TD2778MR

Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter General Description TD2778 Features The TD2778 is a monolithic synchronous buck regulator. The device integrates two 90mΩ MOSFETs, and provides 2.4A of continuous load current over a wide input voltage of 4.75V to 32V. Current mode control provides fast transient response and cycle-by-cycle current limit. An adjustable soft-start prevents inrush current at turn-on, and in shutdown mode the supply current drops to 1µA. This device, available in an SOP8-PP package, provides a very compact solution with minimal external components. • • 2.4A Output Current Wide 4.75V to 32V Operating Input Range • Integrated 90mΩ Power MOSFET Switches • Output Adjustable from 0.923V to 30V • Up to 93% Efficiency • Programmable Soft-Start • Stable with Low ESR Ceramic Output Capacitors • Fixed 200KHz • 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 Package Types Figure 1. Package Types of TD2778 October, 20, 2010. Techcode Semiconductor Limited 1 SOP8 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 Pin Configurations Figure 2 Pin Configuration of TD2778(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. October, 20, 2010. Techcode Semiconductor Limited 2 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 Ordering Information TD2778 □ □ Circuit Type Packing: Blank：Tube R:Type and Reel Package M:SOP8-PP Function Block Figure 3 Function Block Diagram of TD2778 October, 20, 2010. Techcode Semiconductor Limited 3 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 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 Level3 90 45 ºC / W ºC / W All Other Pins Operating Junction Temperature ESD (HBM) MSL Thermal Resistance-Junction to Ambient Thermal Resistance-Junction to Case October, 20, 2010. RθJA RθJC Techcode Semiconductor Limited 4 www.techcodesemi.com Techcode® DATASHEET TD2778 2.4A 32V Synchronous Rectified Step-Down Converter 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 Transconductance 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 4.8 A/V COMP to Current Sense Transconductance Oscillation Frequency Short Circuit Oscillation Frequency Maximum Duty Cycle GCS Fosc1 TD2778 200 KHz Fosc2 VFB = 0V 100 KHz DMAX VFB = 1.0V 90 % 220 ns Minimum On Time * EN Shutdown Threshold Voltage VEN Rising 1.1 1.5 2.0 V EN Shutdown Threshold Voltage Hysteresis 210 EN Lockout Threshold Voltage 2.2 EN Lockout Hysterisis October, 20, 2010. 2.5 210 Techcode Semiconductor Limited 5 mV 2.7 V mV www.techcodesemi.com Techcode® DATASHEET TD2778 2.4A 32V Synchronous Rectified Step-Down Converter 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 1-Vout 2-SW 4-ISW VIN=12V Vout=5V Load=2A 1-Vout 2-SW 4- ISW Figure 4. Steady State Test Figure 5 Steady State Test October, 20, 2010. Techcode Semiconductor Limited 6 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 VIN=12V Vout=5V Load=1A~2A 1-Vout 4-Load VIN=12V Vout=5V 1-Vout 4- ISW Figure 6 Load Transient Test Figure7. Short Circuit Test VIN=12V Vout=5V 1-Vout 4- ISW Figure 8. Short Circuit Recovery October, 20, 2010. Techcode Semiconductor Limited 7 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 Typical Application Circuit Fig9. TD2778 with 5V Output, 470µF/16V Electrolytic Output Capacitor October, 20, 2010. Techcode Semiconductor Limited 8 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 Function Description Component Selection Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin.The voltage divider divides the output voltage down to the feedback voltage by the ratio: Where VFB is the feedback voltage and VOUT is the output voltage.Thus the output voltage is: 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 peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by: October, 20, 2010. Where VOUT is the output voltage, VIN is the input voltage, fS is the switching frequency, and ΔIL is the peak-to-peak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: 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 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. Part Number Voltage/Current Vendor B140 40V, 1A Diodes, Inc. SK14 40V, 1A Diodes, Inc. MBRS140 40V, 1A International Rectifier 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. Techcode Semiconductor Limited 9 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 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 In the case of tantalum or electrolytic capacitors,the estimated by: 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 load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1μF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple for low ESR capacitors can be estimated by: Where C1 is the input capacitance value. Output Capacitor The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by: 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. For simplification, the output voltage ripple can be estimated by: October, 20, 2010. The characteristics of the output capacitor also affect the stability of the regulation system. The TD2778 can be optimized for a wide range of capacitance and ESR values. Compensation Components TD2778 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP pin is the output of the internal transconductance error amplifier. A series 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 AVEA is the error amplifier voltage gain;GCS is the current sense transconductance and RLOAD is the load resistor value. The system has two poles of importance. One is due to the 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 GEA is the error amplifier transconductance. Techcode Semiconductor Limited 10 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero,due to the ESR and capacitance of the output capacitor, is located at: TD2778 Determine the C3 value by the following equation: Where R3 is the compensation resistor. 3. Determine if the second compensation capacitor (C6) is required. It is required if the ESR zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid: If this is the case, then add the second compensation In this case (as shown in Figure 14), a third pole set by capacitor (C6) to set the pole fP3 at the location of the the compensation capacitor (C6) and the ESR zero. Determine the C6 value by the equation: compensation resistor (R3) is used to compensate the effect of the ESR zero on the loop gain. This pole is located at: External Bootstrap Diode An external bootstrap diode may enhance the efficiency of the regulator, the applicable The goal of compensation design is to shape the conditions of external BST diode are: converter transfer function to get a desired loop gain.  VOUT=5V or 3.3V; and The system crossover frequency where the feedback  Duty cycle is high: loop has the unity gain is important. Lower crossover frequencies result in slower line and load transient responses,while higher crossover frequencies could In these cases, an external BST diode is cause system instability. A good rule of thumb is to set recommended from the output of the voltage regulator the crossover frequency below one-tenth of the to BST pin, as shown in Fig.10 switching frequency. To optimize the compensation components, the following procedure can be used. 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: Figure10.Add Optional External Bootstrap Diode to Enhance Efficiency Where fC is the desired crossover frequency which is The recommended external BST diode is IN4148, and typically below one tenth of the switching frequency. the BST cap is 0.1~1μF. 2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ1, below one-forth of the crossover frequency provides sufficient phase margin. October, 20, 2010. Techcode Semiconductor Limited 11 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 Package Information SOP8-pp Package Outline Dimensions October, 20, 2010. Techcode Semiconductor Limited 12 www.techcodesemi.com Techcode® DATASHEET 2.4A 32V Synchronous Rectified Step-Down Converter TD2778 Design Notes October, 20, 2010. Techcode Semiconductor Limited 13 www.techcodesemi.com