otherwise specified: TJ = 25°C, VIN = 2.7V, VEN = VIN, Noise Density 30036110 30036114 IGND vs Load Current IGND(OFF) vs Temperature 30036111 30036112 VADJ vs Temperature Dropout Voltage vs Load Current 30036113 30036115 www.national.com 6 LP38500/2-ADJ, LP38500A/2A-ADJ VEN vs Temperature Turn-on Characteristics 30036116 30036117 Turn-on Time Turn-on Time 30036155 30036156 7 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ Block Diagrams (TO-263, TO-263 THIN) LP38500-ADJ TO-263 Block Diagram 30036150 LP38502-ADJ TO-263 Block Diagram 30036151 Block Diagrams (LLP-8) LP38500-ADJ LLP-8 Block Diagram 30036158 LP38502-ADJ LLP-8 Block Diagram 30036151 www.national.com 8 LP38500/2-ADJ, LP38500A/2A-ADJ Application Information EXTERNAL CAPACITORS The LP3850X requires that at least 10 µF (±20%) capacitors be used at the input and output pins located within one cm of the IC. Larger capacitors may be used without limit on size for both CIN and COUT. Capacitor tolerances such as temperature variation and voltage loading effects must be considered when selecting capacitors to ensure that they will provide the minimum required amount of capacitance under all operating conditions for the application. In general, ceramic capacitors are best for noise bypassing and transient response because of their ultra low ESR. It must be noted that if ceramics are used, only the types with X5R or X7R dielectric ratings should be used (never Z5U or Y5F). Capacitors which have the Z5U or Y5F characteristics will see a drop in capacitance of as much as 50% if their temperature increases from 25°C to 85°C. In addition, the capacitance drops significantly with applied voltage: a typical Z5U or Y5F capacitor can lose as much as 60% of it’s rated capacitance if only half of the rated voltage is applied to it. For these reasons, only X5R and X7R ceramics should be used. INPUT CAPACITOR All linear regulators can be affected by the source impedance of the voltage which is connected to the input. If the source impedance is too high, the reactive component of the source may affect the control loop’s phase margin. To ensure proper loop operation, the ESR of the capacitor used for CIN must not exceed 0.5 Ohms. Any good quality ceramic capacitor will meet this requirement, as well as many good quality tantalums. Aluminum electrolytic capacitors may also work, but can possibly have an ESR which increases significantly at cold temperatures. If the ESR of the input capacitor may exceed 0.5 Ohms, it is recommended that a 2.2 µF ceramic capacitor be used in parallel, as this will assure stable loop operation. OUTPUT CAPACITOR Any type of capacitor may be used for COUT, with no limitations on minimum or maximum ESR, as long as the minimum amount of capacitance is present. The amount of capacitance can be increased without limit. Increasing the size of COUT typically will give improved load transient response. SETTING THE OUTPUT VOLTAGE The output voltage of the LP38500/2-ADJ can be set to any value between 0.6V and 5V using two external resistors shown as R1 and R2 in Figure 1. 30036153 The value of R2 should always be less than or equal to 10 kΩ for good loop compensation. R1 can be selected for a given VOUT using the following formula: VOUT = VADJ (1 + R1/R2) + IADJ (R1) Where VADJ is the adjust pin voltage and IADJ is the bias current flowing into the adjust pin. STABILITY AND PHASE MARGIN Any regulator which operates using a feedback loop must be compensated in such a way as to ensure adequate phase margin, which is defined as the difference between the phase shift and -180 degrees at the frequency where the loop gain crosses unity (0 dB). For most LDO regulators, the ESR of the output capacitor is required to create a zero to add enough phase lead to ensure stable operation. The LP38500/2-ADJ has a unique internal compensation circuit which maintains phase margin regardless of the ESR of the output capacitor, so any type of capacitor may be used. Figure 2 shows the gain/phase plot of the LP38500/2-ADJ with an output of 1.2V, 10 µF ceramic output capacitor, delivering 1.5A of load current. It can be seen that the unity-gain crossover occurs at 150 kHz, and the phase margin is about 40° (which is very stable). FIGURE 2. Gain-Bandwidth Plot for 1.5A Load Figure 3 shows the gain and phase with no external load. In this case, the only load is provided by the gain setting resistors (about 12 kΩ total in this test). It is immediately obvious that the unity-gain frequency is significantly lower (dropping to about 500 Hz), at which point the phase margin is 125°. 