1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout General Description Features The EC3211 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. The device is available in an adjustable version . Supply current during operation is only 20mA ● High Efficiency: Up to 96% and drops to ≤1mA in shutdown. The 2.5V to 5.5V ● 1.2A Output Current input voltage range makes the EC3211 ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. Automatic Burst Mode operation increases efficiency at light loads, further extending battery life. Switching frequency is internally set at 1.5MHz, allowing the use of small surface mount inductors and capacitors. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. Low output voltages are easily supported with the 0.6V feedback reference voltage. The EC3211 is available in TSOT23-5 package. ● 2.5V to 5.5V Input Voltage Range EC3211 ● High Efficiency at light loads ● Very Low Quiescent Current: Max 70uA During Operation ● 1.5MHz Constant Frequency Operation ● No Schottky Diode Required ● Low Dropout Operation: 100% Duty Cycle ● 0.6V Reference Allows Low Output Voltages ● Shutdown Mode Draws ≤1uA Supply Current ● Current Mode Operation for Excellent Line and Load Transient Response ● Over-temperature Protected ● TSOT23-5 Package is Available Applications ●Cellular Telephones ●Personal Information Appliances ●Wireless and DSL Modems ●Digital Still Cameras ●MP3 Players ●Portable Instruments Package Types TSOT23-5 Figure 1. Package Types of EC3211 E-CMOS Corp. (www.ecmos.com.tw) Page 1 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout Pin Assignments EC3211 Pin Name Description 1 RUN Run Control Input. Forcing this pin above 1.5V enables the part. Forcing this pin below 0.3V shuts down the device. In shutdown, all functions are disabled drawing <1μA supply current. Do not leave RUN floating. 2 GND Ground Pin. 3 SW Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. 4 VIN Main Supply Pin. Must be closely decoupled to GND, Pin 2, with a TSOT23‐5 E-CMOS Corp. (www.ecmos.com.tw) 5 VFB 5 VOUT Page 2 of 16 2.2μF or greater ceramic capacitor. Feedback Pin. Receives the feedback voltage from an external resistive divider across the output. Output Voltage Feedback Pin. An internal resistive divider divides the output voltage down for comparison to the internal reference voltage. 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Ordering Information Part Number EC3211ADJT2R Package TSOT23-5 Marking Marking Information 11AJf 1. Starting with underlined 1, a bar is for production year 2012. The next bar is mark on top of A is for year 2013. The next bar is mark on bottom of A is for year 2014.The next bar is mark on top of J is year for 2015. The naming pattern continues with consecutive characters for later years. 2. AJ：Adjustable Voltage 3. f is the week of production. The big character of A~Z is for the week of 1~26, and small a~z is for the week of 27~52. Functional Block Diagram Figure2:Functional Block Diagram of EC3211 E-CMOS Corp. (www.ecmos.com.tw) Page 3 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Type Application Circuit Figure 3. Type Application Circuit of EC3211 Absolute Maximum Ratings Parameter Value Unit Input Supply Voltage -0.3 ~6 V RUN, VFB Voltages -0.3 ~ VIN V SW Voltage -0.3V ~(VIN+0.3) V P-Channel Switch Source Current (DC) 1500 mA N-Channel Switch Sink Current (DC) 1500 mA Peak SW Sink and Source Current 1.8 A Operating Temperature Range -40~+85 ºC Junction Temperature 125 ºC Lead Temperature (Soldering, 10 sec) 260 ºC Storage Temperature Range -65~150 ºC Note1: Stresses greater than those listed under Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation is not implied. Exposure to absolute maximum rating conditions for extended periods may affect reliability. E-CMOS Corp. (www.ecmos.com.tw) Page 4 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 ELECTRICAL CHARACTERISTICS (VIN=3.6V,TA=25°C, Unless otherwise specified) Parameter Feedback current Symbol INFB Regulator Feedback Voltage VFB Reference Voltage Line Regulator VFB Peak Inductor Current IPK Output Voltage Load Regulator Input Voltage Range VLOADREG VIN Active Mode Input DC Bias Current Sleep Mode IS Shut down Oscillator Frequency fOSC RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET RPFET RNFET SW Leakage ILSW RUN Threshold RUN Leakage Current VRUN IRUN E-CMOS Corp. (www.ecmos.com.tw) Conditions TA=25℃ 0℃≦TA≦85℃ -40℃≦TA≦85℃ VIN=2.5V to 5.5V VIN=3V,VFB=0.5V or Vout=90%,Duty Cycles <35% -----VFB=0.5V or Vout=90%, ILoad=0A VFB=0.62V or Vout=103%, ILOAD=0A VRUN=0V,VIN=4.2V VFB=0.6V or Vout=100% VFB=0V or Vout=0V ISW =100mA ISW =-100mA VRUN=0V,VSW=0V or 5V, VIN=5V Page 5 