LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD General Description Features The LM2787 CMOS Negative Regulated Switched Capacitor Voltage Inverter delivers a very low noise adjustable output for an input voltage in the range of +2.7V to +5.5V. Four low cost capacitors are used in this circuit to provide up to 10mA of output current. The regulated output for the LM2787 is adjustable between −1.5V and −5.2V. The LM2787 operates at 260 kHz (typical) switching frequency to reduce output resistance and voltage ripple. With an operating current of only 400 µA (charge pump power efficiency greater than 90% with most loads) and 0.05 µA typical shutdown current, the LM2787 provides ideal performance for cellular phone power amplifier bias and other low current, low noise negative voltage needs. The device comes in a small 8-Bump micro SMD package. n n n n n Inverts and regulates the input supply voltage Small 8-Bump micro SMD package 91% typical charge pump power efficiency at 10mA Low output ripple Shutdown lowers Quiescent current to 0.05 µA (typical) Applications n n n n n Wireless Communication Systems Cellular Phone Power Amplifier Biasing Interface Power Supplies Handheld Instrumentation Laptop Computers and PDA’s Typical Application Circuit and Connection Diagram 10131325 8-Bump micro SMD (Top View) 10131302 © 2002 National Semiconductor Corporation DS101313 www.national.com LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD August 2002 LM2787 Ordering Information Device Order Number Package Number Package Marking* LM2787BP BPA08CCB S8 Tape and Reel (250 units/reel) LM2787BPX BPA08CCB S8 Tape and Reel (3000 units/reel) Supplies As Note: * The small physical size of the micro SMD package does not allow for the full part number marking. Devices will be marked with the designation shown in the column Package Marking. Pin Descriptions Pin No. Name A1 Cap+ B1 VIN C1 VOUT C2 VFB Feedback input. Connect VFB to an external resistor divider between VOUT and a positive adjust voltage VADJ (0≤VADJ≤VIN). DO NOT leave unconnected. C3 SD Active low, logic-level shutdown input. B3 VNEG Negative unregulated output voltage. A3 Cap− Negative terminal for C1. A2 GND Ground. www.national.com Function Positive terminal for C1. Positive power supply input. Regulated negative output voltage. 2 TJMAX (Note 3) (Note 1) θJA (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (VIN to GND or GND to OUT) 150˚C 220˚C/W Operating Input Voltage Range 2.7V to 5.5V Operating Output Current Range Operating Ambient + 5.8V Temp. Range (GND − 0.3V) to (VIN + 0.3V) SD VNEG and VOUT Continuous Output Current 10mA VOUT Short-Circuit Duration to GND (Note 2) 1 sec. Continuous Power Dissipation (TA = 25˚C) (Note 3) 600mW 0mA to 10mA −40˚C to 85˚C Operating Junction Temp. Range −40˚C to 110˚C Storage Temperature −65˚C to 150˚C Lead Temp. (Soldering, 10 sec.) 300˚C ESD Rating (Note 4) 2kV Electrical Characteristics Limits with standard typeface apply for TJ = 25˚C, and limits in boldface type apply over the full temperature range. Unless otherwise specified VIN = 3.6V, C1 = C2 = C3 = 1µF. Symbol Parameter Conditions IQ Supply Current ISD Shutdown Supply Current FSW Switching Frequency (Note 5) VIN = 3.6V ηPOWER Power Efficiency at VNEG IL = 3.6mA IL = 10mA TSTART Start Up time Min Open Circuit, No Load 140 Typ Max 400 950 µA 0.05 1 µA 260 450 94 91 120 Units kHz % 600 µs Output Resistance to VNEG (Note 6) 30 Ω VR Output Voltage Ripple (Note 7) IL =2.5mA, VOUT = −2.7V IL = 10mA, VOUT = −3.8V 1 mV VFB Feedback Pin Reference Voltage IL = 2.5mA (Note 8) Adjustable Output Voltage 5.5V ≥ VIN ≥ 2.7V, 2.5mA ≥ IL 5.5V ≥ VIN ≥ 3.0V, 10mA ≥ IL ≥ 0mA Load Regulation 0 to 10mA, VOUT = − 2.4V 5 mV/mA Line Regulation 5.5V ≥ VIN ≥ 2.7V, IL = 2.5mA 1 mV/V RNEG VOUT VIH Shutdown Pin Input Voltage 5.5V ≥ VIN ≥ 2.7V High VIL Shutdown Pin Input Voltage 5.5V ≥ VIN ≥ 2.7V Low −1.25 −1.20 −1.15 V V − (VIN −0.3V) − (VIN −1.2V) 2.4 V 0.8 V Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to VIN may damage the device and must be avoided. Also, for temperatures above TA = 85˚C, OUT must not be shorted to GND or VIN or device may be damaged. Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is limited by TJMAX (maximum junction temperature), TA (ambient temperature) and θJA (junction-to-ambient thermal resistance). The maximum power dissipation at any temperature is: PDissMAX = (TJMAX — TA)/θJA up to the value listed in the Absolute Maximum Ratings. Note 4: Rating is for the human body model, a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 5: The output switches operate at one half the oscillator frequency, fOSC = 2fSW. Note 6: Current drawn from VNEG pin decreases power efficiency and will increase output voltage ripple. Note 7: In the test circuit, capacitors C1, C2, and C3 are 1µF, 0.30Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, increase output voltage ripple, and reduce efficiency. Note 8: The feedback resistors R1 and R2 are 200kΩ resistors. 