ML4865

July 2000 ML4865 High Voltage High Current Boost Regulator GENERAL DESCRIPTION The ML4865 is a high voltage, 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 ML4865 is capable of start-up with input voltages as low as 1.8V and generates a 12V output with output voltage accuracy of ±4%. Unlike most boost regulators, the ML4865 isolates the load from the battery when the SHDN pin is high. 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 ML4865 requires only one inductor and two capacitors to build a very small regulator circuit capable of achieving conversion efficiencies approaching 90%. FEATURES s Guaranteed full load start-up and operation at 1.8V input s Continuous conduction mode for high output current s Very low quiescent current s Pulse frequency modulation and internal synchronous rectification for high efficiency s Maximum switching frequency > 200kHz s Minimum external components s Low ON resistance internal switching FETs s Fixed 12V output can be adjusted to lower output voltages BLOCK DIAGRAM 4 VL1 6 VL2 SHUTDOWN CONTROL SHDN 7 VIN 3 START-UP SYNCHRONOUS RECTIFIER CONTROL VOUT + – 8 + – SHDN BOOST CONTROL + – FEEDBACK CONTROL 2.42V SENSE 1 PWR GND 5 GND 2 1 ML4865 PIN CONFIGURATION ML4865 8-Pin SOIC (S08) SENSE GND VIN VL1 1 2 3 4 8 7 6 5 VOUT SHDN VL2 PWR GND TOP VIEW PIN DESCRIPTION PIN NAME FUNCTION PIN NAME FUNCTION 1 SENSE Programming pin for setting the output to any value lower than the normal fixed voltage Ground 5 6 PWR GND VL2 SHDN Return for the internal power transistors Boost inductor connection Pulling this pin to VIN through an external resistor shuts down the regulator, isolating the load from the input. Boost regulator output 2 3 4 GND VIN VL1 7 Battery input voltage Boost inductor connection 8 VOUT 2 ML4865 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. Voltage on any Pin ......................... GND – 0.3V to 16.5V Peak Switch Current (IPEAK) ......................................... 2A Average Switch Current (IAVG) ..................................... 1A Junction Temperature .............................................. 150ºC Storage Temperature Range ...................... –65ºC to 150ºC Lead Temperature (Soldering, 10 sec) ..................... 150ºC Thermal Resistance (qJA) .................................... 160ºC/W OPERATING CONDITIONS Temperature Range ML4865CS-2 .............................................. 0ºC to 70ºC ML4865ES-2 ............................................ –20ºC to 70ºC VIN Voltage Range Without external rectifier ............................ 1.8V to 6V With external rectifier ................................ 1.8V to 10V ELECTRICAL CHARACTERISTICS Unless otherwise specified, VIN = Operating Voltage Range, TA = Operating Temperature Range (Note 1) SYMBOL SUPPLY IIN VIN Current VOUT Quiescent Current VL Quiescent Current SHDN = 0 or VIN VOUT = VOUT(MAX) + 5% 0V < VL2 < VOUT –1 10 20 25 30 1 µA µA µA PARAMETER CONDITIONS MIN TYP MAX UNITS PFM REGULATOR IL(PEAK) V OUT IL Peak Current Output Voltage Load Regulation VIN = 5V See Figure 1 VIN = 5V, SENSE = open, IOUT = 0 See Figure 1 VIN = 2.4V, IOUT = 40mA VIN = 5V, IOUT = 160mA 0.8 11.72 11.52 11.52 1.2 12.1 12.0 12.0 1.6 12.48 A V V V FEEDBACK Threshold Voltage Input Bias Current 2.38 –150 2.42 2.48 150 V nA SHUTDOWN Threshold Voltage Input Bias Current Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions. VSHDN = high to low 0.4 –150 0.8 1.6 150 V nA 3 ML4865 ML4865 SENSE VIN GND VIN 100µF VL1 VOUT SHDN VL2 PWR GND 100µF IOUT 27µH (Sumida CD75) Figure 1. Application Test Circuit L1 VIN 3 VIN 4 VL1 6 VL2 Q3 150mΩ SHUTDOWN CONTROL SHDN 7 A1 + – Q2 A2 VOUT + – 8 C1 R1 + VOUT – BOOST CONTROL Q1 SENSE FEEDBACK CONTROL + – 1 R2 2.42V ISET A3 Figure 2. PFM Regulator Detailed Block Diagram IL(MAX) IL ISET 0 VOUT VL 0 Q1 ON Q2 OFF Q1 OFF Q2 ON Figure 3. Inductor Current and Voltage Waveforms 4 ML4865 FUNCTIONAL DESCRIPTION The ML4865 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, eliminate an external component, and allows for load disconnect. Also included on-chip are an NMOS switch and current sense resistor, further reducing the number of external components, which makes the ML4865 very easy to use. 