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Technical content
Features
4.5V to 18V Input Voltage Range Constant-On-Time Mode to Enables Fast Transient Response Low Output Ripple and Allows Ceramic Output Capacitor 500kHz Switching Frequency High Efficient Internal Power MOSFET Switch Optimized for Lower Duty Cycle Applications Integrated 30mΩΩΩΩΩ/20mΩΩΩΩΩ MOSFETs Adjustable Output Voltage from 0.6V to 5V Internal Soft-Start (1.5ms typ.) Input Under Voltage Lockout TSOT23-6 (FC) Package
Applications
Set Top Box Portable TV Access Point Router DSL Modem LCD TVNote : Richtek products are : RoHS compliant and compatible with the current require- ments of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes.
Ordering Information
The RT6257A/B is a high-efficiency, monolithic synchronous step-down DC-DC converter that can deliver up to 6A output current from a 4.5V to 18V input supply. The RT6257A/B adopts ACOT architecture to allow the transient response to be improved and keep in constant frequency. Cycle-by-cycle current limit provides protection against shorted outputs and soft-start eliminates input current surge during start-up. Fault conditions also include output under voltage protection and thermal shutdown. RT6257A/B Package Type J6F : TSOT-23-6 (FC) Lead Plating System G : Green (Halogen Free and Pb Free) UVP Option H : Hiccup PSM/PWM A : PSM/PWM B : Force-PWM Pin Configuration (TOP VIEW) TSOT-23-6 (FC) FB EN GND BOOT LX VIN Marking Information 2K=DNN 2K= : Product Code DNN : Date Code RT6257AHGJ6F RT6257BHGJ6F 2J=DNN 2J= : Product Code DNN : Date Code
©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Table 1. Suggested Component Values de-rating effect, like a DC bias. (2) Considering the noise immunity, it is necessary to add RT = 4.99k between feedback network and chip FB pin. the device and connecting this pin to GND can disable the device. 4 VIN Power input. Supplies the power switches of the device. the output LC filter from LX to the output load. to BOOT to power the high-side switch.
DS6257A/B-00 September 2016 www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Functional Block Diagram Operation The RT6257A/B is a high-efficiency, monolithic synchronous step-down DC-DC converter that can deliver up to 6A output current from a 4.5V to 18V input supply. Using the ACOT control mode can reduce the output capacitance and perform fast transient response. It can minimize the component size without additional external compensation network. Current Limit The RT6257A/B current limit is a cycle-by-cycle “valley” type, measuring the inductor current through the synchronous rectifier during the off-time while the inductor current ramps down. The current is determined by measuring the voltage between Source and Drain of the synchronous rectifier, adding temperature compensation for greater accuracy. If the current exceeds the current limit, the on-time one-shot is inhibited until it drops below the current limit level. If the output current exceeds the available inductor current (controlled by the current limit mechanism), the output voltage will drop. If it drops below the output under-voltage protection level (see next section) the IC will stop switching to avoid excessive heat. Hiccup Mode The RT6257A/B use hiccup mode for UVP. When the protection function is triggered, the IC will shut down for a period of time and then attempt to recover automatically. Hiccup mode allows the circuit to operate safely with low input current and power dissipation, and then resume normal operation as soon as the overload or short circuit is removed. Input Under-Voltage Lockout To protect the chip from operating at insufficient supply voltage, the UVLO is needed. When the input voltage of VIN is lower than the UVLO falling threshold voltage, the device will be lockout. Shut-Down, Start-Up and Enable (EN) The enable input (EN) has a logic-low level. When V EN is below this level the IC enters shutdown mode. When VEN exceeds its logic-high level the IC is fully operational. External Bootstrap Capacitor Connect a 0.1 μF low ESR ceramic capacitor between BOOT and LX. This bootstrap capacitor provides the gate UGATE LGATE Driver LX BOOT VCC Control On-Time EN Comparator LX GND Reg VIBIAS VREF VCC LX VCC Ripple Gen. VIN FB Minoff EN VIN GND LX VIN UV OC
DS6257A/B-00 September 2016www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. driver supply voltage for the high-side N-channel MOSFET switch. Over-Temperature Protection The RT6257A/B includes an Over-Temperature Protection (OTP) circuitry to prevent overheating due to excessive power dissipation. The OTP will shut down switching operation when the junction temperature exceeds 150°C. Once the junction temperature cools down by approximately 15°C, the IC will resume normal operation. For continuous operation, provide adequate cooling so that the junction temperature does not exceed 150°C. UVP Protection The RT6257A/B detects under-voltage conditions by monitoring the feedback voltage on FB pin. When the feedback voltage is lower than 60% of the target voltage, the UVP comparator will go high to turn off both internal high-side and low-side MOSFETs.
