SK6252B

SK6252B 17V 2A 500KHz ECOT Force-CCM Sync Step-Down Regulator General Description Features The SK6252B is a high frequency, synchronous, rectified, step-down, switch-mode converter with internal power MOSFETs. It offers a very compact solution to provide 2A continuous output current over wide input supply range with excellent load and line regulation. ECOT control operation provides very fast transient response and easy loop design as well as very tight output regulation. The SK6252B requires a minimal number of external components, and available in a space saving SOT23-6 package. Applications ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ Digital Set-top Box (STB) Tablet Personal Computer (Pad) Flat-Panel Television and Monitor Wi-Fi Router / AP Digital Video Recorder (DVR) Portable Media Player (PMP) Cable Modem / XDSL General Purposes ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ Wide 4.5V to 17V Operating Input Range 2A Continuous Output Current 500KHz Switching Frequency ECOT Mode Control with Fast Transient Response Built-in Over Current Limit Built-in Over Voltage Protection Continuous Conduction Mode (CCM) Internal Soft-Start 110mΩ/70mΩ Low RDS(ON) Internal Power MOSFETs Output Adjustable from 0.765V No Schottky Diode Required Short Protection with Hiccup-Mode Integrated internal compensation Thermal Shutdown Available in SOT23-6 Package -40°C to +85°C Temperature Range Typical Application Circuit C1 BS VIN IN SW L1 VOUT R1 CIN ON/ OFF EN CFF COUT FB GND R2 Basic Application Circuit 上海洺太电子科技有限公司 -1- SUNTEK MICRO LTD Pin Description Pin Configuration TOP VIEW GND 1 6 BS SW 2 5 EN IN 3 4 FB SOT23-6 Pin Description Pin Name Function 1 GND 2 SW Switching Pin 3 IN Power Supply Pin 4 FB Feedback input. Connect FB to the center of the external resistor divider. 5 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. 6 BS Supply input for the high-side MOSFET gate drive circuit. Connect 0.1 µF (typical) capacitor between BS and SW pins. Ground Pin Ordering Information (1) Part Number Package Mark Quantity/ Reel SK6252B SOT23-6 CSXXX 3000 SK6252B devices are Pb-free and RoHS compliant. 上海洺太电子科技有限公司 -2- SUNTEK MICRO LTD Specifications Absolute Maximum Ratings (1) (2) Item Min Max Unit VIN voltage -0.3 18 V EN voltage -0.3 18 V SW voltage -3 19 V SW voltage ( L) VFB=90% Minimum Off-Time EN Rising Threshold 1.4 V EN Falling Threshold Wake up VIN Voltage Under-Voltage Lockout Threshold Shutdown VIN Voltage 4.3 V 4.5 V 3.8 V 500 mV Soft Start 1.5 mS Thermal Shutdown 160 ℃ Thermal Hysteresis 30 ℃ Hysteresis VIN voltage 3.6 0.8 Note (1): MOSFET on-resistance specifications are guaranteed by correlation to wafer level measurements. Note (2): Thermal shutdown specifications are guaranteed by correlation to the design and characteristics analysis. 上海洺太电子科技有限公司 -5- SUNTEK MICRO LTD Typical Performance Characteristics (1) (2) Note (1): Performance waveforms are tested on the evaluation board. Note (2): VIN =12V, VOUT=3.3V, TA = +25ºC, unless otherwise noted. Efficiency vs Load Current Load Regulation Line Regulation VOUT=5V, 3.3V, 1.2V, 1.1V, 1.0V VOUT=5V, 3.3V, 1.2V, 1.1V, 1.0V VOUT=3.3V ``` Output Ripple Voltage Output Ripple Voltage Output Ripple Voltage VIN=12V, VOUT=3.3V, IOUT=0A VIN=12V, VOUT=3.3V, IOUT=1A VIN=12V, VOUT=3.3V, IOUT=2A Loop Response Output Short Short Circuit Entry VIN=12V, VOUT=3.3V, IOUT=1A-2A VIN=12V, VOUT=3.3V