LM2623 Ratio Adaptive Gated Oscillator Based, DC DC Boost Converter-Low Vin, Low Ripple, General Purpose General Description Features The LM2623 is a high efficiency, general purpose, step-up DC-DC switching regulator for battery-powered and low input voltage systems. It accepts an input voltage between .8 and 14 volts and converts it into a regulated output voltage. The LM2623 can be used to generate up to 14 volts using an inductor and almost any positive voltage using a transformer. It has an internal .17Ω N-Channel MOSFET power switch. Efficiencies up to 90% are achievable with the LM2623. In order to adapt to a number of applications, the LM2623 allows the designer to vary the output voltage, the operating frequency (300kHz to 2 MHz) and duty cycle (17% to 90%) to optimize the part’s performance. The selected values can be fixed or can vary with battery voltage or input to output voltage ratio. Using these techniques according to the design guidelines may improve efficiency or reduce parts cost or both. The LM2623 is a gated oscillator and uses a very simple, on/off regulation mode to produce good efficiency and stable operation over a wide operating range. It normally regulates by skipping switching cycles when it reaches the regulation limit (Pulse Frequency Modulation). In some applications, with the proper selection of external feedback components, it can also regulate cycle to cycle, similar to Pulse Width Modulated parts. It does this by adjusting the off time between switching cycles, while maintaining continuous current through the coil. This cycle to cycle type of PFM results in very low ripple on the output voltage and/or minimum sized output capacitors, similar to PWM parts.This mode also typically maximizes efficiency. n Good Efficiency Over a Very Wide Load Range n Very Low Output Voltage Ripple n Small, Mini-SO-8 Package (Half the Footprint of Standard 8 pin SO Package) n 1.09 mm Package Height n Up to 2 MHz Switching Frequency n .8V to 14V Operating Voltage n 1.1V Start-up Voltage n 1.24V - 14V Adjustable Output Voltage n Up to 2A Load Current n 0.17Ω Internal MOSFET n Up to 90% Regulator Efficiency n 80 µA Typical Operating Current (into Vdd pin of supply) n < 2.5µA Guaranteed Supply Current In Shutdown Applications n n n n n n n Cameras, Pagers and Cell Phones PDAs,Palmtop Computers, GPS devices White LED Drive, TFT or Scanned LCDs Flash Memory Programming Hand-Held Instruments 1, 2, 3 or 4 Cell Alkaline Systems 1, 2 or 3 Cell Lithium-ion Systems Typical Application Circuit 20038801 © 2002 National Semiconductor Corporation DS200388 www.national.com LM2623 Ratio Adaptive Gated (RAG) Oscillator Based, DC to DC Converter Low Vin, Low Ripple, General Purpose, High Efficiency, Boost April 2002 LM2623 Connection Diagram Mini SO-8 (MM) Package 20038818 Top View Ordering Information Order Number Package Type NSC Package Drawing Package Marking Supplied As LM2623MMX Mini SO-8 MUA08A S46B 3000 Units on Tape and Reel LM2623AMMX Mini SO-8 MUA08A S46A 3000 Units on Tape and Reel LM2623AMM Mini SO-8 MUA08A S46A 1000 Units on Tape and Reel LM2623MM Mini SO-8 MUA08A S46B 1000 Units on Tape and Reel Pin Description Pin Name Function 1 PGND Power Ground 2 EN Active-Low Shutdown Input 3 FREQ Frequency Adjust. An external resistor connected between this pin and a voltage source sets the switching frequency of the LM2623. 4 FB Output Voltage Feedback 5 SGND Signal Ground 6 VDD Power Supply for Internal Circuitry 7 BOOT Bootstrap Supply for the Gate Drive of Internal MOSFET Power Switch 8 SW Drain of the Internal MOSFET Power Switch www.national.com 2 Power Dissipation (TA =25˚C) (Note 2) (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. SW Pin Voltage 500mW ESD Rating (Note 3) 2kV −0.5 V to 14.5V BOOT, VDD, EN and FB Pins −0.5V to 10V FREQ Pin Operating Conditions (Note 1) 100µA θJA (Note 2) 240˚C/W TJmax (Note 2) 150˚C Storage Temperature Range 3V to 5V FB, EN Pins 0 to VDD BOOT Pin −65˚C to +150˚C Lead Temp. (Soldering, 5 sec) VDD Pin 0 to 10V Ambient Temperature (TA) 260˚C −40˚C to +85˚C Electrical Characteristics Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range of −40˚C to +85˚C. Unless otherwise specified: VDD = VOUT = 3.3V. Symbol Parameter Condition Typ VDD_ST Start-Up Supply Voltage 25˚C ILOAD = 0mA (Note 4) VIN_OP Minimum Operating Supply Voltage (once started) ILOAD = 0mA VFB FB Pin Voltage VOUT_MAX Maximum Output Voltage η Efficiency 0.65 1.24 1.2028 Max Units 1.1 V .8 V 1.2772 V 14 VIN = 3.6V; VOUT = 5V; ILOAD = 500mA VIN = 2.5V; VOUT = 3.3V; ILOAD = 200mA D Min Switch Duty Cycle V 87 % 87 17 IDD Operating Quiescent Current (Note 5) FB Pin > 1.3V; EN Pin at VDD ISD Shutdown Quiescent Current (Note 6) % 80 110 µA VDD, BOOT and SW Pins at 5.0V; EN Pin < 200mV 0.01 2.5 µA 2. 