2.4A/32V Synchronous Rectified Step-Down Converter General Description EC3278 Features The EC3278 is a monolithic synchronous buck regulator. • The device integrates two 90mΩ MOSFETs, and provides • 2.4A Output Current 2.4A of continuous load current over a wide input voltage • • of 4.75V to 32V. Current mode control provides fast Integrated 90mΩ Power MOSFET Switches transient response and cycle-by-cycle current limit. Wide 4.75V to 32V Operating Input Range Output Adjustable from 0.923V to 30V • Up to 93% Efficiency An adjustable soft-start prevents inrush current at turn-on, • and in shutdown mode the supply current drops to 1µA. • Programmable Soft-Start SOP-8(Exposed PAD) • package, provides a very compact solution with minimal • • external components. Fixed 200KHz Frequency This device, available in Stable with Low ESR Ceramic Output Capacitors an Cycle-by-Cycle Over Current Protection Input Under Voltage Lockout Applications • • Distributed Power Systems Networking Systems • FPGA, DSP, ASIC Power Supplies • Green Electronics/ Appliances • Notebook Computers Typical Application Circuit Fig1. EC3278 with 5V Output, 470µF/16V Electrolytic Output Capacitor E-CMOS Corp. (www.ecmos.com.tw) Page 1 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Pin Configurations Figure 2 Pin Configuration of EC3278(Top View) Pin Description Pin Number Pin Name Description High-Side Gate Drive Boost Input. BS supplies the drive for the high-side N-Channel 1 BS MOSFET switch. Connect a 0.01µF or greater capacitor from SW to BS to power the high side switch. Power Input. IN supplies the power to the IC, as well as the step-down converter 2 IN switches. Drive IN with a 4.75V to 32V power source. Bypass IN to GND with a suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor. Power Switching Output. SW is the switching node that supplies power to the output. 3 SW Connect the output LC filter from SW to the output load. Note that a capacitor is required from SW to BS to power the high-side switch. 4 GND 5 FB Ground. Feedback Input. FB senses the output voltage to regulate that voltage. Drive FB with a resistive voltage divider from the output voltage. The feedback threshold is 0.923V. See Setting the Output Voltage. Compensation Node. COMP is used to compensate the regulation control loop. 6 COMP Connect a series RC network from COMP to GND to compensate the regulation control loop. In some cases, an additional capacitor from COMP to GND is required. See Compensation Components. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to 7 EN turn on the regulator, drive it low to turn it off. Pull up with 100kΩ resistor for automatic startup. Soft-Start Control Input. SS controls the soft start period. Connect a capacitor from SS 8 SS to GND to set the soft-start period. A 0.1µF capacitor sets the soft-start period to 15ms. To disable the soft-start feature, leave SS unconnected. E-CMOS Corp. (www.ecmos.com.tw) Page 2 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Ordering Information Part Number Package Marking EC3278NNMHR SOP-8L (Exposed PAD) EC3278 LLLLL YYWWT Marking Information LLLLL is Lot Number YYWW is date code T is internal tracking code Package Types Figure 3. Package Types of EC3278 E-CMOS Corp. (www.ecmos.com.tw) Page 3 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Function Block Diagram Figure 4 Function Block Diagram of EC3278 Absolute Maximum Ratings Parameter Symbol Value Unit Supply Voltage VIN -0.3 to 32 V Switch Node Voltage VSW 30 V Boost Voltage VBS VSW – 0.3V to VSW +6V V Output Voltage VOUT 0.923V to 30 V –0.3V to +6V V TJ 150 ºC Storage Temperature TSTG -65 to 150 ºC Lead Temperature (Soldering, 10 sec) TLEAD 260 ºC 2000 V All Other Pins Operating Junction Temperature ESD (HBM) MSL Level3 RθJA RθJC Thermal Resistance-Junction to Ambient Thermal Resistance-Junction to Case E-CMOS Corp. (www.ecmos.com.tw) Page 4 of 11 90 45 ºC / W ºC / W 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Electrical Characteristics VIN = 12V, Ta = 25℃ unless otherwise specified. Parameters Symbol Shutdown Supply Current Min. VEN = 0V VEN = 2.0V; VFB = Supply Current Feedback Voltage Test Condition 1.0V VFB 4.75V ≤ VIN ≤ 30V 0.900 Feedback Overvoltage Threshold Typ. Max. Unit 1 3.0 µA 1.3 1.5 mA 0.923 0.946 V 1.1 V 400 V/V 800 µA/V Error Amplifier Voltage Gain * AEA Error Amplifier Trans-conductance GEA High-Side Switch On Resistance * RDS(ON)1 90 mΩ Low-Side Switch On Resistance * RDS(ON)2 90 mΩ High-Side Switch Leakage ∆IC = ±10µA VEN = 0V, VSW = 0V Current 10 µA Upper Switch Current Limit Minimum Duty Cycle 2.9 A Lower Switch Current Limit From Drain to Source 1.1 A GCS 4.8 A/V Fosc1 200 KHz COMP to Current Sense Trans-conductance Oscillation Frequency Short Circuit Oscillation Frequency Maximum Duty Cycle Fosc2 VFB = 0V 100 KHz DMAX VFB = 1.0V 90 % 220 ns Minimum On Time * EN Shutdown Threshold Voltage VEN Rising 1.1 EN Shutdown Threshold Voltage 2.0 210 Hysteresis EN Lockout Threshold Voltage 2.2 EN Lockout Hysterisis E-CMOS Corp. (www.ecmos.com.tw) 1.5 2.5 210 Page 5 of 11 V mV 2.7 V mV 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Electrical Characteristics(Cont.) VIN = 12V, Ta = 25℃ unless otherwise specified. Parameters Symbol Input Under Voltage Lockout Threshold Test Condition VIN Rising Input Under Voltage Lockout Min. 3.80 Typ. 4.10 Max. 4.40 Unit V 210 mV 6 µA Threshold Hysteresis Soft-Start Current VSS = 0V Soft-Start Period CSS = 0.1µF Thermal Shutdown * 15 ms 160 °C Typical Performance Characteristics VIN=12V Vout=5V No Load VIN=12V Vout=5V Load=2A 1-Vout 2-SW 4-ISW 1-Vout 2-SW 4- ISW Figure 5. Steady State Test E-CMOS Corp. (www.ecmos.com.tw) Figure 6. Steady State Test Page 6 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter VIN=12V Vout=5V Load=1A~2A 1-Vout 4-Load VIN=12V Vout=5V 1-Vout 4- ISW Figure 7. Load Transient Test VIN=12V EC3278 Figure 8. Short Circuit Test Vout=5V 1-Vout 4- ISW Figure 9. Short Circuit Recovery E-CMOS Corp. (www.ecmos.com.tw) Page 7 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Function