VIN=12V, VOUT=3.3V 上海洺太电子科技有限公司 -6- SUNTEK MICRO LTD Short Circuit Recovery Enable Startup at No Load Enable Shutdown at No Load VIN=12V, VOUT=3.3V VIN=12V, VOUT=3.3V, IOUT=0A VIN=12V, VOUT=3.3V, IOUT=0A Enable Startup at Full Load Enable Shutdown at Full Load Power Up at No Load VIN=12V, VOUT=3.3V, IOUT=2A VIN=12V, VOUT=3.3V, IOUT=2A VIN=12V, VOUT=3.3V, IOUT=0A Power Up at Full Load VIN=12V, VOUT=3.3V, IOUT=2A 上海洺太电子科技有限公司 -7- SUNTEK MICRO LTD Functional Block Diagram IN Internal Regulator EN Bias&Voltage Reference VCC BS UVLO HICCUP VSHORT Reference FB On-Time Control OC PWM Reference SW Driver Ripple Gen OCL Reference SW GND Block Diagram Functions Description Internal Regulator The SK6252B is an ECOT mode step down DC/DC converter which 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 500KHz 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 reference (VFB) and outputs a current proportional to the difference. This output current is then used to charge or discharge the internal compensation network, 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. Under-Voltage Lockout (UVLO) Under-voltage lockout (UVLO) protects the chip from operating at insufficient supply voltage. UVLO protection monitors the internal regulator voltage. When the voltage is lower than UVLO threshold voltage, the device shuts off. When the voltage is higher than UVLO threshold voltage, the device is enabled again. Thermal Shutdown Thermal shutdown prevents the chip from operating at exceedingly high temperature. When the silicon die temperature exceeds 160°C, it shuts down the whole chip. When the temperature falls below its lower threshold (Typ. 130°C) the chip is enabled again. 上海洺太电子科技有限公司 -8- SUNTEK MICRO LTD 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 0.765V. When it is lower than the internal reference (REF), SS overrides REF so that the error amplifier uses SS as the reference. When SS is higher than REF, REF regains control. The SS time is internally set to 1.5ms. Over Current Protection and Hiccup The SK6252B employs a cycle-by-cycle over current limit; when the inductor current valley value exceeds the set current limit threshold, meanwhile, output voltage starts to drop until FB is below the Under-Voltage (UV) threshold, a UV event is triggered, the SK6252B enters hiccup mode to periodically restart. 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 SK6252B exits the hiccup mode once the over current condition is removed. Startup and Shutdown If both VIN and EN are higher than the appropriate thresholds, the chip starts. The reference block starts first, generates 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. 上海洺太电子科技有限公司 -9- SUNTEK MICRO LTD Applications Information Setting the Output Voltage SK6252B requires an input capacitor, an output capacitor and an inductor. These components are CRYTICAL to the performance of the device. SK6252B is internally compensated and does not require any external components to achieve a stable operation. The output voltage can be programmed by resistor divider. 