30036161 FIGURE 1. 9 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ 30036157 30036154 FIGURE 3. Gain-Bandwidth Plot for No Load The reduction in unity-gain bandwidth as load current is reduced is normal for any LDO regulator using a P-FET or PNP pass transistor, because they have a pole in the loop gain function given by: FIGURE 4. Load Transient Response In cases where extremely fast load changes occur, the output capacitance may have to be increased. When selecting capacitors, it must be understood that the better performing ones usually cost the most. For fast changing loads, the internal parasitics of ESR (equivalent series resistance) and ESL (equivalent series inductance) degrade the capacitor’s ability to source current quickly to the load. The best capacitor types for transient performance are (in order): 1. Multilayer Ceramic: with the lowest values of ESR and ESL, they can have ESR values in the range of a few milli Ohms. Disadvantage: capacitance values above about 22 µF significantly increase in cost. 2. Low-ESR Aluminum Electrolytics: these are aluminum types (like OSCON) with a special electrolyte which provides extremely low ESR values, and are the closest to ceramic performance while still providing large amounts of capacitance. These are cheaper (by capacitance) than ceramic. 3. Solid tantalum: can provide several hundred µF of capacitance, transient performance is slightly worse than OSCON type capacitors, cheaper than ceramic in large values. 4. General purpose aluminum electrolytics: cheap and provide a lot of capacitance, but give the worst performance. In general, managing load transients is done by paralleling ceramic capacitance with a larger bulk capacitance. In this way, the ceramic can source current during the rapidly changing edge and the bulk capacitor can support the load current after the first initial spike in current. PRINTED CIRCUIT BOARD LAYOUT Good layout practices will minimize voltage error and prevent instability which can result from ground loops. The input and output capacitors should be directly connected to the IC pins with short traces that have no other current flowing in them (Kelvin connect). The best way to do this is to place the capacitors very near the IC and make connections directly to the IC pins via short traces on the top layer of the PCB. The regulator’s ground pin should be connected through vias to the internal or backside ground plane so that the regulator has a single point ground. The external resistors which set the output voltage must also be located very near the IC with all connections directly tied via short traces to the pins of the IC (Kelvin connect). Do not connect the resistive divider to the load point or DC error will be induced. 10 This illustrates how the pole goes to the highest frequency when RL is minimum value (maximum load current). In general, LDO’s have maximum bandwidth (and lowest phase margin) at full load current. In the case of the LP38500/2-ADJ, it can be seen that it has good phase margin even when using ceramic capacitors with ESR values of only a few milli Ohms. LOAD TRANSIENT RESPONSE Load transient response is defined as the change in regulated output voltage which occurs as a result of a change in load current. Many applications have loads which vary, and the control loop of the voltage regulator must adjust the current in the pass FET transistor in response to load current changes. For this reason, regulators with wider bandwidths often have better transient response. The LP38500/2-ADJ employs an internal feedforward design which makes the load transient response much faster than would be predicted simply by loop speed: this feedforward means any voltage changes appearing on the output are coupled through to the high-speed driver used to control the gate of the pass FET along a signal path using very fast FET devices. Because of this, the pass transistor’s current can change very quickly. Figure 3 shows the output voltage load transient which occurs on a 1.8V output when the load changes from 0.1A to 1.5A at an average slew rate of 0.5A/µs. As shown, the peak output voltage change from nominal is about 40 mV, which is about 2.2%. www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ RFI/EMI SUSCEPTIBILITY RFI (Radio Frequency