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| Tem ommercial Extended C 0C to 70C -2 quipment Handheld E for Portable G FEATURIN rature Range pe July 2000 ML4872 High Current Boost Regulator with Shutdown GENERAL DESCRIPTION The ML4872 is a continuous conduction boost regulator designed for DC to DC conversion in multiple cell battery powered systems. Continuous conduction allows the regulator to maximize output current for a given inductor. The maximum switching frequency can exceed 200kHz, allowing the use of small, low cost inductors. The ML4872 is capable of start-up with input voltages as low as 1.8V and is available in 5V and 3.3V output versions with an output voltage accuracy of 3%. An integrated synchronous rectifier eliminates the need for an external Schottky diode and provides a lower forward voltage drop, resulting in higher conversion efficiency. In addition, low quiescent battery current and variable frequency operation result in high efficiency even at light loads. The ML4872 requires only one inductor and two capacitors to build a very small regulator circuit capable of achieving conversion efficiencies approaching 90%. The SHDN input allows the user to stop the regulator from switching and powers down the control circuitry. FEATURES s Guaranteed full load start-up and operation at 1.8V Input Continuous conduction mode for high output current Very low supply current (20A output referenced) for micropower operation Pulse frequency modulation and internal synchronous rectification for high efficiency Maximum switching frequency > 200kHz Minimum external components Low ON resistance internal switching FETs 5V and 3.3V output versions s s s s s s s BLOCK DIAGRAM 1 VIN 2 VL1 6 VL2 START-UP SYNCHRONOUS RECTIFIER CONTROL VOUT + - 5 + - BOOST CONTROL + - 1.25V SHDN 4 PWR GND 8 GND 3 1 ML4872 PIN CONFIGURATION ML4872 8-Pin SOIC (S08) VL1 VIN GND SHDN 1 2 3 4 TOP VIEW 8 7 6 5 PWR GND NC VL2 VOUT PIN DESCRIPTION PIN NAME FUNCTION PIN NAME FUNCTION 1 2 3 4 VL1 VIN GND SHDN Boost inductor connection Battery input voltage Ground Pulling this pin to VIN causes the regulator to stop switching, and powers down the control circuitry 5 6 7 8 VOUT V L2 NC PWR GND Boost regulator output Boost inductor connection No connection Return for the NMOS output transistor 2 ML4872 ABSOLUTE MAXIMUM RATINGS Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. VOUT .......................................................................... 7V Voltage on any other pin ..... GND - 0.3V to VOUT + 0.3V Peak Switch Current (IPEAK) ......................................... 2A Average Switch Current (IAVG) ..................................... 1A Junction Temperature .............................................. 150C Storage Temperature Range ...................... -65C to 150C Lead Temperature (Soldering 10 sec) ...................... 260C Thermal Resistance (qJA) .................................... 160C/W OPERATING CONDITIONS Temperature Range ML4872CS-X .............................................. 0C to 70C ML4872ES-X ........................................... -20C to 70C VIN Operating Range ML4872CS-X ................................ 1.8V to VOUT - 0.2V ML4872ES-X ................................ 2.0V to VOUT - 0.2V ELECTRICAL CHARACTERISTICS Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1). SYMBOL SUPPLY IIN IOUT(Q) VIN Current VOUT Quiescent Current VIN = VOUT - 0.2V SHDN = 0V SHDN = VIN IL(Q) VL Quiescent Current 2 25 15 5 35 22 1 A A A A PARAMETER CONDITIONS MIN TYP MAX UNITS PFM REGULATOR IL Peak Current V OUT Output Voltage IL(PEAK) = 0 -3 Suffix -5 Suffix Load Regulation See Figure 1, -3 Suffix VIN = 2.4V, IOUT = 400mA See Figure 1, -5 Suffix VIN = 2.4V, IOUT = 220mA 1.2 3.30 4.95 3.20 4.85 1.4 3.35 5.05 3.25 4.95 1.7 3.40 5.15 3.40 5.15 A V V V V SHUTDOWN Input Bias Current Shutdown Threshold VSHDN = high to low -100 0.4 0.6 100 1.1 nA V Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions. 3 ML4872 20H (Sumida CD75) ML4870 VIN VL1 VIN 100F GND SHDN PWR GND NC VL2 VOUT 200F IOUT Figure 1. Application Test Circuit IL 1 VL1 VIN 2 RSENSE 6 VL2 START-UP SYNCHRONOUS RECTIFIER CONTROL Q2 A2 + - VOUT 5 VOUT + - A3 BOOST CONTROL + A1 SHDN 4 Q1 ISET - 2.4V PWR GND 8 GND 3 Figure 2. PFM Regulator Block Diagram IL(MAX) IL ISET 0 VOUT VL2 0 Q1 ON Q2 OFF Q1 OFF Q2 ON Figure 3. Inductor Current and Voltage Waveforms 4 ML4872 FUNCTIONAL DESCRIPTION The ML4872 combines a unique form of current mode control with a synchronous rectifier to create a boost converter that can deliver high currents while maintaining high efficiency. Current mode control allows the use of a very small, high frequency inductor and output capacitor. Synchronous rectification replaces the conventional external Schottky diode with an