85 ICL Switch Peak Current Limit LM2623A IC Switch Peak Current Limit LM2623 RDS_ON MOSFET Switch On Resistance 2.2 A 1.2 A 0.17 0.26 Ω 0.15VDD V Enable Section VEN_LO EN Pin Voltage Low (Note 7) VEN_HI EN Pin Voltage High (Note 7) 0.7VDD 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 outside of its rated operating conditions. Note 2: The maximum power dissipation must be derated at elevated temperatures and is dictated by Tjmax (maximum junction temperature), θJA (junction to ambient thermal resistance), and TA (ambient temperature). The maximum allowable power dissipation at any temperature is Pdmax = (Tjmax - TA)/ θJA or the number given in the Absolute Maximum Ratings, whichever is lower. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. For Pin 8 (SW) the ESD rating is 1.0 kV. Note 4: VDD tied to Boot and EN pins. Frequency pin tied to VDD through 121K resistor. VDD_ST = VDD when startu-up occurs. VIN is VDD + D1 voltage (usually 10-50 mv at start-up) Note 5: This is the current into the VDD pin. Note 6: This is the total current into pins VDD, BOOT, SW and FREQ. Note 7: When the EN pin is below VEN_LO, the regulator is shut down; when it is above VEN_HI, the regulator is operating. 3 www.national.com LM2623 Absolute Maximum Ratings LM2623 Typical Performance Characteristics Efficiency vs VIN VOUT = 5.0V VFB vs Temperature 20038829 20038831 Frequency vs VIN Maximum Start Up Voltage vs Temperature 20038828 20038826 Typical RDS(ON) vs Temperature Typical Current Limit vs Temperature 20038825 www.national.com 20038827 4 This device is optimized for use in cellular phones and other applications requiring a small size, low profile, as well as low quiescent current for maximum battery life during stand-by and shutdown. A high-efficiency gated-oscillator topology offers an output of up to 2A. Operating Principle The LM2623 is designed to provide step-up DC-DC voltage regulation in battery-powered and low-input voltage systems. It combines a step-up switching regulator, N-channel power MOSFET, built-in current limit, thermal limit, and voltage reference in a single 8-pin MSOP package Figure 1. The switching DC-DC regulator boosts an input voltage between .8V and 14V to a regulated output voltage between 1.24V and 14V. The LM2623 starts from a low 1.1V input and remains operational down to below .8V. Additional features include a built-in peak switch current limit, and thermal protection circuitry. 20038814 FIGURE 1. Functional Diagram tion to the capacitor allows the duty cycle to dynamically compensate for changes to the input/output voltage ratio. We call this a Ratio Adaptive Gated Oscillator circuit. See the Application Notes for sample application circuits. Using the correct RC components to adjust the oscillator allows the part to run with low ripple and high efficiency over a wide range of loads and input/output voltages. Gated Oscilaltor Control Scheme The on/off regulation mode of the LM2623, along with its ultra-low quiescent current, results in good efficiency over a very wide load range. The internal oscillator frequency can be programmed using an external resistor to be constant or vary with the battery voltage. Adding a capacitor to program the frequency allows the designer to adjust the duty cycle and optimize it for the application. Adding a resistor in addi- 5 www.national.com LM2623 DETAILED DESCRIPTION LM2623 DETAILED DESCRIPTION (Continued) 20038815 FIGURE 2. Typical Step-Up Regulator Waveforms never goes to zero like it frequently does in the hysteretic operating mode of circuits with wide load variations or duty cycles that aren’t matched to the input/output voltage ratio. Optimizing the duty cycle for a given set of input/output voltages conditions can be done by using the circuit values in the Application Notes. Pulse Frequency Modulation (PFM) Pulse Frequency Modulation is typically accomplished by switching continuously until the voltage limit is reached and skipping cycles after that to just maintain it. This results in a somewhat hysteretic mode of operation. The coil stores more energy each cycle as the current ramps up to high levels. When the voltage limit is reached, the system usually overshoots to a higher voltage than required, due to the stored energy in the coil (see figure 2). The system will also undershoot somewhat when it starts switching again because it has depleted all the stored energy in the coil and needs to store more energy to reach equilibrium with the load. Larger output capacitors and smaller inductors reduce the ripple in these situations. The frequency being filtered, however, is not the basic switching frequency. It is a lower frequency determined by the load, the input/output voltage and the circuit parameters. This mode of operation is useful in situations where the load variation is significant. Power managed computer systems, for instance, may vary from zero to full load while the system is on and this is usually the preferred regulation mode for such systems. Low Voltage Start-Up The LM2623 can start-up from voltages as low as 1.1 volts. On start-up, the control circuitry switches the N-channel MOSFET continuously until the output reaches 2.5 volts. After this output voltage