July 2000 ML4866* 3.3V Output DC–DC Step-Down Converter GENERAL DESCRIPTION FEATURES The ML4866 is a high efficiency pulse width modulated (PWM) buck regulator designed for use in 5V systems or portable equipment that need a compact, efficienct 3.3V supply. It has a switching frequency of 120kHz and uses synchronous rectification to optimize power conversion efficiency. Unlike other solutions, the ML4866 requires no external diodes or FETs. ■ High power conversion efficiency over 2 decades of load current ■ No external FETs or diodes; minimum external components ■ 3.5V to 6.5V input voltage range The ML4866 can provide up to 500mA of output current, and operates over an input voltage range of 3.5V to 6.5V (3 to 4 cells or a 5 VDC supply). A complete switched mode power converter can be quickly and easily implemented with few external components. Thanks to a built-in autoburst mode, power conversion efficiency of this DC–DC converter can exceed 90% over more than 2 decades of output load current. ■ Significantly extends battery life over linear regulator based solutions ■ Micropower operation ■ Low shutdown mode quiescent current Stability and fast loop response are provided by current programming and a current sense circuit. The ML4866 also has a SHDN pin for use in systems which have power management control. Undervoltage lockout and soft start are also built in. (* Indicates Part is End Of Life as of July 1, 2000) BLOCK DIAGRAM 5 7 VIN 1 VOUT VL CURRENT SENSE BUCK CONTROL UVLO/ SHUTDOWN OSC ERROR AMPLIFIER SLOPE COMPENSATION REFERENCE – – BURST SHDN 6 3 VREF + + BURST 4 VREF COMP 2 GND 8 1 ML4866 PIN CONFIGURATION ML4866 8-Pin SOIC (S08) VOUT 1 8 GND COMP 2 7 VL VREF 3 6 SHDN BURST 4 5 VIN TOP VIEW PIN DESCRIPTION PIN NAME FUNCTION 1 VOUT Regulated 3.3V output 2 COMP 3 4 2 PIN NAME FUNCTION 5 V IN Input voltage Connection point for an external compensation network 6 SHDN Pulling this pin low shuts down the regulator V REF 1.25V reference output 7 VL Buck inductor connection BURST This pin controls when the control circuit switches between PWM and PFM modes of operation 8 GND Ground ML4866 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 ML4866CS ................................................. 0ºC to 70ºC ML4866ES .............................................. -20ºC to 70ºC ML4866IS ............................................... -40ºC to 85ºC VIN Operating Range ................................... 3.5V to 6.5V VIN ................................................................................................... 7V Voltage on any other pin ......... GND - 0.3V to VIN + 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 = 5V, L = 50µH, COUT = 100µF, RCOMP = 390kW, CCOMP = 15nF, TA = Operating Temperature Range (Note 1) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0 < I(VREF) < –5µA, IOUT = 0mA 1.22 1.25 1.27 V Oscillator Initial Accuracy IOUT = 200mA, TA = 25°C 100 115 165 kHz Oscillator Total Variation Line and Temp 90 130 185 kHz 3 5 ms REFERENCE VREF Output Voltage PWM REGULATOR fOSC Soft Start VIN to VOUT Delay BURST Burst Mode Threshold 250 BURST PWM Mode Threshold 400 500 BURST Bias Current Output Voltage mV 850 mV 35 µA IOUT = 200mA 3.2 3.3 3.4 V IOUT = 20mA, BURST = 0V 3.28 3.38 3.48 V ±2 % Line Regulation VIN = 4V to 6.5V, TA = 25°C Load Regulation IOUT = 100mA to 500mA, TA = 25°C ±2.5 % IOUT = 5mA to 100mA, BURST = 0V, TA = 25°C ±2.5 % Temperature Stability TA = -40°C to 85°C ±1 % Total Variation Line, Load, Temp ±5 % SHUTDOWN UVLO Startup Threshold 3.2 3.5 V UVLO Shutdown Threshold 2.9 3.1 V SHDN Threshold SHDN Bias Current 2 V –5 µA 3 ML4866 ELECTRICAL CHARACTERISTICS SYMBOL (Continued) PARAMETER CONDITIONS MIN TYP MAX UNITS IOUT = 0mA, BURST = 5V 400 500 µA IOUT = 0mA, BURST = 0V 120 220 µA SHDN = 0V 20 35 µA SUPPLY IIN VIN Current Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions. FUNCTIONAL DESCRIPTION The ML4866 is a current-mode, step-down (buck) converter designed to keep the buck inductor current in the continuous conduction mode (CCM). Current-mode operation provides faster output response to input voltage and output current changes along with cycle-by-cycle current limiting. CCM inductor current is preferred when the highest conversion efficiencies are required. ramping the inductor current down to 0mA. This action is repeated until the output voltage returns to its nominal setting and begins again when the output drops below its nominal setting. The rate or frequency at which this “bursting” occurs is directly proportional to the output current. When the average output current rises above 130mA, the ML4866 returns to PWM operation. For high efficiencies at low output current, the ML4866 contains an autoburst function which automatically switches