𝑉𝑂𝑈𝑇 = 𝑉𝐹𝐵 × 𝑅1 + 𝑅2 𝑅2 VOUT(V) R1(KΩ) R2(KΩ) L1(μH) C1(nF) CIN(μF) COUT(μF) CFF (pF) Opt. 1.0 3.07 10 2.2 100 22 22×2 CFF Chapter 1.05 3.73 10 2.2 100 22 22×2 CFF Chapter 1.2 5.69 10 2.2 100 22 22×2 CFF Chapter 1.5 9.61 10 2.2 100 22 22×2 CFF Chapter 1.8 13.53 10 3.3 100 22 22×2 CFF Chapter 2.5 22.68 10 4.7 100 22 22×2 CFF Chapter 3.3 33.14 10 4.7 100 22 22×2 CFF Chapter 5.0 55.36 10 4.7 100 22 22×2 CFF Chapter All the external components are the suggested values, the final values are based on the application testing results. Selecting the Inductor The recommended inductor values are shown in the table above. It is important to guarantee the inductor core does not saturate during any foreseeable situation. The inductor should be rated to handle the maximum inductor peak current: Care should be taken when reviewing the different saturation current ratings that are specified by different manufacturers. Saturation current ratings are typically specified at 25°C, so ratings at maximum ambient temperature of the application should be requested from the manufacturer. The inductor value can be calculated with: 𝐿= 𝑉𝑂𝑈𝑇 × (𝑉𝐼𝑁 − 𝑉𝑂𝑈𝑇 ) 𝑉𝐼𝑁 × ∆𝐼𝐿 × 𝐹𝑂𝑆𝐶 Where ΔIL is the inductor ripple current. Choose inductor ripple current to be approximately 30% to 50% of the maximum load current. The maximum inductor peak current can be estimated as: 𝐼𝐿(𝑀𝐴𝑋) = 𝐼𝐿𝑂𝐴𝐷 + ∆𝐼𝐿 2 Under light load conditions below 100mA, larger inductance is recommended for improved efficiency. Larger inductances lead to smaller ripple current and voltage, but they also have larger physical dimensions, lower saturation currents and higher linear impedance. Therefore, the choice of inductance should be compromised according to the specific application. 上海洺太电子科技有限公司 - 10 - SUNTEK MICRO LTD Selecting the Input Capacitor The input current to the step-down converter is discontinuous and therefore requires a capacitor to supply the AC current while maintaining the DC input voltage. For a better performance, use ceramic capacitors placed as close to VIN as possible. A 0.1µF input capacitor to filter out high frequency interference is recommended. Capacitors with X5R and X7R ceramic dielectrics are recommended because they are stable with temperature fluctuations. The capacitors must also have a ripple current rating greater than the maximum input ripple current of the converter. The input ripple current can be estimated with Equation: 𝐼𝐶𝐼𝑁 = 𝐼𝑂𝑈𝑇 × √ 𝑉𝑂𝑈𝑇 𝑉𝑂𝑈𝑇 × (1 − ) 𝑉𝐼𝑁 𝑉𝐼𝑁 From the above equation, it can be concluded that the input ripple current reaches its maximum at VIN=2VOUT where 𝐼 I𝐶𝐼𝑁 = 𝑂𝑈𝑇. For simplification, choose an input capacitor with an RMS current rating greater than half of the 2 maximum load current. The input capacitance value determines the input voltage ripple of the converter. If there is an input voltage ripple requirement in the system, choose the input capacitor that meets the specification. The input voltage ripple can be estimate with Equation: ∆𝑉𝐼𝑁 = 𝐼𝑂𝑈𝑇 𝑉𝑂𝑈𝑇 𝑉𝑂𝑈𝑇 × × (1 − ) 𝐹𝑂𝑆𝐶 × 𝐶𝐼𝑁 𝑉𝐼𝑁 𝑉𝐼𝑁 Similarly, when VIN=2VOUT, input voltage ripple reaches its maximum of ∆𝑉𝐼𝑁 = 1 4 × 𝐼𝑂𝑈𝑇 𝐹𝑂𝑆𝐶 ×𝐶𝐼𝑁 . Selecting the Output Capacitor An output capacitor is required to maintain the DC output voltage. The output voltage ripple can be estimated with Equation: ∆𝑉𝑂𝑈𝑇 = 𝑉𝑂𝑈𝑇 𝑉𝑂𝑈𝑇 1 × (1 − ) × (𝑅𝐸𝑆𝑅 + ) 𝐹𝑂𝑆𝐶 × 𝐿 𝑉𝐼𝑁 8 × 𝐹𝑂𝑆𝐶 × 𝐶𝑂𝑈𝑇 There are some differences between different types of capacitors. 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 with Equation: ∆𝑉𝑂𝑈𝑇 = 𝑉𝑂𝑈𝑇 2 8 × 𝐹𝑂𝑆𝐶 × 𝐿 × 𝐶𝑂𝑈𝑇 × (1 − 𝑉𝑂𝑈𝑇 ) 𝑉𝐼𝑁 A larger output capacitor can achieve a better load transient response, but the maximum output capacitor limitation should also be considered in the design application. If the output capacitor value is too high, the output voltage will not be able to reach the design value during the soft-start time and will fail to regulate. The maximum output capacitor value (COUT_MAX) can be limited approximately with Equation: 𝐶𝑂𝑈𝑇_𝑀𝐴𝑋 = (𝐼𝐿𝐼𝑀_𝐴𝑉𝐺 − 𝐼𝑂𝑈𝑇 ) × 𝑇𝑆𝑆/𝑉𝑂𝑈𝑇 Where LLIM_AVG is the average start-up current during the soft-start period, and TSS is the soft- start time. 