Description Component Selection Where VOUT is the output voltage, VIN is the input voltage, fS is the switching frequency, and ΔIL is the peak-to-peak Setting the Output Voltage The output voltage is set using a resistive voltage divider inductor ripple current. Choose an inductor that will not saturate under the from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by maximum inductor peak current. The peak inductor current can be calculated by: the ratio: Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is: Where ILOAD is the load current. The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirements. Optional Schottky Diode During the transition between high-side switch and low-side Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the switch, the body diode of the lowside power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 1 lists example Schottky diodes and their Manufacturers. Table 1： Part Number Voltage/Current Vendor B140 40V, 1A Diodes, Inc. Also, make sure that the peak inductor current is below the SK14 40V, 1A Diodes, Inc. maximum switch current limit. The inductance value can be MBRS140 40V, 1A International Rectifier peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. calculated by: Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose X5R or X7R dielectrics when using ceramic capacitors. E-CMOS Corp. (www.ecmos.com.tw) Page 8 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: The worst-case condition occurs at VIN = 2VOUT,where IC1 = ILOAD/2. For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum The characteristics of the output capacitor also affect the load current. stability of the regulation system. The EC3278 can be The input capacitor can be electrolytic, tantalum or ceramic. optimized for a wide range of capacitance and ESR values. When using electrolytic or tantalum capacitors, a small, high Compensation Components quality ceramic capacitor, i.e. 0.1μF, should be placed as EC3278 employs current mode control for easy close to the IC as possible. When using ceramic capacitors, compensation and fast transient response. The system make sure that they have enough capacitance to provide stability and transient response are controlled through the sufficient charge to prevent excessive voltage ripple at COMP pin. COMP pin is the output of the internal input. The input voltage ripple for low ESR capacitors can trans-conductance error amplifier. A series be estimated by: capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. The DC gain of the voltage feedback loop is given by: Where C1 is the input capacitance value. Output Capacitor The output capacitor is required to maintain the DC output Where AVEA is the error amplifier voltage gain; GCS is the voltage. Ceramic, tantalum, or low ESR electrolytic current sense transconductance and RLOAD is the load capacitors are recommended. Low ESR capacitors are resistor value. preferred to keep the output voltage ripple low. The output The system has two poles of importance. One is due to the voltage ripple can be estimated by: compensation capacitor (C3) and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at: Where C2 is the output capacitance value and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. Where GEA is the error amplifier transconductance. For simplification, the output voltage ripple can be estimated by: E-CMOS Corp. (www.ecmos.com.tw) Page 9 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter The system has one zero of importance, due to the EC3278 Determine the C3 value by the following equation: compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: Where R3 is the compensation resistor. 3. Determine if the second compensation capacitor (C6) is The system may have another zero of importance, if the required. It is required if the ESR zero of the output output capacitor has a large capacitance and/or a high ESR capacitor is located at less than half of the switching value. The zero, due to the ESR and capacitance of the frequency, or the following relationship is valid: output capacitor, is located at: If this is the case, then add the second compensation capacitor (C6) to set the pole fP3 at the location of the ESR In this case (as shown in Figure 10), a third pole set by the zero. Determine the C6 value by the equation: compensation capacitor (C6) and the compensation resistor (R3) is used to compensate the effect of the ESR zero on External Bootstrap Diode the loop gain. This pole is located at: An external bootstrap diode may enhance the efficiency of the regulator, the applicable The goal of compensation design is to shape the converter conditions of external BST diode are: transfer function to get a desired loop gain. The system VOUT=5V or 3.3V; and crossover frequency where the feedback loop has the unity Duty cycle is high: gain is important. Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system instability. A good rule of thumb is to set the crossover frequency below In these cases, an external BST diode is recommended one-tenth of the switching frequency. from the output of the voltage regulator to BST pin, as To optimize the compensation components, the following shown in Fig.10 procedure can be used. 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: Where fC is the desired crossover frequency which is Figure10.Add Optional External Bootstrap Diode to Enhance typically below one tenth of the switching frequency. Efficiency 2. Choose the compensation capacitor (C3) to achieve the The recommended external BST diode is IN4148, and the desired phase margin. For applications with typical inductor BST cap is 0.1~1μF. values, setting the compensation zero, fZ1, below one-forth of the crossover frequency provides sufficient phase margin. E-CMOS Corp. (www.ecmos.com.tw) Page 10 of 11 3I13N-Rev.P001 2.4A/32V Synchronous Rectified Step-Down Converter EC3278 Package Information SOP-8(Exposed PAD) Package Outline Dimensions E-CMOS Corp. (www.ecmos.com.tw) Page 11 of 11 3I13N-Rev.P001