Interference) and EMI (Electro-Magnetic Interference) can degrade any integrated circuit's performance because of the small dimensions of the geometries inside the device. In applications where circuit sources are present which generate signals with significant high frequency energy content (> 1 MHz), care must be taken to ensure that this does not affect the IC regulator. If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes from the output of a switching regulator), good ceramic bypass capacitors must be used at the input pin of the IC to reduce the amount of EMI conducted into the IC. If the LP38500/2-ADJ output is connected to a load which switches at high speed (such as a clock), the high-frequency current pulses required by the load must be supplied by the capacitors on the IC output. Since the bandwidth of the regulator loop is less than 300 kHz, the control circuitry cannot respond to load changes above that frequency. This means the effective output impedance of the IC at frequencies above 300 kHz is determined only by the output capacitor(s). Ceramic capacitors provide the best performance in this type of application. In applications where the load is switching at high speed, the output of the IC may need RF isolation from the load. In such cases, it is recommended that some inductance be placed between the output capacitor and the load, and good RF bypass capacitors be placed directly across the load. PCB layout is also critical in high noise environments, since RFI/ EMI is easily radiated directly into PC traces. Noisy circuitry should be isolated from "clean" circuits where possible, and grounded through a separate path. At MHz frequencies, ground planes begin to look inductive and RFI/EMI can cause ground bounce across the ground plane. In multi-layer PC Board applications, care should be taken in layout so that noisy power and ground planes do not radiate directly into adjacent layers which carry analog power and ground. OUTPUT NOISE Noise is specified in two ways: Spot Noise or Output noise density is the RMS sum of all noise sources, measured at the regulator output, at a specific frequency (measured with a 1Hz bandwidth). This type of noise is usually plotted on a curve as a function of frequency. Total output noise voltage or Broadband noise is the RMS sum of spot noise over a specified bandwidth, usually several decades of frequencies. Attention should be paid to the units of measurement. Spot noise is measured in units µV/√Hz or nV/√Hz and total output noise is measured in µV(rms). The primary source of noise in low-dropout regulators is the internal reference. In CMOS regulators, noise has a low frequency component and a high frequency component, which depend strongly on the silicon area and quiescent current. Noise can generally be reduced in two ways: increase the transistor area or increase the reference current. However, enlarging the transisitors will increase die size, and increasing the reference current means higher total supply current (ground pin current). SHORT-CIRCUIT PROTECTION The LP38500/2-ADJ contains internal current limiting which will reduce output current to a safe value if the output is overloaded or shorted. Depending upon the value of VIN, thermal limiting may also become active as the average power dissipated causes the die temperature to increase to the limit value 11 (about 170°C). The hysteresis of the thermal shutdown circuitry can result in a “cyclic” behavior on the output as the die temperature heats and cools. ENABLE OPERATION (LP38502-ADJ Only) The Enable pin (EN) must be actively terminated by either a 10 kΩ pull-up resistor to VIN, or a driver which actively pulls high and low (such as a CMOS rail to rail comparator). If active drive is used, the pull-up resistor is not required. This pin must be tied to VIN if not used (it must not be left floating). DROPOUT VOLTAGE The dropout voltage of a regulator is defined as the input-tooutput differential required by the regulator to keep the output voltage within 2% of the nominal value. For CMOS LDOs, the dropout voltage is the product of the load current and the RDS(on) of the internal MOSFET pass element. Since the output voltage is beginning to “drop out” of regulation when it drops by 2%, electrical performance of the device will be reduced compared to the values listed in the Electrical Characteristics table for some parameters (line and load regulation and