上海洺太电子科技有限公司 - 11 - SUNTEK MICRO LTD On the other hand, special attention should be paid when selecting these components. The DC bias of these capacitors can result in a capacitance value that falls below the minimum value given in the recommended capacitor specifications table. The ceramic capacitor’s actual capacitance can vary with temperature. The capacitor type X7R, which operates over a temperature range of −55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R has a similar tolerance over a reduced temperature range of −55°C to +85°C. Many large value ceramic capacitors, larger than 1uF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore, X5R or X7R is recommended over Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C. Feed-Forward Capacitor (CFF) SK6252B has internal loop compensation, so adding CFF is optional. For specific applications, if necessary, consider whether to add feed-forward capacitors according to the situation. The use of a feed-forward capacitor (CFF) in the feedback network is to improve the transient response or higher phase margin. For optimizing the feed-forward capacitor, knowing the cross frequency is the first thing. The cross frequency (or the converter bandwidth) can be determined by using a network analyzer. When getting the cross frequency with no feed-forward capacitor identified, the value of feed-forward capacitor (CFF) can be calculated with the following equation: 𝐶𝐹𝐹 = 1 1 1 1 ×√ ×( + ) 2𝜋 × 𝐹𝐶𝑅𝑂𝑆𝑆 𝑅1 𝑅1 𝑅2 Where FCROSS is the cross frequency. To reduce transient ripple, the feed-forward capacitor value can be increased to push the cross frequency to higher region. Although this can improve transient response, it also decreases phase margin and cause more ringing. In the other hand, if more phase margin is desired, the feed-forward capacitor value can be decreased to push the cross frequency to lower region. 上海洺太电子科技有限公司 - 12 - SUNTEK MICRO LTD PC Board Layout Consideration PCB layout is very important to achieve stable operation and better performance. It is highly recommended to duplicate EVB layout for optimum performance. If change is necessary, please follow these guidelines 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. Top Layer Bottom Layer Sample Board Layout 上海洺太电子科技有限公司 - 13 - SUNTEK MICRO LTD Package Description: SOT23-6 Unit: mm Symbol Dimensions In Millimeters Min Typ Max A 2.80 2.90 3.00 B 2.60 2.80 B1 1.50 C Symbol Dimensions In Millimeters Min Typ Max L3 1.800 1.900 2.000 3.00 L4 0.077 0.127 0.177 1.60 1.70 L5 - 0.250 - - - 1.05 L6 - 0.600 - C1 0.60 0.80 1.00 θ 0° C2 0.35 0.40 0.45 Θ1 10° 12° 14° C4 0.223 0.273 0.323 Θ2 10° 12° 14° K 0.000 0.075 0.150 R - 0.100 - L 0.325 0.400 0.475 R1 - 0.100 - L1 0.325 0.450 0.550 R2 - 0.100 - L2 0.850 0.950 1.050 0° Note: 1) All dimensions are in millimeters. 2) Package length does not include mold flash, protrusion or gate burr. 3) Package width does not include inter lead flash or protrusion. 4) Lead popularity (bottom of leads after forming) shall be 0.10 millimeters max. 上海洺太电子科技有限公司 - 14 - SUNTEK MICRO LTD