DS6257A/B-00 September 2016 www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation.
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified) Absolute Maximum Ratings (Note 1) Power Dissipation, PD @ TA = 25°C Package Thermal Resistance (Note 2) ESD Susceptibility (Note 3) Recommended Operating Conditions (Note 4) Parameter Symbol Conditions Min Typ Max Unit Supply Voltage VIN Supply Input Operating Voltage VIN 4.5 -- 18 V VIN Under-Voltage Lockout Threshold-Rising VUVLO V IN rising 3.9 4.1 4.3 V VIN Under-Voltage Lockout Threshold-Hysteresis VUVLO -- 0.3 -- V Supply Current Supply Current (Shutdown) I SHDN V EN = 0 -- 3 -- A Supply Current (Quiescent) I Q IOUT = 0 VFB = VREF x 105%, not switching -- 115 -- A Soft-Start Soft-Start Time t SS -- 1.5 -- ms Enable Voltage EN Input High Voltage V EN_H 1.5 -- -- V EN Input Low Voltage V EN_L -- -- 0.4 V
DS6257A/B-00 September 2016www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Note 1. Stresses beyond those listed “Absolute Maximum Ratings ” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect device reliability. Note 2. θ JA is measured under natural convection (still air) at T A = 25 °C with the component mounted on a high effective- thermal-conductivity four-layer test board on a JEDEC 51-7 thermal measurement standard. The first layer is filled with copper. θJC is measured at the lead of the package. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Parameter Symbol Conditions Min Typ Max Unit Feedback Voltage Feedback Voltage V FB 0.594 0.6 0.606 V Feedback Current I FB V FB = 4V 50 50 nA Internal MOSFET Switch-On Resistance High-Side R DS(ON)_H V BOOT VLX = 4.8V -- 30 -- m Low-Side R DS(ON)_L -- 20 -- m Discharge FET RON R DISCHG -- 50 -- Current Limit Hide-Side Switch Current Limit I LIM_H -- 11 -- A Low-Side Switch Valley Current Limit I LIM_L 6.3 8.5 10.7 A Switching Frequency Switching Frequency f SW 400 500 600 kHz On-Time Timer Control Minimum On-Time t ON(MIN) V IN = VIN(MAX) -- 60 -- ns Minimum Off-Time t OFF(MIN) -- 200 -- ns Thermal Shutdown Thermal Shutdown Threshold T SD -- 150 -- C Thermal Shutdown Hysteresis TSD -- 15 -- C Output Under Voltage UVP Trip Threshold UVP detect -- 60 -- % Hysteresis -- 10 -- %
DS6257A/B-00 September 2016 www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Typical Operating Characteristics Efficiency vs. Output Current 100 0.001 0.01 0.1 1 10 Output Current (A) Efficiency (%) VIN = 4.5V VIN = 12V VIN = 18V VOUT = 1V, L = 1.2μH (744325120) Output Voltage vs. Output Current 0.950 0.975 1.000 1.025 1.050 1.075 1.100 Output Current (A) Output Voltage (V) VOUT = 1V VIN = 18V VIN = 12V VIN = 4.5V Efficiency vs. Output Current 100 0.001 0.01 0.1 1 10 Output Current (A) Efficiency (%) VOUT = 5V, L = 3.3μH (74437368033) VIN = 8.5V VIN = 12V VIN = 18V UVLO Threshold vs. Temperature 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 -50 -25 0 25 50 75 100 125 Temperature (°C) UVLO Threshold (V) Rising Falling VOUT = 1V, IOUT = 1A EN Threshold vs. Temperature 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 -50 -25 0 25 50 75 100 125 Temperature (°C) EN Threshold (V) EN_H EN_L VOUT = 1V, IOUT = 0A Output Voltage vs. Output Current 4.9 5.0 5.1 5.2 5.3 Output Current (A) Output Voltage (V) VOUT = 5V VIN = 18V VIN = 12V VIN = 8.5V
DS6257A/B-00 September 2016www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Time (2 μs/Div) Output Ripple Voltage VOUT (20mV/Div) VLX (5V/Div) VIN = 12V, VOUT = 1V, IOUT = 6A, L = 1.2μH Time (5ms/Div) Power On from EN VOUT (1V/Div) VLX (10V/Div) VEN (2V/Div) IOUT (5A/Div) VIN = 12V, VOUT = 1V, IOUT = 6A, L = 1.2μH Time (200 μs/Div) Power Off from EN VOUT (1V/Div) VLX (10V/Div) VEN (2V/Div) IOUT (5A/Div) VIN = 12V, VOUT = 1V, IOUT = 6A, L = 1.2μH Output Voltage vs. Temperature 4.850 4.875 4.900 4.925 4.950 4.975 5.000 5.025 5.050 5.075 5.100 5.125 5.150 -50 -25 0 25 50 75 100 125 Temperature (°C) Output Voltage (V) VIN =12V VIN = 18V VIN = 8.5V VOUT = 5V, IOUT = 1.8A Output Voltage vs. Temperature 0.96 0.98 1.00 1.02 1.04 1.06 -50 -25 0 25 50 75 100 125 Temperature (°C) Output Voltage (V)VIN = 18V VIN = 12V VIN = 4.5V VOUT = 1V, IOUT = 1.5A Load Transient Response Time (100 μs/Div) VOUT (20mV/Div) IOUT (2A/Div) VIN = 12V, VOUT = 1V, IOUT = 0A to 6A, L = 1.2μH