PSRR would be affected). REVERSE CURRENT PATH The internal MOSFET pass element in the LP38500/2-ADJ has an inherent parasitic diode. During normal operation, the input voltage is higher than the output voltage and the parasitic diode is reverse biased. However, if the output is pulled above the input in an application, then current flows from the output to the input as the parasitic diode gets forward biased. The output can be pulled above the input as long as the current in the parasitic diode is limited to 200 mA continuous and 1A peak. The regulator output pin should not be taken below ground potential. If the LP38500/2-ADJ is used in a dual-supply system where the regulator load is returned to a negative supply, the output must be diode-clamped to ground. POWER DISSIPATION/HEATSINKING The maximum power dissipation (PD(MAX)) of the LP38500/2ADJ is limited by the maximum junction temperature of 125°C, along with the maximum ambient temperature (TA (MAX)) of the application, and the thermal resistance (θJA) of the package. Under all possible conditions, the junction temperature (TJ) must be within the range specified in the Operating Ratings. The total power dissipation of the device is given by: PD = ((VIN − VOUT) x IOUT) + (VIN x IGND) (1) where IGND is the operating ground current of the device (specified under Electrical Characteristics). The maximum allowable junction temperature rise (ΔTJ) depends on the maximum expected ambient temperature (TA(MAX)) of the application, and the maximum allowable junction temperature (TJ(MAX)): ΔTJ = TJ(MAX)− TA(MAX) (2) The maximum allowable value for junction to ambient Thermal Resistance, θJA, can be calculated using the formula: θJA = ΔTJ / PD(MAX) (3) www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ The LP38500/2-ADJ is available in the TO-263 and LLP-8 packages. The thermal resistance depends on the amount of copper area allocated to heat transfer. HEATSINKING TO-263 and TO-263 THIN PACKAGES The TO-263 package and TO-263 THIN package use the copper plane on the PCB as a heatsink. The DAP of the package is soldered to the copper plane for heat sinking. Figure 5 shows a typical curve for the θJA of the TO-263 package for different copper area sizes (the thermal performance of both TO-263 and TO-263 THIN are the same). The tests were done using a PCB with 1 ounce copper on top side only, with copper patterns which were square in shape. As shown in the figure, increasing the copper area beyond 1.5 square inch produces very little improvement. HEATSINKING LLP-8 PACKAGE The junction-to-ambient thermal resistance for the LLP-8 package is dependent on how much PCB copper is present to conduct heat away from the device. The LP38502SD-ADJ evaluation board (980600046-100) was tested and gave a result of about 80°C/W with a power dissipation of 1W and no external airflow. This evaluation board is a two layer board using two ounce copper, and the copper area on topside for heatsinking is approximately two square inches. Multiple vias under the DAP also thermally connect to the backside layer which has about three square inches of copper dedicated to heatsinking. Finite modeling of the LP38502SD-ADJ with a four layer board (JEDEC JESD51-7 and JESD51-5) with one thermal via directly under the DAP to the first copper plane predicts a θJA of 72°C/W. With four thermal vias directly under the DAP to the first copper plane, the modeling predicts a θJA of 50°C/W. Adding a dog-bone copper area with four additional thermal vias in the dog-bone area to the first copper plane can improve θJA to 45C°C/W. See Application Note AN-1520 A Guide to Board Layout for Best Thermal Resistance for Exposed Packages for additional thermal considerations for printed circuit board layouts. 30036152 FIGURE 5. θJA vs Copper Area for TO-263 Package www.national.com 12 LP38500/2-ADJ, LP38500A/2A-ADJ Physical Dimensions inches (millimeters) unless otherwise noted TO-263 5-Lead, Molded, Surface Mount Package NS Package Number TS5B TO-263 THIN 5-Lead, Molded, Surface Mount Package NS Package Number TJ5A 13 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ 8-Lead LLP Package NS Package Number SDA08C www.national.com 14 LP38500/2-ADJ, LP38500A/2A-ADJ Notes 15 www.national.com LP38500/2-ADJ, LP38500A/2A-ADJ 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs 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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