July 2000 G FEATURINperature Range ML4871 Tem ommercial -20˚C to 70˚C quipment Handheld E for Portable Extended C High Current Boost Regulator GENERAL DESCRIPTION FEATURES The ML4871 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 ML4871 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%. ■ Guaranteed full load start-up and operation at 1.8V Input ■ Continuous conduction mode for high output current ■ Very low supply current (20µA 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 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 ML4871 requires only one inductor and two capacitors to build a very small regulator circuit capable of achieving conversion efficiencies approaching 90%. The circuit also contains a RESET output which goes low when the DETECT input drops below 1.25V. BLOCK DIAGRAM 1 VL1 7 4 6 RESET DETECT VL2 + COMP – VREF VIN 2 SYNCHRONOUS RECTIFIER CONTROL START-UP VOUT + 5 – + – + BOOST CONTROL – PWR GND 1.25V GND 8 3 1 ML4871 PIN CONFIGURATION ML4871 8-Pin SOIC (S08) VL1 1 8 PWR GND VIN 2 7 RESET GND 3 6 VL2 DETECT 4 5 VOUT TOP VIEW PIN DESCRIPTION NO. NAME 1 VL1 Boost inductor connection 2 VIN 3 4 2 FUNCTION NO. NAME FUNCTION 5 VOUT Boost regulator output Battery input voltage 6 VL2 Boost inductor connection GND Ground 7 RESET Output goes low when DETECT goes below 1.25V DETECT Pulling this pin below 1.25V causes the RESET pin to go low 8 PWR GND Return for the NMOS output transistor ML4871 ABSOLUTE MAXIMUM RATINGS OPERATING CONDITIONS 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. Temperature Range ML4871CS-X ............................................. 0ºC to 70ºC ML4871ES-X .......................................... –20ºC to 70ºC VIN Operating Range ML4871CS-X ................................ 1.8V to VOUT – 0.2V ML4871ES-X ................................. 2.0V to VOUT – 0.2V 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 .............................................. 150ºC Storage Temperature Range...................... –65ºC to 150ºC Lead Temperature (Soldering 10 sec) ....................... 260ºC Thermal Resistance (qJA) .................................... 160ºC/W ELECTRICAL CHARACTERISTICS Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1). SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS SUPPLY IIN IOUT(Q) IL(Q) VIN Current VIN = VOUT – 0.2V 2 5 µA VOUT Quiescent Current DETECT = VIN 30 40 µA DETECT = 0V 25 35 µA 1 µA VL Quiescent Current PFM REGULATOR IL Peak Current VOUT 1.2 1.4 1.7 A -3 Suffix 3.30 3.35 3.40 V -5 Suffix 4.95 5.05 5.15 V See Figure 1, -3 Suffix VIN = 2.4V, IOUT £ 400mA 3.20 3.25 3.40 V See Figure 1, -5 Suffix VIN = 2.4V, IOUT £ 220mA 4.85 4.95 5.15 V DETECT Threshold 1.18 1.25 1.28 V DETECT Hysteresis 25 35 45 mV 100 nA Output Voltage Load Regulation IL(PEAK) = 0 RESET COMPARATOR DETECT Bias Current Note 1: –100 RESET Output High Voltage IRESET = –200µA RESET Output Low Voltage IRESET = 500µA VOUT – 0.2 V 0.2 V Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions. 3 ML4871 20µH (Sumida CD75) ML4871 VIN VL1 PWR GND VIN RESET GND 100µF IOUT VL2 DETECT VOUT 200µF Figure 1. Application Test Circuit. IL 1 6 VL1 VIN 2 VL2 RSENSE Q2 SYNCHRONOUS RECTIFIER CONTROL START-UP + 5 – A3 + + BOOST CONTROL – A1 Q1 – ISET PWR GND 8 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 VOUT VOUT A2 1.25V GND 3 ML4871 FUNCTIONAL DESCRIPTION DESIGN CONSIDERATIONS The ML4871 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 ML4871 very easy to use. OUTPUT CURRENT CAPABILITY REGULATOR OPERATION The ML4871 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 20µA of supply current is drawn from the output. This allows the part to remain efficient even when the load current drops below 200µA. 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. 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: IOUT( MAX) = 125 . V V IN( MIN) OUT 0.7A (1) INDUCTOR SELECTION The ML4871 is able to operate over a wide range of inductor values. A value of 10µH is a good choice, but any value between 5µH and 33µH 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 10µH 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 µH 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 ML4871 under worst case conditions to determine its suitability. Several manufacturers supply standard inductance values in surface mount packages: Coilcraft (847) 639-6400 Coiltronics (561) 241-7876 Dale (605) 665-9301 Sumida (847) 956-0666 RESET COMPARATOR An additional comparator is provided to detect low VIN, low VOUT, or any other error condition that the user may want to sense. The inverting input of the comparator is connected to the 1.25V reference, and the non-inverting input is connected to the DETECT pin. The output of this comparator is connected to the RESET pin of the device and can swing from VOUT to ground. 