is reached, the normal step-up regulator feedback and gated oscillator control scheme take over. Once the device is in regulation, it can operate down to below .8V input, since the internal power for the IC can be boot-strapped from the output using the Vdd pin. Shut Down The LM2623 features a shutdown mode that reduces the quiescent current to less than a guaranteed 2.5 uA over temperature. This extends the life of the battery in battery powered applications. During shutdown, all feedback and control circuitry is turned off. The regulator’s output voltage drops to one diode drop below the input voltage. Entry into the shutdown mode is controlled by the active-low logic input pin EN (pinh- 2). When the logic input to this pin is pulled below .15Vdd, the device goes into shutdown mode. The logic input to this pin should be above .7Vdd for the device to work in normal stepup mode. Cycle To Cycle PFM When the load doesn’t vary over a wide range (like zero to full load), ratio adaptive circuit techniques can be used to achieve cycle to cycle PFM regulation and lower ripple (or smaller output capacitors). The key to success here is matching the duty cycle of the circuit closely to what is required by the input to output voltage ratio. This ratio then needs to be dynamically adjusted for input voltage changes (usually caused by batteries running down). The chosen ratio should allow most of the energy in each switching cycle to be delivered to the load and only a small amount to be stored. When the regulation limit is reached, the overshoot will be small and the system will settle at an equilibrium point where it adjusts the off time in each switching cycle to meet the current requirements of the load. The off time adjustment is done by exceeding the regulation limit during each switching cycle and waiting until the voltage drops below the limit again to start the next switching cycle. The current in the coil www.national.com Internal Current Limit And Thermal Protection An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85A typical, approximately 4A maximum) so as not to come into effect during normal operating conditions. An internal thermal protection circuit disables the MOSFET power switch when the junction temperature (TJ) exceeds about 160˚C. The switch is re-enabled when TJ drops below approximately 135˚C. 6 Setting The Output Voltage quency vs RFQ“ in the Typical Operating Characteristics section of the datasheet for choosing the RFQ value to achieve the desired switching frequency. A high switching frequency allows the use of very small surface mount inductors and capacitors and results in a very small solution size. A switching frequency between 300kHz and 2MHz is recommended. The output voltage of the step-up regulator can be set by connecting a feedback resistive divider made of RF1 and RF2. The resistor values are selected as follows: RF2 = RF1 /[(VOUT/ 1.24) −1] A value of 50k to 100k is suggested for Rf2. Then, Rf1 can be selected using the above equation. Output Diode Selection A Schottky diode should be used for the output diode. The forward current rating of the diode should be higher than the peak input current, and the reverse voltage rating must be higher than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer. Table 1 shows a list of the diode manufacturers. Vdd Supply A Vdd supply of 3 to 5 volts is recommended for the LM2623. This voltage can be bootstrapped from a much lower input voltage by simply connecting the Vdd pin to Vout. It can also be another voltage in the system or the battery voltage if they are in the right voltage range. In the event that the Vdd supply voltage is not a low ripple voltage source (less than 200 millivolts), it may be advisable to use an RC filter to clean it up. Excessive ripple on Vdd may reduce the efficiency. Setting The Switching Frequency The switching frequency of the oscillator is selected by choosing an external resistor (RFQ) connected between FREQ and VDD pins. See the graph titled „Switching Fre- TABLE 1. Suggested Manufacturers List Inductors Capacitors Diodes Coilcraft Tel: (800) 322-2645 Fax: (708) 639-1469 Sprague/ Vishay Tel: (207) 324-4140 Fax: (207) 324-7223 Motorola Tel: (800) 521-6274 Fax: (602) 244-6609 Coiltronics Tel: (407) 241-7876 Fax: (407) 241-9339 Kemet Tel: (864) 963-6300 Fax: (864) 963-6521 International Rectifier (IR) Tel: (310) 322-3331 Fax: (310) 322-3332 Pulse Engineering Tel: (619) 674-8100 Fax: (619) 674-8262 Nichicon Tel: (847) 843-7500 Fax: (847) 843-2798 General Semiconductor Tel: (516) 847-3222 Fax: (516) 847-3150 7 www.national.com LM2623 Design Procedure LM2623 Ratio Adaptive Gated (RAG) Oscillator Based, DC to DC Converter Low Vin, Low Ripple, General Purpose, High Efficiency, Boost Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead Mini SO-8 (MM) NS Package Number MUA08A For Order Numbers, refer to the table in the ’Ordering Information’ section of this document. 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