of 16 Min --0.5880 0.5865 0.5850 --- Typ --0.6000 0.6000 0.6000 0.04 Max 30 0.6120 0.6135 0.6150 0.4 Unit nA V V V %/V 1.55 1.6 1.7 A --2.5 0.5 --- --5.5 % V --- 300 400 uA --- 45 70 uA --1.2 ------- 0.1 1.5 400 0.3 0.3 1 1.8 --0.4 0.4 uA MHz KHz Ω Ω --- 0.01 1 uA 0.3 --- 1 0.01 1.5 1 V uA 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Typical Performance Characteristics Reference Voltage Oscillator Frequency Oscillator Frequency vs Supply Voltage RDS(ON) vs Temperature E-CMOS Corp. (www.ecmos.com.tw) Page 6 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Typical Performance Characteristics(Cont.) RDS(ON) vs Input Voltage Efficiency vs Output Current Efficiency vs Output Current Efficiency vs Output Current E-CMOS Corp. (www.ecmos.com.tw) Page 7 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Typical Performance Characteristics(Cont.) Efficiency vs Output Current Output Voltage vs Output Current Efficiency vs Input Voltage Dynamic Supply Current vs Supply Voltage E-CMOS Corp. (www.ecmos.com.tw) Page 8 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Typical Performance Characteristics(Cont.) P-FET Leakage vs Temperature E-CMOS Corp. (www.ecmos.com.tw) N-FET Leakage vs Temperature Page 9 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Function Description Main Control Loop The EC3211 uses a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback voltage, FB, relative to the 0.6V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. the EA amplifier’s output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator is reduced to about 400kHz, 1/4 the nominal frequency. This frequency fold-back ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator’s frequency will progressively increase to 1.5MHz when VFB or VOUT rises above 0V. Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel Burst Mode Operation MOSFET and the inductor. An important detail to remember is that at low input The EC3211 is capable of Burst Mode operation in which supply voltages, the RDS(ON) of the P-channel switch the internal power MOSFETs operate intermittently increases (see Typical Performance Characteristics). based on load demand. Therefore, the user should calculate the power In Burst Mode operation, the peak current of the inductor dissipation when the EC3211 is used at 100% duty cycle with low input voltage (See Thermal Considerations in is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light the Applications Information section). loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 20mA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, E-CMOS Corp. (www.ecmos.com.tw) Page 10 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Function Description(Cont.) The basic EC3211 application circuit is shown in Figure 3. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Low Supply Operation The EC3211 will operate with input supply voltages as low as 2.5V, but the maximum allowable output current is reduced at this low voltage. Figure 4 shows the reduction Inductor Selection in the maximum output current as a function of input voltage for various output voltages. For most applications, the value of the inductor will fall in the range of 1uH to 4.7uH. Its value is chosen based on the desired ripple current. Large value inductors lower Slope Compensation and Inductor Peak ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the Current ripple current as shown in equation 1. A reasonable starting point for setting ripple current is DIL = 480mA Slope compensation provides stability in constant (40% of 1200mA). frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak The DC current rating of the inductor should be at least current for duty cycles >40%. However, the EC3211 uses equal to the maximum load current plus half the ripple a patent-pending scheme that counteracts this current to prevent core saturation. Thus, a 1320mA rated compensating ramp, which allows the maximum inductor inductor should be enough for most applications peak current to remain unaffected throughout all duty (1200mA + 120mA). For better efficiency, choose a low cycles. DC-resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 200mA. Lower inductor values (higher DIL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Figure 4.Maximum Output Current vs Input Voltage E-CMOS Corp. (www.ecmos.com.tw) Page 11 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Function Description(Cont.) This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is Inductor Core Selection commonly used for design because even significant deviations do not offer much relief. Note that the Different core materials and shapes will change the capacitor