5 ML4871 DESIGN CONSIDERATIONS (Continued) OUTPUT CAPACITOR current in the output capacitor ramps quickly to 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. 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 Suitable tantalum capacitors can be obtained from the following vendors: (2) 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 AVX (207) 282-5111 Kemet (846) 963-6300 Sprague (207) 324-4140 90 1000 800 VOUT = 3.3V EFFICIENCY (%) IOUT (mA) VOUT = 3.3V 600 VOUT = 5V 400 80 70 VOUT = 5V 200 0 1.0 2.0 3.0 60 5.0 4.0 VIN = 2.4V 1 10 VIN (V) Figure 4. IOUT vs. VIN Using the Circuit of Figure 8 1000 Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8 90 VOUT = 5V IIN (µA) 60 VOUT = 3.3V 30 0 1.0 2.0 3.0 4.0 5.0 VIN (V) Figure 6. No Load Input Current vs. VIN 6 100 IOUT (mA) ML4871 DESIGN CONSIDERATIONS (Continued) LAYOUT INPUT CAPACITOR Good layout practices will ensure the proper operation of the ML4871. Some layout guidelines follow: 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 47µF to 100µF. This filtering prevents the input ripple from affecting the ML4871 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 ML4871 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. • Use adequate ground and power traces or planes • Keep components as close as possible to the ML4871 • 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 ML4871 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 8. BATTERY MONITORING The condition of the batteries can be monitored using the DETECT pin. For primary batteries, the comparator can be used to signal that the batteries will soon need to be replaced. For rechargeable batteries, the comparator can be used to signal the start of a charging cycle. For input voltages greater than the minimum operating voltage, the RESET pin can be set to go low at a specified battery voltage by connecting a resistor divider across the battery stack and to the DETECT pin of the ML4871 as shown in Figure 7. The low battery trip voltage is determined by first choosing a minimum battery voltage, VIN(MIN), and then calculating the values of RA and RB: VIN( MIN) = 125 . 1R A + RB RB 6 (3) 7 VIN RESET 2 RA DETECT 4 + COMP RB VREF – FROM START-UP CIRCUITRY Figure 7. Battery Monitoring Circuit The values of RA and RB should be sufficiently large to minimize the power dissipation in the divider. Also, use care when selecting the low battery trip point. Too high a trip voltage can lead to memory effects in the battery, while too low a trip point can lead to reduced service life or polarity reversal. Refer to the manufacturer’s data sheets for more information on selecting and designing battery systems. Figure 8. Sample PC Board Layout 7 ML4871 DESIGN EXAMPLE In order to design a boost converter using the ML4871, 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. The complete circuit is shown in Figure 9. As mentioned previously, the use of an input supply bypass capacitor is highly recommended. First, it must be determined whether the ML4871 is capable of delivering the output current. This is done using Equation 1: IOUT( MAX) = 125 . 10µH (Sumida CD75) 30. V 0.7A = 0.53A 50. V ML4871 Next, select an inductor. As previously mentioned, the recommended inductance is 10µH. 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. VIN VL1 PWR GND VIN RESET GND 100µF DETECT Finally, the value of the output capacitor is determined using Equation 2: COUT = 44 10mH = 88mF 5.0V VL2 VOUT VOUT 100µF Figure 9. Typical Application Circuit The closest standard value would be a 100µF capacitor with an ESR rating of 100mW. If such a low ESR value cannot be found, two 47µF capacitors in parallel could also be used. IOUT(MAX) (mA) VIN (V) VOUT = 3.3V VOUT = 5.0V 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 4.8 386.2 451.9 521.5 585.9 651.0 716.5 782.0 286.2 332.1 379.1 430.0 479.0 525.4 571.8 618.5 665.0 711.7 758.7 805.3 851.9 899.0 946.1 992.7 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 Table 1. Typical IOUT and Efficiency vs. VIN 8 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 ML4871 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) 0º - 8º 0.055 - 0.061 (1.40 - 1.55) 0.012 - 0.020 (0.30 - 0.51) 0.004 - 0.010 (0.10 - 0.26) 0.015 - 0.035 (0.38 - 0.89) 0.006 - 0.010 (0.15 - 0.26) SEATING PLANE ORDERING INFORMATION PART NUMBER OUTPUT VOLTAGE TEMPERATURE RANGE PACKAGE ML4871CS-3 ML4871CS-5 3.3V 5.0V 0ºC to 70ºC 0ºC to 70ºC 8-Pin SOIC (S08) 8-Pin SOIC (S08) ML4871ES-3 ML4871ES-5 3.3V 5.0V –20ºC to 70ºC –20ºC to 70ºC 8-Pin SOIC (S08) 8-Pin SOIC (S08) © Micro Linear 1997. 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. 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. DS4871-01 2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com 9