LM2705_07

LM2705 Micropower Step-up DC/DC Converter with 150mA Peak Current Limit August 2007 LM2705 Micropower Step-up DC/DC Converter with 150mA Peak Current Limit General Description The LM2705 is a micropower step-up DC/DC in a small 5-lead SOT-23 package. A current limited, fixed off-time control scheme conserves operating current resulting in high efficiency over a wide range of load conditions. The 21V switch allows for output voltages as high as 20V. The low 400ns offtime permits the use of tiny, low profile inductors and capacitors to minimize footprint and cost in space-conscious portable applications. The LM2705 is ideal for LCD panels requiring low current and high efficiency as well as white LED applications for cellular phone back-lighting. The LM2705 can drive up to 3 white LEDs from a single Li-Ion battery. The low peak inductor current of the LM2705 makes it ideal for USB applications. Features ■ ■ ■ ■ ■ ■ ■ 150mA, 0.7Ω, internal switch Uses small surface mount components Adjustable output voltage up to 20V 2.2V to 7V input range Input undervoltage lockout 0.01µA shutdown current Small 5-Lead SOT-23 package Applications ■ ■ ■ ■ ■ LCD Bias Supplies White LED Back-Lighting Handheld Devices Digital Cameras Portable Applications Typical Application Circuit 20039701 FIGURE 1. Typical 20V Application © 2007 National Semiconductor Corporation 200397 www.national.com LM2705 Connection Diagram Top View 20039702 SOT23-5 TJmax = 125°C, θJA = 220°C/W (Note 2) Ordering Information Order Number LM2705MF-ADJ LM2705MFX-ADJ Package Type SOT23-5 SOT23-5 NSC Package Drawing MA05B MA05B Top Mark S59B S59B Supplied As 1000 Units, Tape and Reel 3000 Units, Tape and Reel Pin Descriptions/Functions Pin 1 2 3 4 5 Name SW GND FB SHDN VIN Power Switch input. Ground. Output voltage feedback input. Shutdown control input, active low. Analog and Power input. Connect the ground of the feedback network to an AGND plane which should be tied directly to the GND pin. SHDN(Pin 4): Shutdown Pin. The shutdown pin is an active low control. Tie this pin above 1.1V to enable the device. Tie this pin below 0.3V to turn off the device. VIN(Pin 5): Input Supply Pin. Bypass this pin with a capacitor as close to the device as possible. Function SW(Pin 1): Switch Pin. This is the drain of the internal NMOS power switch. Minimize the metal trace area connected to this pin to minimize EMI. GND(Pin 2): Ground Pin. Tie directly to ground plane. FB(Pin 3): Feedback Pin. Set the output voltage by selecting values for R1 and R2 using: www.national.com 2 LM2705 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 SW Voltage FB Voltage SHDN Voltage Maximum Junction Temp. TJ (Note 2) Lead Temperature (Soldering 10 sec.) Vapor Phase (60 sec.) 7.5V 21V 2V 7.5V 150°C Infrared (15 sec.) ESD Ratings (Note 3) Human Body Model Machine Model (Note 4) 220°C 2kV 200V Operating Conditions Junction Temperature (Note 5) Supply Voltage SW Voltage Max. −40°C to +125°C 2.2V to 7V 20.5V 300°C 215°C Electrical Characteristics Specifications in standard type face are for TJ = 25°C and those in boldface type apply over the full Operating Temperature Range (TJ = −40°C to +125°C). Unless otherwise specified VIN =2.2V. Symbol IQ Parameter Device Disabled Device Enabled Shutdown FeedbackTrip Point Switch Current Limit FB Pin Bias Current Input Voltage Range Switch RDSON Switch Off Time SHDN Pin Current SHDN = VIN, TJ = 25°C SHDN = VIN, TJ = 125°C SHDN = GND IL UVP VFB Hysteresis SHDN Threshold Switch Leakage Current Input Undervoltage Lockout Feedback Hysteresis SHDN low SHDN High 1.1 VSW = 20V ON/OFF Threshold FB = 1.23V (Note 7) 2.2 0.7 400 0 15 0 0.05 1.8 8 0.7 0.7 220 0.3 5 µA V mV 80 nA Conditions FB = 1.3V FB = 1.2V SHDN = 0V 1.189 110 100 Min (Note 5) Typ (Note 6) 40 235 0.01 1.237 150 30 Max (Note 5) 70 300 2.5 1.269 175 180 120 7.0 1.6 Units µA V mA nA V Ω ns VFB ICL IB VIN RDSON TOFF ISD V °C/W θJA SOT23-5 Thermal Resistance Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged directly into each pin. Note 4: ESD susceptibility