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. REGULATOR OPERATION The ML4865 is a variable frequency, current mode switching regulator. Its unique control scheme converts efficiently over more than three decades of load current. A detailed block diagram of the boost converter is shown in Figure 2. Error amplifier A3 converts deviations in the desired output voltage to a small current, ISET. The inductor current is measured through a 150mW 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 1.2A. 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 25µA of supply current is drawn from the output. This allows the part to remain efficient even when the load current drops below 250µA. SHUTDOWN The SHDN pin should be held low for normal operation. Raising the shutdown voltage above the threshold level will disable the synchronous rectifier, Q2 and Q3, and force ISET to zero. This prevents switching from occurring and disconnects the body diode of Q2 from the output. As a result, the output voltage is allowed to drop below the input voltage and current is prevented from flowing from the input to the output. FEEDBACK The SENSE pin should be left open or bypassed to ground for normal operation. The addition of the resistor divider R1 and R2 causes the input of error amplifier A3 to reach the threshold voltage before the internal resistors do. This allows the ML4865 to provide output voltages lower than the preset 12V if desired. 5 ML4865 DESIGN CONSIDERATIONS INPUT VOLTAGE RANGE The input voltage range determines whether an external Schottky diode is necessary or optional. If the input voltage is 6V or lower, the ML4865 can be operated as a stand alone boost regulator with a shutdown that fully isolates the input from the output. Adding an optional Schottky diode extends the input voltage range up to 10V, and improves the efficiency and the output current capability. However, the external diode now provides a leakage path from the input to the output during shutdown. 900 WITH EXTERNAL SCHOTTKY 700 IOUT (mA) 500 300 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 dependent on the input voltage. The full load efficiency may be as low as 65% when the ML4865 is used without a Schottky diode and can exhibit an input voltage dependence when an external diode is used. 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 empirical equation: IOUT( MAX) @ VIN ™ IIN ™ h (A) VOUT (1) 100 0 0 2 4 WITHOUT EXTERNAL SCHOTTKY 6 VIN (V) 8 10 Figure 4. Output Current vs. Input Voltage 100 VIN = 10V 90 VOUT = 12V EFFICIENCY (%) 80 VIN = 5V Where, for applications using the internal synchronous rectifier: IOUT( MAX) V @ IN ´ 0.05 ´ VIN + 0.4 ´ 0.65 VOUT 70 VIN = 2V 21 6 7 60 with Schottky without Schottky 50 1 10 IOUT (mA) 100 1000 IIN = 0.05 ´ VIN + 0.4 h = 0.65 And for applications using an external Schottky: IOUT( MAX) @ VIN ™ 0.07 ™ VIN + 0.4 ™ 0.025 ™ VIN + 0.65 VOUT Figure 5. Efficiency vs. Output Current 300 21 6 7 21 6 7 IIN (mA) 250 IIN = 0.07 ´ VIN + 0.4 200 WITH EXTERNAL SCHOTTKY h = 0.025 ´ VIN + 0.65 The curves and the equations are based on the operating circuit shown in Figure 7. It is recommended to verify the current capability and efficiency for the components selected. 150 100 50 WITHOUT EXTERNAL SCHOTTKY 0 2 4 VIN (V) 6 8 10 0 Figure 6. No Load Input Current vs. Input Voltage for the Circuit of Figure 7 6 ML4865 DESIGN CONSIDERATIONS INDUCTOR SELECTION The ML4865 is able to operate over a wide range of inductor values. A value of 22µH or 33µH is a good choice, but any value between 15µH and 50µH 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 15µH 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 ML4865 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 (Continued) COUT = 10 ™ L VOUT (F) (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 200mW of ESR. Suitable tantalum capacitors can be obtained from the following vendors: AVX Sprague Kemet TPS Series 593D Series T495 Series (207) 282-5111 (207) 324-4140 (864) 963-6300 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 22µF to 68µF. This filtering prevents the input ripple from affecting the ML4865 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 ML4865 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. 