on-chip PMOS FET to reduce losses and eliminate an external component. Also included on-chip are an NMOS switch and current sense resistor, further reducing the number of external components, which makes the ML4872 very easy to use. DESIGN CONSIDERATIONS OUTPUT CURRENT CAPABILITY The maximum current available at the output of the regulator is related to the maximum inductor current by the ratio of the input to output voltage and the full load efficiency. The maximum inductor current is approximately 1.25A and the full load efficiency may be as low as 70%. The maximum output current can be determined by using the typical performance curves shown in Figures 4 and 5, or by calculation using the following equation: REGULATOR OPERATION The ML4872 is a variable frequency, current mode switching regulator. Its unique control scheme converts efficiently over more than three decades of load current. A block diagram of the boost converter is shown in Figure 2. Error amp A3 converts deviations in the desired output voltage to a small current, ISET. The inductor current is measured through a 50mW resistor which is amplified by A1. The boost control block matches the average inductor current to a multiple of the ISET current by switching Q1 on and off. The peak inductor current is limited by the controller to about 1.5A. At light loads, ISET will momentarily reach zero after an inductor discharge cycle , causing Q1 to stop switching. Depending on the load, this idle time can extend to tenths of seconds. While the circuit is not switching, only 20A of supply current is drawn from the output. This allows the part to remain efficient even when the load current drops below 200A. Amplifier A2 and the PMOS transistor Q2 work together to form a low drop diode. When transistor Q1 turns off, the current flowing in the inductor causes pin 6 to go high. As the voltage on VL2 rises above VOUT, amplifier A2 allows the PMOS transistor Q2 to turn on. In discontinuous operation, (where IL always returns to zero), A2 uses the resistive drop across the PMOS switch Q2 to sense zero inductor current and turns the PMOS switch off. In continuous operation, the PMOS turn off is independent of A2, and is determined by the boost control circuitry. Typical inductor current and voltage waveforms are shown in Figure 3. IOUT( MAX) = 125 . V V IN( MIN) OUT 0.7A (1) INDUCTOR SELECTION The ML4872 is able to operate over a wide range of inductor values. A value of 10H is a good choice, but any value between 5H and 33H is acceptable. As the inductor value is changed the control circuitry will automatically adjust to keep the inductor current under control. Choosing an inductance value of less than 10H will reduce the component's footprint, but the efficiency and maximum output current may drop. It is important to use an inductor that is rated to handle 1.5A peak currents without saturating. Also look for an inductor with low winding resistance. A good rule of thumb is to allow 5 to 10mW of resistance for each H of inductance. The final selection of the inductor will be based on tradeoffs between size, cost and efficiency. Inductor tolerance, core and copper loss will vary with the type of inductor selected and should be evaluated with a ML4872 under worst case conditions to determine its suitability. Several manufacturers supply standard inductance values in surface mount packages: Coilcraft Coiltronics Dale Sumida (847) 639-6400 (561) 241-7876 (605) 665-9301 (847) 956-0666 SHUTDOWN The SHDN pin should be held low for normal operation. Raising the shutdown voltage above the threshold level will disable the synchronous rectifier and force ISET to zero. This prevents switching from occurring, and the output voltage becomes VIN - VDIODE. 5 ML4872 DESIGN CONSIDERATIONS OUTPUT CAPACITOR The output capacitor filters the pulses of current from the switching regulator. Since the switching frequency will vary with inductance, the minimum output capacitance required to reduce the output ripple to an acceptable level will be a function of the inductor used. Therefore, to maintain an output voltage with less than 100mV of ripple at full load current, use the following equation: COUT = 44 L VOUT (2) (Continued) between 0.5A and 1.5A. This fast change in current through the capacitor's ESL causes a high frequency (5ns) spike to appear on the output. After the ESL spike settles, the output still has a ripple component equal to the inductor discharge current times the ESR. To minimize these effects, choose an output capacitor with less than 10nH of ESL and 100mW of ESR. Suitable tantalum capacitors can be obtained from the following vendors: AVX Kemet Sprague (207) 282-5111 (846) 963-6300 (207) 324-4140 The output capacitor's Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL), also