July 2000 ML4771* High Current Boost Regulator GENERAL DESCRIPTION FEATURES The ML4771 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 ML4771 is capable of start-up with input voltages as low as 1.8V, and the output voltage can be set anywhere between 3.0V and 5.5V by an external resistor divider connected to the SENSE pin. ■ 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 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 ML4771 requires a minimum number of external components to build a very small regulator circuit capable of achieving conversion efficiencies exceeding 85%. ■ Maximum switching frequency > 200kHz ■ Minimum external components ■ Low ON resistance internal switching FETs ■ Adjustable output voltage (3.0V to 5.5V) (* Indicates Part is End Of Life as of July 1, 2000) BLOCK DIAGRAM L1 1 6 VL2 VL1 VIN 2 VOUT *CIN VOUT 5 R1 VBAT GND 3 BOOST CONTROL SENSE COUT *CFB 4 R2 PWR GND 8 1 ML4771 PIN CONFIGURATION ML4771 8-Pin SOIC (S08) VL1 1 8 PWR GND VIN 2 7 NC GND 3 6 VL2 SENSE 4 5 VOUT TOP VIEW PIN DESCRIPTION PIN NAME FUNCTION PIN NAME FUNCTION 1 VL1 Boost inductor connection 5 VOUT Output of the boost regulator 2 VIN Battery input voltage 6 VL2 Boost inductor connection 3 GND Ground 7 NC No connection 4 SENSE Programming pin for setting the output voltage 8 PWR GND Return for the NMOS output transistor 2 ML4771 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 ML4771CS ................................................. 0ºC to 70ºC ML4771ES ............................................. –20ºC to 70ºC VIN Operating Range ML4771CS .................................... 1.8V to VOUT – 0.2V ML4771ES .................................... 2.0V to VOUT – 0.2V VOUT Operating Range ................................. 3.0V to 5.5V 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 2 5 µA 25 35 µA 1 µA SUPPLY IIN VIN Current IOUT(Q) IL(Q) VIN = VOUT – 0.2V VOUT Quiescent Current VL Quiescent Current PFM REGULATOR IL Peak Current 1.2 1.4 1.7 A VSENSE SENSE Comparator Threshold Voltage 2.52 2.57 2.62 V VOUT Output Voltage See Figure 1, IOUT = 0 4.95 5.05 5.15 V Load Regulation See Figure 1, VIN = 2.4V, IOUT = 220mA 4.85 4.95 5.15 V Note 1: Limits are guaranteed by 100% testing, sampling or correlation with worst case test conditions. 3 ML4771 20µH (Sumida CD75) ML4771 VIN VL1 PWR GND VIN NC GND 100µF IOUT VL2 SENSE VIN VOUT 200µF 259kΩ 268kΩ Figure 1. Application Test Circuit. IL 1 VIN 2 6 VL2 VL1 RSENSE Q2 SYNCHRONOUS RECTIFIER CONTROL START-UP VOUT VOUT A2 + 5 – R1 SENSE A3 + BOOST CONTROL – A1 + Q1 – ISET GND PWR GND 3 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 4 2.57V R2 ML4771 FUNCTIONAL DESCRIPTION DESIGN CONSIDERATIONS The ML4771 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 ML4771 very easy to use. OUTPUT CURRENT CAPABILITY REGULATOR OPERATION The ML4771 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 ML4771 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 ML4771 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 5 ML4771 DESIGN CONSIDERATIONS (Continued) 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) 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 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 (207) 282-5111 Kemet (846) 963-6300 Sprague (207) 324-4140 90 1000 600 VOUT = 3.0V VOUT = 3.0V EFFICIENCY (%) IOUT (mA) 800 VOUT = 5.5V 400 80 VOUT = 5.5V 70 200 0 1.5 2.5 3.5 4.5 5.5 VIN (V) 60 VIN = 2.4V 1 10 100 Figure 4. IOUT vs. VIN Using the Circuit of Figure 8 Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8 160 VOUT = 5.5V IIN (µA) 120 80 40 0 1.5 VOUT = 3.0V 2.5 3.5 4.5 5.5 VIN (V) Figure 6. No Load Input Current vs. VIN 6 1000 IOUT (mA) Figure 7. Sample PC Board Layout ML4771 DESIGN CONSIDERATIONS DESIGN EXAMPLE (Continued) 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 47µF to 100µF. This filtering prevents the input ripple from affecting the ML4771 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 ML4771 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. 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. First, it must be determined whether the ML4871 is capable of delivering the output current. This is done using Equation 1: IOUT( MAX) = 125 . 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. Then, the value of the output capacitor is determined using Equation 2: SETTING THE OUTPUT VOLTAGE The adjustable output of the ML4771 requires an external feedback resistor divider to set VOUT. The output voltage can be determined from the following equation: VOUT 1R + R 6 = 257 . 1 2 R2 30. V 0.7A = 0.53A 50. V (3) where R1 and R2 are connected as shown in Figure 2. The value of R2 should be 250kW or less to minimize bias current errors. Choose an appropriate value for R2 and calculate R1. COUT = 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. Finally, the values of R1 and R2 are calculated using equation 3, assuming that R2 = 250kW: R1 = LAYOUT Good layout practices will ensure the proper operation of the ML4771. Some layout guidelines follow: 44 10mH = 88mF 5.0V 5.0 250kW - 250kW = 236kW 257 . The complete circuit is shown in Figure 8. As mentioned previously, the use of an input supply bypass capacitor is highly recommended. • Use adequate ground and power traces or planes • Keep components as close as possible to the ML4771 10µH (Sumida CD75) • 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 ML4771 ground pin, and the input and output capacitors ML4771 VL1 • Separate the ground for the converter circuitry from the ground of the load circuitry and connect at a single point VIN 100µF VIN NC GND VL2 SENSE A sample layout is shown in Figure 7. PWR GND VOUT VOUT 100µF 236kΩ 250kΩ Figure 8. Typical Application Circuit 7 ML4771 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 TEMPERATURE RANGE PACKAGE ML4771CS (End Of Life) ML4771ES (Obsolete) 0ºC to 70ºC –20ºC to 70ºC 8-Pin SOIC (S08) 8-Pin SOIC (S08) © Micro Linear 2000. 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; 5,747,977; 5,754,012; 5,757,174; 5,767,653; 5,777,514; 5,793,168; 5,798,635; 5,804,950; 5,808,455; 5,811,999; 5,818,207; 5,818,669; 5,825,165; 5,825,223; 5,838,723; 5.844,378; 5,844,941. Japan: 2,598,946; 2,619,299; 2,704,176; 2,821,714. Other patents are pending. DS4771-01 8 2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 www.microlinear.com