manufacturer’s ripple current ratings are often size/current and price/current relationship of an inductor. based on 2000 hours of life. This makes it advisable to Toroid or shielded pot cores in ferrite or permalloy further derate the capacitor, or choose a capacitor rated materials are small and don’t radiate much energy, but at a higher temperature than required. Always consult generally cost more than powdered iron core inductors the manufacturer if there is any question. with similar electrical characteristics. The choice of which The selection of COUT is driven by the required effective style inductor to use often depends more on the price vs series resistance (ESR). Typically, once the ESR size requirements and any radiated field/EMI requirement for COUT has been met, the RMS current requirements than on what the EC3211 requires to rating generally far exceeds the IRIPPLE(P-P) operate. Table 1 shows some typical surface mount requirement. The output ripple DVOUT is determined by: inductors that work well in EC3211 applications. Table 1. Representative Surface Mount Inductors CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: E-CMOS Corp. (www.ecmos.com.tw) where f = operating frequency, COUT = output capacitance and DIL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since DIL increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. Page 12 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Function Description(Cont.) Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the EC3211’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. However, care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. Figure 5:Setting the output Voltage Vout R1 R2 1.2V 150K 150K 1.5V 160K 240K 1.8V 150K 300K 2.5V 150K 470K 3.3V 150K 680K Table 2. Vout VS. R1, R2, Cf Select Table Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Output Voltage Programming Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the In the adjustable version, the output voltage is set by a losses in EC3211 circuits: VIN quiescent current and I2R resistive divider according to the following formula: losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve The external resistive divider is connected to the output, at very low load currents can be misleading since the allowing remote voltage sensing as shown in Figure5. actual power lost is of no consequence as illustrated in Figure 6. E-CMOS Corp. (www.ecmos.com.tw) Page 13 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Function Description(Cont.) RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. Thermal Considerations Figure 6:Power Lost VS Load Current 1. The VIN quiescent current is due to two components: the DC bias current as given in the electrical characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. In continuous mode, IGATECHG =f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is “chopped” between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: E-CMOS Corp. (www.ecmos.com.tw) In most applications the EC3211 does not dissipate much heat due to its high efficiency. But, in applications where the EC3211 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150°C, both power switches will be turned off and the SW node will become high impedance. To avoid the EC3211 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(qJA) where PD is the power dissipated by the regulator and qJA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the EC3211 in dropout at an input voltage of 2.7V, a load current of 800mA and an ambient temperature of 70°C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70°C is approximately 0.52W. Page 14 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Function Description(Cont.) Therefore, power dissipated by the part is: PD = ILOAD 2 • RDS(ON) = 187.2mW For the TSOT-23 package, the qJA is 250°C/ W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.1872)(250) = 116.8°C which is below the maximum junction temperature of 125°C. Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to (ΔILOAD • ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its steady state value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory. A second, more severe transient is caused by switching in loads with large (>1μF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 • CLOAD).Thus, a 10μF capacitor charging to 3.3V would require a 250μs rise time, limiting the charging current to about 130mA. E-CMOS Corp. (www.ecmos.com.tw) Page 15 of 16 3I12N-Rev.F001 1.5MHz, 1.2A, Synchronous Step-Down Regulator Dropout EC3211 Package Information TSOT23-5 E-CMOS Corp. (www.ecmos.com.tw) Package Outline Dimensions Page 16 of 16 3I12N-Rev.F001