using the machine model is 150V for SW pin. Note 5: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 6: Typical numbers are at 25°C and represent the most likely norm. Note 7: Feedback current flows into the pin. 3 www.national.com LM2705 Typical Performance Characteristics Enable Current vs VIN (Part Switching) Disable Current vs VIN (Part Not Switching) 20039705 20039706 Efficiency vs Load Current Efficiency vs Load Current 20039724 20039725 SHDN Threshold vs VIN Switch Current Limit vs VIN 20039713 20039737 www.national.com 4 LM2705 Switch RDSON vs VIN FB Trip Point and FB Pin Current vs Temperature 20039715 20039723 Output Voltage vs Load Current Off Time vs Temperature 20039727 20039741 Step Response Start-Up/Shutdown 20039728 20039729 VOUT = 20V, VIN = 3.0V 1) Load, 0.5mA to 5mA to 0.5mA, DC 2) VOUT, 200mV/div, AC 3) IL, 100mA/div, DC T = 100µs/div VOUT = 20V, VIN = 3.0V 1) SHDN, 1V/div, DC 2) IL, 100mA/div, DC 3) VOUT, 10V/div, DC T = 400µs/div RL = 3.9kΩ 5 www.national.com LM2705 Operation 20039704 FIGURE 2. LM2705 Block Diagram 20039730 VOUT = 20V, VIN = 2.7V, IOUT = 2.5mA 1) VSW, 20V/div, DC 2) Inductor Current, 100mA/div, DC 3) VOUT, 200mV/div, AC T = 10µs/div FIGURE 3. Typical Switching Waveform www.national.com 6 LM2705 The LM2705 features a constant off-time control scheme. Operation can be best understood by referring to Figure 2 and Figure 3. Transistors Q1 and Q2 and resistors R3 and R4 of Figure 2 form a bandgap reference used to control the output voltage. When the voltage at the FB pin is less than 1.237V, the Enable Comp in Figure 2 enables the device and the NMOS switch is turned on pulling the SW pin to ground. When the NMOS switch is on, current begins to flow through inductor L while the load current is supplied by the output capacitor COUT. Once the current in the inductor reaches the current limit, the CL Comp trips and the 400ns One Shot turns off the NMOS switch.The SW voltage will then rise to the output voltage plus a diode drop and the inductor current will begin to decrease as shown in Figure 3. During this time the energy stored in the inductor is transferred to COUT and the load. After the 400ns off-time the NMOS switch is turned on and energy is stored in the inductor again. This energy transfer from the inductor to the output causes a stepping effect in the output ripple as shown in Figure 3. This cycle is continued until the voltage at FB reaches 1.237V. When FB reaches this voltage, the enable comparator then disables the device turning off the NMOS switch and reducing the Iq of the device to 40uA. The load current is then supplied solely by COUT indicated by the gradually decreasing slope at the output as shown in Figure 3. When the FB pin drops slightly below 1.237V, the enable comparator enables the device and begins the cycle described previously. The SHDN pin can be used to turn off the LM2705 and reduce the Iq to 0.01µA. In shutdown mode the output voltage will be a diode drop lower than the input voltage. discharge the energy in the inductor and transfer the energy to the output capacitor and load. This can cause a ramping effect in the inductor current waveform and an increased ripple on the output voltage. Using a smaller inductor will cause the IPK to increase and will increase the output voltage ripple further. For typical curves and evaluation purposes the DT1608C series inductors from Coilcraft were used. Other acceptable inductors would include, but are not limited to, the SLF6020T series from TDK, the NP05D series from Taiyo Yuden, the CDRH4D18 series from Sumida, and the P1166 series from Pulse. INDUCTOR SELECTION - SEPIC REGULATOR The following equation can be used to calculate the approximate inductor value for a SEPIC regulator: The boost inductor, L1, can be smaller or larger but is generally chosen to be the same value as L2. See Figure 8and Figure 9for typical SEPIC applications. DIODE SELECTION To maintain high efficiency, the average current rating of the schottky diode should be larger than the peak inductor current, IPK. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown of the schottky diode larger than the output voltage. CAPACITOR SELECTION Choose low ESR capacitors for the output to minimize output voltage ripple. Multilayer ceramic capacitors are the best choice. For most applications, a 1µF ceramic capacitor is sufficient. For some applications a reduction in output voltage ripple can be achieved by increasing the output capacitor. Output voltage ripple can further be reduced by adding a 4.7pF feed-forward capacitor in the feedback network placed in parallel with RF1, see Figure 2. Local bypassing for the input is needed on the LM2705. Multilayer ceramic capacitors are a good choice for this as well. A 4.7µF capacitor is sufficient for most applications. For additional bypassing, a 100nF ceramic capacitor can be used to shunt high frequency ripple on the input. LAYOUT CONSIDERATIONS The input bypass capacitor CIN, as shown in Figure 1, must be placed close to the IC. This will reduce copper trace resistance which effects input voltage ripple of the IC. For additional input voltage filtering, a 100nF bypass capacitor can be placed in parallel with CIN to shunt any high frequency noise to ground. The output capacitor, COUT, should also be placed close to the IC. Any copper trace connections for the Cout capacitor can increase the series resistance, which directly effects output voltage ripple. The feedback network, resistors R1 and R2, should be kept close to the FB pin to minimize copper trace connections that can inject noise into the system. The ground connection for the feedback resistor network should connect directly to an analog ground plane. The analog ground plane should tie directly to the GND pin. If no analog ground plane is available, the ground connection for the feedback network should tie directly to the GND pin. Trace connections made to the inductor and schottky diode Application Information INDUCTOR SELECTION - BOOST REGULATOR The appropriate inductor for a given application is calculated using the following equation: where VD is the schottky diode voltage, ICL is the switch current limit found in the Typical Performance Characteristics section, and TOFF is the switch off time. When using this equation be sure to use the minimum input voltage for the application, such as for battery powered applications. For the LM2705 constant-off time control scheme, the NMOS power switch is turned off when the current limit is reached. There is approximately a 100ns delay from the time the current limit is reached in the NMOS power switch and when the internal logic actually turns off the switch. During this 100ns delay, the peak inductor current will increase. This increase in inductor current demands a larger saturation current rating for the inductor. This saturation current can be approximated by the following equation: Choosing inductors with low ESR decrease power losses and increase efficiency. Care should be taken when choosing an inductor. For applications that require an input voltage that approaches the output voltage, such as when converting a Li-Ion battery voltage to 5V, the 400ns off time may not be enough time to 7 www.national.com LM2705 should be minimized to reduce power dissipation and increase overall efficiency. 20039742 20039709 FIGURE 4. 2 White LED Application and Efficiency 20039743 20039734 FIGURE 5. 3 White LED Application and Efficiency www.national.com 8 LM2705 20039735 FIGURE 6. Li-Ion 12V Application 20039736 FIGURE 7. 5V to 12V Application 9 www.national.com LM2705 20039739 FIGURE 8. 3.3V SEPIC Application 20039740 FIGURE 9. 5V SEPIC Application www.national.com 10 LM2705 Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead Small Outline Package (M5) For Ordering, Refer to Ordering Information Table NS Package Number MA05B 11 www.national.com LM2705 Micropower Step-up DC/DC Converter with 150mA Peak Current Limit Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. 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