SD8942

SD8942 600KHz, 16V，2A Synchronous Step-Down Converter GENERAL DESCRIPTION FEATURES The SD8942 is a fully integrated, high– efficiency 2A synchronous rectified step-down converter. The SD8942 operates at high efficiency over a wide output current load range. This device offers two operation modes, PWM control and PFM Mode switching control, which allows a high efficiency over the wider range of the load. The SD8942 requires a minimum number of readily available standard external components and is available in an 6-pin SOT23 ROHS compliant package. ** ** *** ** ** ** APPLICATIONS ** ** * Wireless and DSL Modems Notebook Computer Distributed Power Systems Digital Set Top Boxes Flat Panel Television and Monitors 4.5V to 16V Input Voltage Range 600KHz Frequency Operation 2A Output Current No Schottky Diode Required High Efficiency: Up to 96% 0.6V Reference Slope Compensated Current Mode Control for Excellent Line and Load Transient Response Integrated internal compensation Stable with Low ESR Ceramic Output Capacitors Over Current Protection with Hiccup-Mode Thermal Shutdown Inrush Current Limit and Soft Start -40°C to +85°C Temperature Range TYPICAL APPLICATION R3 100kΩ EN FB 4 3 IN GND 5 INPUT C1 10μF/25V CERAMIC SD8942 2 SW BS 6 1 R2 15kΩ 1% R1 110kΩ 1% C3 22nF C2 22μF/6.3V CERAMIC X2 5V/2A L1 4.7μH OUTPUT Figure 1. Basic Application Circuit ABSOLUTE MAXIMUM RATINGS(Note1) Input Supply Voltage ……….… -0.3V to 17V EN,FB Voltages …………….…-0.3 to 6V SW Voltage ……..……-0.3V to (Vin+0.5V) BS Voltage ……….. (Vsw-0.3) to (Vsw+5V) www.shouding.net Operating Temperature Range … -40°C to +85°C Lead Temperature(Soldering,10s) ….…....+300°C Storage Temperature Range ….…-65°C to 150°C 1 SD8942 PIN DESCRIPTION PIN NAME FUNCTION 1 BS Bootstrap. A capacitor connected between SW and BST pins is required to form a floating supply across the high-side switch driver. 2 GND 3 FB Adjustable version feedback input. Connect FB to the center point of the external resistor divider. 4 EN Drive this pin to a logic-high to enable the IC. Drive to a logic-low to disable the IC and enter micro-power shutdown mode. 5 VIN Power supply Pin 6 SW Switching Pin Ground PIN CONFLGURATION BS 1 6 SW GND 2 5 IN FB 3 4 EN SOT23-6 (SD8942) www.shouding.net 2 SD8942 ELECTRICAL CHARACTERISTICS(Note3) (VIN=12V, VOUT=5V, TA = 25°C, unless otherwise noted.) Parameter Conditions Input Voltage Range MIN TYP 3.3 Supply Current in Operation VEN=2.0V, VFB=1.1V Supply Current in Shutdown VEN =0 or EN = GND Regulated Feedback TA = 25°C, 4.5V≤VIN ≤18V Voltage High-Side Switch On-Resistance Low-Side Switch On-Resistance High-Side Switch Leakage VEN=0V, VSW=0V Current 0.4 MAX unit 16 V 0.6 mA 1 0.588 0.6 uA 0.612 V 90 mΩ 70 mΩ 0 10 uA Upper Switch Current Limit Minimum Duty Cycle 3 A Oscillation Frequency 0.6 MHz 92 60 % nS 160 ℃ Maximum Duty Cycle Minimum On-Time Thermal Shutdown www.shouding.net VFB=0.6V 3 SD8942 YPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, VOUT = 5V, L = 10μH, TA = 25ºC, unless otherwise noted. FUNCTIONAL BLOCK DIAGRAM Short protection Figure 2. SD8942 Block Diagram www.shouding.net 4 SD8942 FUNCTIONAL DESCRIPTION Internal Regulator The SD8942 is a current mode step down DC/DC converter that provides excellent transient response with no extra external compensation components. This device contains an internal, low resistance, high voltage power MOSFET, and operates at a high 600K operating frequency to ensure a compact, high efficiency design with excellent AC and DC performance. Error Amplifier The error amplifier compares the FB pin voltage with the internal FB reference (VFB) and outputs a current proportional to the difference between the two. This output current is then used to charge or discharge the internal compensation network to form the COMP voltage, which is used to control the power MOSFET current. The optimized internal compensation network minimizes the external component counts and simplifies the control loop design. Internal Soft-Start The soft-start is implemented to prevent the converter output voltage from overshooting during