from pulse width modulation (PWM) to pulsed frequency modulation (PFM) operation when the output current drops below 100mA. Selection of either mode is possible by applying the correct logic level signal to the BURST pin. When operating in PWM mode, loop compensation of the ML4866 is simplified due to its transconductance type error amplifier. For applications having a load current range of less than 100mA and greater than 130mA, the BURST pin should be left open and bypassed to ground with a 15nF or larger capacitor. It is possible to tailor an application for the highest possible efficiency by externally forcing the ML4866 into either control mode. Applying a logic low level to BURST forces the IC into PFM mode. Conversely, a logic high places it in PWM mode. Care should be taken to avoid reducing the efficiency by placing the controller in the least efficient mode for a given output current. An under voltage lockout (UVLO) circuit within the ML4866 enables the converter when the input voltage is greater than 3.25V and disables it when the input voltage is below 3.10V. The IC can also be disabled externally by applying a logic low signal to the SHDN pin. When disabled, the ML4866 draws less than 20µA of current. The internal 1.25V bandgap reference is made available via the VREF pin, and may be used for general applications requiring less than 10µA of current. For proper operation, this pin must always be bypassed to GND with a 100nF capacitor. BURST MODE Burst (PFM) mode is a method of regulating the output voltage by applying a variable frequency modulation technique to the buck inductor. This method maintains higher efficiencies at light loads than if PWM were used. If BURST is left open, the ML4866 switches from PWM mode to PFM mode when the output current falls below 100mA. When the output voltage falls out of regulation while in PFM mode, the internal buck switch turns on and ramps the inductor current up to 300mA. The buck switch then turns off and the synchronous switch turns on, 4 VIN – VOUT L –VOUT L ∆I Figure 1. Inductor Current ML4866 DESIGN CONSIDERATIONS 100 98 Figure 1 shows the inductor current in a step-down converter operating in CCM. Note that the inductor current does not reach zero during each switching cycle. This is unlike discontinuous conduction mode (DCM) where the inductor current is allowed to reach zero. CCM operation generally results in lower peak to peak output ripple voltage and higher circuit efficiencies because of lower peak and RMS currents in the switching FETs and buck inductor. The minimum value of inductance required for CCM operation with a 6.5V input and a load range of 100mA to 500mA is: 96 L> L> VOUT ( VIN ( MAX) - VOUT ) 2 VIN ( MAX) IOUT IL(P –P) = ( MAX) ( VIN ( MIN) - VOUT 88 86 ( MAX) ) (2) 2 3.465V ( 4.0V - 3.465V) = 103mA 4.0V 90kHz 100mH ( MAX ) + VOUT IL(PEAK) = IOUT ( MAX) + ( MAX ) ( VIN ( MIN) - VOUT ( MAX) ) VIN ( MIN) fSW L (3) For the highest efficiency, inductor core and copper losses must be minimized. Good high frequency core material such as Kool-Mu, ferrite or Molyperm are popular choices for this converter. Disregarding physical size requirements, the lowest loss inductor will generally be the one with the highest peak current rating. Figure 2 displays the efficiency of the ML4866 under various input voltage and output current conditions. These results were obtained using a Coiltronics CTX100-4 inductor having the following specifications: Peak Current Rating - 950mA DC Resistance - 175mW 3.5 4.0 4.5 5.0 5.5 6.0 6.5 A partial listing of inductor manufacturers with standard parts which meet the criteria for use with the ML4866 is given below. Coiltronics Dale Coilcraft XFMRS, Inc Sumida (561) 241-7876 (605) 665-9301 (847) 639-6400 (317) 834-1066 (847) 956-0666 CAPACITOR SELECTION 3.465V ( 4.0V - 3.465V) = 550mA 4.0V 120kHz 100mH Nominal Inductance - 100µH IOUT = 500mA Figure 2. Efficiency vs. Input Voltage VIN ( MIN) fSW ( MIN) L IL(PEAK) = IOUT IOUT = 10mA 92 (1) To guarantee reliable operation, the peak inductor current must be between 80% and 85% of its maximum rated value. This value is the sum of the inductor peak to peak current and the maximum output current: 2 VOUT 94 90 3.3V (6.5V - 33 . V) > 68mH 2 6.5V 100mA 120kHz IL(P –P) = IOUT = 100mA INPUT VOLTAGE (V) fSW ( MIN) EFFICIENCY (%) INDUCTOR SELECTION A typical digital system requires a peak to peak output ripple voltage of no greater than 1% to 3% of the nominal output voltage. In a step-down converter, the largest contributor to ripple voltage is almost always the product of the inductor peak-to-peak current times the output capacitor’s equivalent series resistance. To select the correct capacitor, first calculate the minimum capacitance value required: C OUT > C OUT > VOUT ( VIN ( MAX) - VOUT ) (4) 8 VP -P ( MAX) VIN ( MAX) L fSW 2 . V) 3.3 (6.5V - 33 8 33mV 6.5V 100mH 120kHz 2 > 4.27mF Next, calculate the maximum permissible ESR of the output capacitor: ESR < (0.033) < 0.33W (0.1) (5) When limited space is available, tantalum capacitors are the best choice. Electrolytic capacitors can be used and will be less expensive, but the ESR for low capacitance values as needed here will be much higher than for the same value tantalum. Table 2 lists the ESR values for a number of general purpose tantalum capacitors which are widely available from a number of sources. A 47µF capacitor was chosen for the design example. 