contribute to the ripple. Just after the NMOS transistor, Q1, turns off, the current in the output capacitor ramps quickly to 1000 90 800 VOUT = 3.3V IOUT (mA) 600 VOUT = 5V EFFICIENCY (%) 80 VOUT = 3.3V 400 70 VOUT = 5V 200 VIN = 2.4V 1 10 IOUT (mA) 100 1000 0 1.0 2.0 3.0 VIN (V) 4.0 5.0 60 Figure 4. IOUT vs. VIN Using the Circuit of Figure 8 Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8 90 VOUT = 5V 60 IIN (A) VOUT = 3.3V 30 0 1.0 2.0 3.0 VIN (V) 4.0 5.0 Figure 6. No Load Input Current vs. VIN 6 ML4872 DESIGN CONSIDERATIONS INPUT CAPACITOR Due to the high input current drawn at startup and possibly during operation, it is recommended to decouple the input with a capacitor with a value of 47F to 100F. This filtering prevents the input ripple from affecting the ML4872 control circuitry, and also improves the efficiency by reducing the I squared R losses during the charge cycle of the inductor. Again, a low ESR capacitor (such as tantalum) is recommended. It is also recommended that low source impedance batteries be used. Otherwise, the voltage drop across the source impedance during high input current situations will cause the ML4872 to fail to start-up or to operate unreliably. In general, for two cell applications the source impedance should be less than 200mW, which means that small alkaline cells should be avoided. (Continued) DESIGN EXAMPLE In order to design a boost converter using the ML4872, it is necessary to define a few parameters. For this example, assume that VIN = 3.0V to 3.6V, VOUT = 5.0V, and IOUT(MAX) = 500mA. First, it must be determined whether the ML4871 is capable of delivering the output current. This is done using Equation 1: IOUT( MAX) = 125 . . 30V 0.7A = 0.53A 50V . Next, select an inductor. As previously mentioned, the recommended inductance is 10H. Make sure that the peak current rating of the inductor is at least 1.5A, and that the DC resistance of the inductor is in the range of 50 to 100mW. Finally, the value of the output capacitor is determined using Equation 2: COUT = 44 10mH = 88mF 5.0V LAYOUT Good layout practices will ensure the proper operation of the ML4872. Some layout guidelines follow: * Use adequate ground and power traces or planes * Keep components as close as possible to the ML4872 * Use short trace lengths from the inductor to the VL1 and VL2 pins and from the output capacitor to the VOUT pin * Use a single point ground for the ML4872 ground pin, and the input and output capacitors * Separate the ground for the converter circuitry from the ground of the load circuitry and connect at a single point A sample layout is shown in Figure 7. The closest standard value would be a 100F capacitor with an ESR rating of 100mW. If such a low ESR value cannot be found, two 47F capacitors in parallel could also be used. The complete circuit is shown in Figure 8. As mentioned previously, the use of an input supply bypass capacitor is highly recommended. 10H (Sumida CD75) ML4872 VIN VL1 VIN 100F GND SHDN PWR GND NC VL2 VOUT 100F VOUT Figure 7. Sample PC Board Layout Figure 8. Typical Application Circuit 7 ML4872 IOUT(MAX) (mA) VIN (V) 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 IOUT (mA) VIN = 2.4V, VOUT = 3.3V 1.0 2.0 5.0 10.0 20.0 50.0 100.0 200.0 586.0 VIN = 2.4V, VOUT = 5.0V 1.0 2.0 5.0 10.0 20.0 50.0 100.0 200.0 485.0 EFFICIENCY PERCENTAGE 82.0 84.4 87.0 87.6 87.9 88.3 88.6 88.2 65.1 84.4 87.0 87.7 88.4 88.9 89.1 88.9 87.5 71.6 VOUT = 3.3V 386.2 451.9 521.5 585.9 651.0 716.5 782.0 VOUT = 5.0V 286.2 332.1 379.1 430.0 479.0 525.4 571.8 618.5 665.0 711.7 758.7 805.3 851.9 899.0 946.1 992.7 Table 1. Typical IOUT and Efficiency vs. VIN 8 ML4872 PHYSICAL DIMENSIONS inches (millimeters) Package: S08 8-Pin SOIC 0.189 - 0.199 (4.80 - 5.06) 8 PIN 1 ID 0.148 - 0.158 0.228 - 0.244 (3.76 - 4.01) (5.79 - 6.20) 1 0.017 - 0.027 (0.43 - 0.69) (4 PLACES) 0.050 BSC (1.27 BSC) 0.059 - 0.069 (1.49 - 1.75) 0 - 8 0.055 - 0.061 (1.40 - 1.55) 0.012 - 0.020 (0.30 - 0.51) SEATING PLANE 0.004 - 0.010 (0.10 - 0.26) 0.015 - 0.035 (0.38 - 0.89) 0.006 - 0.010 (0.15 - 0.26) ORDERING INFORMATION PART NUMBER ML4872CS-3 ML4872CS-5 ML4872ES-3 ML4872ES-5 OUTPUT VOLTAGE 3.3V 5.0V 3.3V 5.0V TEMPERATURE RANGE 0C to 70C 0C to 70C -20C to 70C -20C to 70C PACKAGE 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) (c) Micro Linear 1998. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners. Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897; 5,717,798; 5,742,151; 5,754,012; 5,757,174; 5,767,653. Japan: 2,598,946; 2,619,299; 2,704,176. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application. 2092 Concourse Drive San Jose, CA 95131 Tel: (408) 433-5200 Fax: (408) 432-0295 http://www.microlinear.com DS4872-01 9 |
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