startup. When the chip starts, the internal circuitry generates a soft-start voltage (SS) ramping up from 0V to 0.6V. When it is lower than the internal reference (REF), SS overrides REF so the error amplifier uses SS as the reference. When SS is higher than REF, REF regains control. The SS time is internally fixed to 1 ms. Over-Current-Protection and Hiccup Startup and Shutdown The SD8942 has cycle-by-cycle over current limit when the inductor current peak value exceeds the set current limit threshold. Meanwhile, output voltage starts to drop until FB is below the Under-Voltage (UV) threshold, typically 30% below the reference. Once a UV is triggered, the SD8942 enters hiccup mode to periodically restart the part. This protection mode is especially useful when the output is dead-short to ground. The average short circuit current is greatly reduced to alleviate the thermal issue and to protect the regulator. The SD8942 exits the hiccup mode once the over current condition is removed. If both VIN and EN are higher than their appropriate thresholds, the chip starts. The reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. The regulator provides stable supply for the remaining circuitries. Three events can shut down the chip: EN low, VIN low and thermal shutdown. In the shutdown procedure, the signaling path is first blocked to avoid any fault triggering. The COMP voltage and the internal supply rail are then pulled down. The floating driver is not subject to this shutdown command. www.shouding.net 5 SD8942 APPLICATION INFORMATION Setting the Output Voltage Selecting the Output Capacitor The external resistor divider is used to set the output voltage (see Typical Application on page 1). The feedback resistor R1 also sets the feedback loop bandwidth with the internal compensation capacitor. Choose R1 to be around 100kΩ for optimal transient response. R2 is then given by: The output capacitor (C2) 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: R2 = R1 Vout / VFB − 1 Selecting the Inductor A 4.7μH to 22μH inductor with a DC current rating of at least 25% percent higher than the maximum load current is recommended for most applications. For highest efficiency, the inductor DC resistance should be less than 15mΩ. For most designs, the inductance value can be derived from the following equation. L= Vout × (Vin − Vout ) Vin × ∆I L × fOSC Where ΔIL is the inductor ripple current. Choose inductor ripple current to be approximately 30% if the maximum load current, 2A. The maximum inductor peak current is: I L ( MAX ) ∆I L = I LOAD + 2 ∆VOUT =  VOUT  VOUT   1 × 1 −  ×  RESR +  f S × L  VIN   8 × f S × C2  Where L is the inductor 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: ∆VOUT =  VOUT  VOUT × 1 −  2 8 × f S × L × C2  VIN  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: ∆VOUT = VOUT  VOUT  × 1 −  × RESR f S × L  VIN  The characteristics of the output capacitor also affect the stability of the regulation system. The SD 8942 can be optimized for a wide range of capacitance and ESR values. Under light load conditions below 100mA, larger inductance is recommended for improved efficiency. www.shouding.net 6 SD8942 PCB Layout Guide PCB layout is very important to achieve stable operation. It is highly recommended to duplicate EVB layout for optimum performance. If change is necessary, please follow these guidelines and take Figure 4 for reference. 1) Keep the path of switching current short and minimize the loop area formed by Input capacitor, high-side MOSFET and low-side MOSFET. 2) Bypass ceramic capacitors are suggested to be put close to the Vin Pin. 3) Ensure all feedback connections are short and direct. Place the feedback resistors and compensation components as close to the chip as possible. 4) VOUT, SW away from sensitive analog areas such as FB. 5) Connect IN, SW, and especially GND respectively to a large copper area to cool the chip to improve thermal performance and long-term reliability. 6) An example of 2-layer PCB layout is shown in Figure 4 for reference. PACKAGE INFORMATION ****** www.shouding.net 7