5 ML4866 DESIGN CONSIDERATIONS (Continued) FREQUENCY COMPENSATION VARYING LOAD CURRENT Frequency compensation of the ML4866 is required when the converter is operating in PWM mode. Two simple methods are provided to ensure the converter is frequency stable. Both these methods will work only if the inductor current is selected to be in CCM at the maximum load current (see Inductor Selection). The first, called dominant pole compensation, is used when non-varying loads are expected. This method requires a single capacitor connected from the error amplifier output (COMP Pin) to ground. To minimize output voltage variations due to rapidly changing load currents, use the series RC zero compensation method to find the compensation network component values that will improve the output voltage response to load transients. For loads which change suddenly, the transient response (or bandwidth) of the circuit must be increased to prevent the output voltage from going outside of the regulation band. The method used to accomplish this is called zero/pole compensation and requires a series resistor capacitor network from COMP to ground. To determine which method works best for a given application, apply the components found from the zero/pole compensation method to an actual circuit and examine the output voltage variation. If the voltage variation is acceptable, connect the simpler, single capacitor and re-check the output voltage for acceptable load transient response. The unity gain bandwidth of the converter is extended to 15kHz using an RC network determined by: R COMP > f G , where G = O gm fCOMP (7) C COMP = 1 50p R COMP (8) Where f0 = 15kHz, fCOMP = 640Hz, RCOMP > 375kW (use 390kW, 5%), and CCOMP = 16nF (use 15nF). Either method of compensation for CCM mode with result in continued stability as the ML4866 changes to DCM mode at lighter load currents. Figure 3 shows a typical application circuit for the ML4866. NON-VARYING LOAD CURRENT For the best possible response to load transients using only a single capacitor, dominant pole compensation is implemented with a single capacitor value of: C COMP = gm 2 fCOMP (6) Where fCOMP is the unity gain crossover point (640Hz), gm = 62.5µmho, and CCOMP > 15.5nF (choose a standard 18nF or 22nF capacitor). The value of CCOMP can be increased but at the risk of increased output voltage variations with transient loads. VOUT 3.3V 33µF CAPACITANCE VOLTAGE RATING SIZE ESR @ 100kHz 4.7µF 16V 3216 0.490W 10µF 6.3V 3216 0.368W 22µF 16V 7343 0.149W 33µF 6.3V 6032 0.291W 47µF 10V 7343 0.144W 100µF 6.3V 7343 0.088W Table 2. ESR Values for Low Cost Tantalum Capacitors 6 100µH ML4866 VOUT 390kΩ COMP VREF BURST 15nF 100nF 15nF 1 8 2 7 3 6 4 5 GND VL SHDN VIN 100µF VIN 3.5V to 6.5V 100nF Figure 3. Typical Application Circuit ML4866 LAYOUT For proper performance, all components should be placed as close to the ML4866 as possible. Particular attention should be paid to minimize the length of the connections between the COMP and VREF pins to GND. Also avoid bringing these traces and the associated components close to VL. It is always recommended that a 10µF or greater capacitor be connected to VIN of the ML4866. A 33µF tantalum capacitor and 100nF film or ceramic capacitor is recommended when powering the ML4866 from Lithium or Alkaline cells. Ground and power planes must be large enough to carry the current the converter has been designed to supply. A sample PC board layout is shown in Figure 4. Figure 4. Sample PC Board Layout 7 ML4866 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 0.050 BSC (1.27 BSC) 0.017 - 0.027 (0.43 - 0.69) (4 PLACES) 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 ML4866CS (End Of Life) ML4866ES (EOL) ML4866IS (Obsolete) 0ºC to 70ºC -20ºC to 70ºC -40ºC to 85ºC 8-Pin SOIC (S08) 8-Pin SOIC (S08) 8-Pin SOIC (S08) DS4866-01 © 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. Japan: 2,598,946; 2,619,299. 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. 8 2092 Concourse Drive San Jose, CA 95131 Tel: (408) 433-5200 Fax: (408) 432-0295 www.microlinear.com 9/8/97 Printed in U.S.A.