DS6257A/B-00 September 2016 www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Time (10ms/Div) Power On from VIN VOUT (1V/Div) VLX (10V/Div) VIN (10V/Div) IOUT (5A/Div) VIN = 12V, VOUT = 1V, IOUT = 6A, L = 1.2μH Time (10ms/Div) Power Off from VIN VOUT (1V/Div) VLX (10V/Div) VIN (10V/Div) IOUT (5A/Div) VIN = 12V, VOUT = 1V, IOUT = 6A, L = 1.2μH
DS6257A/B-00 September 2016www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation.
Application Information
The consideration of inductor selection includes inductance, RMS current rating and, saturation current rating. The inductance selection is generally flexible and is optimized for the low cost, low physical size, and high system performance. Choosing lower inductance to reduce physical size and cost, and it is useful to improve the transient response. However, it causes the higher inductor peak current and output ripple voltage to decrease system efficiency. Conversely, higher inductance increase system efficiency, but the physical size of inductor will become larger and transient response will be slow because more transient time is required to change current (up or down) by inductor. A good compromise between size, efficiency, and transient response is to set a inductor ripple current ( ΔIL) about 20% to 50% of the desired full output load current. Calculate the approximate inductance by the input voltage, output voltage, switching frequency (fSW), maximum rated output current (IOUT(MAX)) and inductor ripple current (ΔIL). OUT IN OUT IN SW L VV VL = Vf I Once the inductance is chosen, the inductor ripple current (ΔIL) and peak inductor current can be calculated. OUT IN OUT L IN SW L(PEAK) OUT(MAX) L L(VALLY) OUT(MAX) L VV VI= Vf L 1I = I I 2 1I = I I 2 The typical operating circuit design for the RT6257A/B, the output voltage is 5V, maximum rated output current is 6A, input voltage is 12V, and inductor ripple current is 1.8A which is 30% of the maximum rated output current, the calculated inductance value is : 51 2 5L = = 3.24 μH 12 500 10 1.8 The inductor ripple current set at 1.8A and so we select 3.3μH inductance. The actual inductor ripple current and required peak current is shown as below : L 3- 6 51 2 5I = = 1.77A 12 500 10 3.3 10 L(PEAK) OUT(MAX) L 11 . 7 7I = I I = 6 + = 6.885A22 Inductor saturation current should be chosen over IC's valley current limit. Input Capacitor Selection The effective input capacitance is a function of the input voltage (VIN), output voltage (VOUT), rated output current (IOUT), switching frequency (fSW), and input ripple voltage of the regulator (ΔVINP) : OUT OUTOUT IN ININ(MIN) SW INP VVI1 VVC = fV Ceramic capacitors are most often used because of their low cost, small size, high RMS current ratings, and robust surge current capabilities. It should pay attention that value of capacitors change as temperature, bias voltage, and operating frequency change. For example the capacitance value of a capacitor decreases as the dc bias across the capacitor increases. Several ceramic capacitors may be paralleled to meet the RMS current, size, and height requirements of the application. Considering the DC bias effects for the input capacitor, the typical operating circuit used two 10μF low ESR ceramic capacitors on the VIN pin and an additional 0.1μF is recommended to place as close as possible to the IC input side for high frequency filtering. Output Capacitor Selection The RT6257A/B is optimized for output terminal with ceramic capacitors application and best performance will be obtained using them. The total output capacitance value is usually determined by the desired output ripple voltage level and transient response requirements for sag which is undershoot on positive load steps and soar which is overshoot on negative load steps.