3 www.national.com LM2787 Absolute Maximum Ratings LM2787 10131326 FIGURE 1. Standard Application Circuit Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, VOUT = −2.5V. Output Voltage vs. Output Current Output Voltage vs. Input Voltage 10131305 10131306 Maximum VNEG Current vs. Input Voltage No Load Supply Current vs. Input Voltage 10131308 www.national.com 10131309 4 LM2787 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, VOUT = −2.5V. (Continued) Switching Frequency vs. Input Voltage VFB vs. Temperature 10131311 10131315 Start-up Time vs. Input Voltage Start-up from Shutdown (no load) 10131312 10131310 Output Ripple Output Noise Spectrum 10131313 10131324 5 www.national.com LM2787 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, VOUT = −2.5V. (Continued) Line Transient Response Load Transient Response 10131317 10131318 10131327 FIGURE 2. Functional Block Diagram www.national.com 6 The LM2787 is an inverting, regulated charge-pump power converter. It features low noise, small physical size, and is simple to use. It is an ideal solution for biasing GaAsFET devices such as power amplifier modules found in portable devices and cellular phones. A switched capacitor charge-pump circuit is used to invert the input voltage VIN to its corresponding negative value which is seen at VNEG. This voltage is regulated by a low dropout linear regulator at VOUT (Figure 2). The output voltage can be regulated anywhere from −1.5V to −5.2V and is determined by a pair of feedback resistors (see Setting the Output Voltage). The PSRR of the linear regulator reduces the output voltage ripple produced by the charge-pump inverter at the output VOUT. The regulator also attenuates noise from the incoming supply due to its high PSRR. In summation, larger value capacitors with lower ESR will give the lowest output noise and ripple. C1, C2, and C3 should be 1.0µF minimum with less than 0.3Ω ESR. Larger values may be used for any or all capacitors. All capacitors should be either ceramic, surface-mount chip tantalum, or polymer electrolytic. Shutdown The LM2787 features a logic-level shutdown feature. The function is active-low and will reduce the supply current to 0.05µA (typical) when engaged. When shutdown is active VOUT and VNEG are switched to ground. Output Noise and Ripple Low output noise and output voltage ripple are two of the attractive features of the LM2787. Because they are small, the noise and ripple can be hard to measure accurately. Ground loop error between the circuit and the oscilloscope caused by the switching of the charge-pump produces ground currents in the probe wires. This causes sharp voltage spikes on the oscilloscope waveform. To reduce this error, measure the output directly at the output capacitor (C3) and use the shortest wires possible. Also, do not use the ground lead on the probe. Take the tip cover off of the probe and touch the grounding ring of the probe directly to the output ground. This should give the most accurate reading of the actual output waveform. Application Information Setting the Output Voltage The output voltage on the LM2787 is set by using a resistor divider between the output, the feedback pin, and an arbitrary voltage VADJ (Figure 2). VADJ can range from GND to any positive voltage up to VIN. VADJ is usually chosen to be GND and should not be connected to a different voltage unless it is well regulated so the output will stay constant. The feedback pin is held at a constant voltage VFB which equals −1.2V. The output voltage can be selected using the equation: Micro SMD Mounting The micro SMD package requires specific mounting techniques which are detailed in National Semiconductor Application Note # 1112. Referring to the section Surface Mount Technology (SMT) Assembly Considerations, it should be noted that the pad style which must be used with the 8-pin package is the NSMD (non-solder mask defined) type. For best results during assembly, alignment ordinals on the PC board may be used to facilitate placement of the micro SMD device. The current into the feedback pin IFB is in the range of 10nA to 100nA. Therefore using a value of 500kΩ or smaller for R1 should make this current of little concern when setting the output voltage. For best accuracy, use resistors with 1% or better tolerance. Capacitor Selection Selecting the right capacitors for your circuit is important. The capacitors affect the output resistance of the charge-pump, the output voltage ripple, and the overall dropout voltage (VIN-|VOUT|) of the circuit. The output resistance of the charge-pump inverter is: Micro SMD Light Sensitivity Exposing the micro SMD device to direct sunlight may cause misoperation of the device. Light sources such as Halogen lamps can also affect electrical performance if brought near the device. The wavelengths which have the most detrimental effect are reds and infra-reds. The fluorescent lighting used inside of most buildings has very little effect on performance. The switching frequency is fixed at 260kHz and RSW (the combined resistance of the internal switches) is typically 7 www.national.com LM2787 10Ω. It is clear from this equation that low ESR capacitors are desirable and that larger values of C1 will further reduce the output resistance. The output resistance of the entire circuit (in dropout) is: ROUT = RNEG + Rregulator Rregulator (the output impedance of the linear regulator) is approximately 10Ω. When the circuit is in regulation, the overall output resistance is equal to the linear regulator load regulation (5mV/mA). The dropout voltage is therefore affected by the capacitors used since it is simply defined as IOUT*ROUT. A larger value of capacitor and lower ESR for C2 will lower the output voltage ripple of the charge-pump. This ripple will then be subject to the PSRR of the linear regulator and reduced at VOUT. Device Description LM2787 Low Noise Regulated Switched Capacitor Voltage Inverter in micro SMD Physical Dimensions inches (millimeters) unless otherwise noted 8-Bump micro SMD NS Package Number BPA08CCB For Ordering, Refer to Ordering Information Table X1 = 1.346 X2 = 1.346 X3 = 0.850 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation Americas Email: support@nsc.com www.national.com National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. www.s-manuals.com