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 (due to capacitance) at full load current, use the following equation: ML4865 SENSE VIN C1 47µF GND VIN VL1 VOUT SHDN VL2 PWR GND R1 1MΩ 22µH (Sumida CD75) D1 MBR0520L VOUT C2 47µF Figure 7. Typical Application Circuit. 7 ML4865 DESIGN CONSIDERATIONS SHUTDOWN The SHDN pin is a high impedance input and is noise sensitive. Either drive the SHDN input from a low impedance source or bypass the pin to GND with a 10nF ceramic capacitor. (Continued) If an external Schottky is required, look for a device with a voltage rating of 20V or greater. The average forward current rating should be at least 500mA, and the forward voltage should be 600mV or less. Suitable Schottky rectifiers can be obtained from the following vendors: Diodes, Inc B120 (805) 446-4800 (310) 322-3331 (602) 897-5056 SENSE Int’l Rectifier 10BQ040 The SENSE pin should be left open or bypassed to ground for normal operation. The output can be set to voltages lower than the preset value by adding a resistor divider. The output voltage can be determined from the following equation: VOUT R1 + R2 = 2.42 ´ (V) R2 (3) Motorola MBR0520L LAYOUT Good layout practices will ensure the proper operation of the ML4865. Some layout guidelines follow: • Use adequate ground and power traces or planes • Keep components as close as possible to the ML4865 • 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 ML4865 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. where R1 and R2 are connected as shown in Figure 2. The value of R2 should be 1MW or less to minimize bias current errors. Choose an appropriate value of R2 and calculate the value of R1. R1 = R2 ´  V - 1 (W)  2.42  OUT (4) EXTERNAL SCHOTTKY RECTIFIER Due to excessive power dissipation, an external Schottky rectifier is required when operating at input voltages above 6V. Even for applications where the input voltage is below 6V, the use of an external rectifier may be necessary to achive efficiency or output current requirements. Figure 8. Sample ML4865 Layout 8 ML4865 DESIGN EXAMPLE In order to design a boost converter using the ML4865, it is necessary to define the values of a few parameters. For this example, assume the following design parameters: VIN = 4.75 to 5.25V VOUT = 12V IOUT(MAX) = 150mA Shutdown required First, it must be determined whether the ML4865 is capable of delivering the output current without an external Schottky rectifier. This is done using Equation 1: IOUT( MAX) @ IOUT( MAX) @ VIN ™ 0.05 ™ VIN + 0.4 ™ 0.65 VOUT 5.25 ™ 0.05 ™ 5.25 + 0.4 ™ 0.65 = 188mA 12 21 6 7 20 5 7 Next, select an inductor. As previously mentioned, the recommended inductance is 22µH. 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 110 to 220mW. A Sumida CD75-220 meets these requirements. Finally, the value of the output capacitor is determined using Equation 2: COUT = 10 ™ L 10 ™ 22mH = = 18.3mF VOUT 12V The closest standard value would be a 22µF capacitor with an ESR rating of 200mW. An AVX TPSD226M025R0200 would be a good choice. As mentioned previously, the use of an input supply bypass capacitor is highly recommended. Since the output capacitance meets the minimum input capacitance recommended it can also be used for the input. 9 ML4865 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 ML4865CS-2 ML4865ES-2 (Obsolete) OUTPUT VOLTAGE 12V 12V TEMPERATURE RANGE 0ºC to 70ºC –20ºC to 70ºC PACKAGE 8-Pin SOIC (S08) 8-Pin SOIC (S08) © 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. 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 www.microlinear.com DS4865-01 1