DS6257A/B-00 September 2016 www.richtek.com ©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. RIPPLE RIPPLE(ESR) RIPPLE(C) RIPPLE(ESR) L ESR LRIPPLE(C) OUT SW V = V V V = I R IV = 8C f The typical operating circuit design for the RT6257A/B, the output voltage is 5V, inductor ripple current is 1.77A, and using 2 pieces of 22μF output capacitor with about 5mΩ ESR, the output voltage ripple components are : RIPPLE(ESR) L ESR LRIPPLE(C) OUT SW RIPPLE RIPPLE(ESR) RIPPLE(C) V = I R = 1.77A 5m = 8.85mV I 1.77AV = = 8C f 84 4 μF5 0 0 k H z = 10mV V = V V = 18.85mV Output Transient Undershoot and Overshoot In addition to output ripple voltage at the switching frequency, the output capacitor and its ESR also affect the voltage sag (undershoot) and soar (overshoot) when the load steps up and down abruptly. The ACOT transient response is very quick and output transients are usually small. However, the combination of small ceramic output capacitors (with little capacitance), low output voltages (with little stored charge in the output capacitors), and low duty cycle applications (which require high inductance to get reasonable ripple currents with high input voltages) increases the size of voltage variations in response to very quick load changes. Typically, load changes occur slowly with respect to the IC's 500kHz switching frequency. But some modern digital loads can exhibit nearly instantaneous load changes and the following section shows how to calculate the worst-case voltage swings in response to very fast load steps. The output voltage transient undershoot and overshoot each have two components : the voltage steps caused by the Output Ripple Voltage Output ripple voltage at the switching frequency is caused by the inductor current ripple and its effect on the output capacitor's ESR and stored charge. These two ripple components are called ESR ripple and capacitive ripple. Since ceramic capacitors have extremely low ESR and relatively little capacitance, both components are similar in amplitude and both should be considered if ripple is critical. output capacitor's ESR, and the voltage sag and soar due to the finite output capacitance and the inductor current slew rate. Use the following formulas to check if the ESR is low enough (typically not a problem with ceramic capacitors) and the output capacitance is large enough to prevent excessive sag and soar on very fast load step edges, with the chosen inductor value. The amplitude of the ESR step up or down is a function of the load step and the ESR of the output capacitor : ESR_STEP OUT ESRV = I R The amplitude of the capacitive sag is a function of the load step, the output capacitor value, the inductor value, the input-to-output voltage differential, and the maximum duty cycle. The maximum duty cycle during a fast transient is a function of the on-time and the minimum off-time since the ACOT TM control scheme will ramp the current using on-times spaced apart with minimum off-times, which is as fast as a llowed. Calculate the approximate on-time (neglecting parasitics) and maximum duty cycle for a given input and output voltage a s : OUT ONON MAX IN SW ON OFF(MIN) Vtt = and D = Vf t t The actual on-time will be slightly longer as the IC compensates for voltage drops in the circuit, but we can neglect both of these since the on-time increase compensates for the voltage losses. Calculate the output voltage sag as : OUTSAG OUT IN(MIN) MAX OUT L( I )V = 2C V D V The amplitude of the capacitive soar is a function of the load step, the output capacitor value, the inductor value and the output voltage : OUTSOAR OUT OUT L( I )V = 2C V
©Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Ric htek Technology Corporation. Put the input capacitor as close as possible to VIN pin. from the LX node to prevent stray capacitive noise pickup. Figure 8. PCB Layout Guide connected as close to the device as possible. components away from this trace. Suggestion layout trace wider for thermal. Connect feedback network behind the output capacitors. shown in Figure 8 for references.
DS6257A/B-00 September 2016 www.richtek.com Richtek Technology Corporation 14F, No. 8, Tai Yuen 1st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnish ed by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringeme nts of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of R ichtek or its subsidiaries. Min. Max. Min. Max. A 0.700 1.000 0.028 0.039 A1 0.000 0.100 0.000 0.004 B 1.397 1.803 0.055 0.071 b 0.300 0.559 0.012 0.022 C 2.591 3.000 0.102 0.118 D 2.692 3.099 0.106 0.122 e H 0.080 0.254 0.003 0.010 L 0.300 0.610 0.012 0.024 Symbol Dimensions In Millimeters Dimensions In Inches 0.950 0.037 TSOT-23-6 (FC) Surface Mount Package Outline Dimension