RT7255 1.5A, 18V, 500kHz ACOTTM Synchronous Step-down Converter Features General Description The RT7255 is a synchronous step-down converter with Advanced Constant On-Time (ACOTTM) mode control. The ACOTTM provides a very fast transient response with few external components. The low impedance internal MOSFET supports high efficiency operation with wide input voltage range from 4.3V to 18V. The proprietary circuit of the RT7255 supports all ceramic capacitors. The output voltage can be adjusted between 0.6V and 8V. Ordering Information Input Under-Voltage Lockout Hiccup Mode Under-Voltage Protection Thermal Shutdown RT7255 Package Type V8 : SOT-23-8 4.3V to 18V Input Voltage Range Adjustable Soft-Start Time PGOOD Function 1.5A Output Current Advanced Constant On-Time Control Fast Transient Response Support All Ceramic Capacitors Up to 95% Efficiency 500kHz Switching Frequency Adjustable Output Voltage from 0.6V to 8V Cycle-by-Cycle Current Limit Applications Lead Plating System G : Green (Halogen Free and Pb Free) A : Hiccup + PSM B : Non-UVP + PSM C : Hiccup + PWM D : Non-UVP + PWM Industrial and Commercial Low Power Systems Computer Peripherals LCD Monitors and TVs Green Electronics/Appliances Point of Load Regulation for High-Performance DSPs, FPGAs, and ASICs Note : Richtek products are : RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Simplified Application Circuit L RT7255 VIN VIN VOUT SW CBOOT CIN BOOT RPG CFF R1 COUT PGOOD 5V EN Enable SS FB R2 GND CSS Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT7255 Pin Configurations Marking Information EN PGOOD 0B=DNN VIN RT7255AGV8 SW (TOP VIEW) 8 7 6 5 0B= : Product Code DNN : Date Code RT7255BGV8 0A=DNN SS GND BOOT 4 RT7255CGV8 FB 3 2 0A= : Product Code DNN : Date Code 09=DNN 09= : Product Code DNN : Date Code SOT-23-8 RT7255DGV8 06=DNN 06= : Product Code DNN : Date Code Functional Pin Description Pin No. Pin Name Pin Function 1 BOOT Bootstrap Supply for High-Side Gate Driver. Connect a 1F ceramic capacitor between the BOOT and SW pins. 2 GND Power Ground. 3 SS Soft-Start Time Setting. Connect a capacitor from SS to GND to set the soft-start period. 4 FB Feedback Voltage Input. The pin is used to set the output voltage of the converter via a resistive divider. The converter regulates VFB to 0.6V. 5 PGOOD Power Good Indicator Output. 6 EN Enable Control Input. Connect EN to a logic-high voltage to enable the IC or to a logic-low voltage to disable. Do not leave this high impedance input unconnected. 7 VIN Power Input. The input voltage range is from 4.3V to 18V. Must bypass with a suitable large ceramic capacitor at this pin. 8 SW Switch Node. Connect to external L-C filter. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 Function Block Diagram BOOT VIN PVCC Reg VIBIAS Min. Off PVCC VIN VREF UGATE Control OC Driver SW LGATE UV & OV GND SW PVCC SW Ripple Gen. SS FB VIN SW PGOOD + Comparator On-Time GND PWRGN EN EN Operation The RT7255 is a synchronous step-down converter with advanced constant on-time control mode. Using the ACOT control mode can reduce the output capacitance and fast transient response. It can minimize the component size without additional external compensation network. UVLO Protection 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. Thermal Shutdown Current Protection When the junction temperature exceeds the OTP The inductor current is monitored via the internal switches cycle-by-cycle. Once the output voltage drops under UV threshold, the RT7255 will enter hiccup mode. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 threshold value, the IC will shut down the switching operation. Once the junction temperature cools down and is lower than the OTP lower threshold, the converter will autocratically resume switching. is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT7255 Absolute Maximum Ratings (Note 1) VIN to GND ------------------------------------------------------------------------------------------------------------- 0.3V to 20V SW to GND ------------------------------------------------------------------------------------------------------------- 0.3V to 20V <10ns --------------------------------------------------------------------------------------------------------------------- 5V to 25V BOOT to GND ---------------------------------------------------------------------------------------------------------- 0.3V to 26V BOOT to SW------------------------------------------------------------------------------------------------------------ 0.3V to 6V Other Pins --------------------------------------------------------------------------------------------------------------- 0.3V to 6V Power Dissipation, PD @ TA = 25C SOT-23-8 ---------------------------------------------------------------------------------------------------------------- 0.53W Package Thermal Resistance (Note 2) SOT-23-8, JA ---------------------------------------------------------------------------------------------------------- 186.2C/W SOT-23-8, JC ---------------------------------------------------------------------------------------------------------- 47.4C/W Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------- 260C Junction Temperature ------------------------------------------------------------------------------------------------ 150C Storage Temperature Range --------------------------------------------------------------------------------------- 65C to 150C ESD Susceptibility (Note 3) HBM (Human Body Model) ----------------------------------------------------------------------------------------- 2kV Recommended Operating Conditions (Note 4) Supply Input Voltage ------------------------------------------------------------------------------------------------- 4.3V to 18V Ambient Temperature Range--------------------------------------------------------------------------------------- 40C to 85C Junction Temperature Range -------------------------------------------------------------------------------------- 40C to 125C Electrical Characteristics (VIN = 12V, TA = 25C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Supply Current (Shutdown) ISHDN VEN = 0V -- -- 12 A Supply Current (Quiescent) IQ VEN = 2V, VFB = 1V -- 0.5 -- mA High-Side RDS(ON)_H VBST SW = 4.8V -- 230 -- Low-Side RDS(ON)_L PVIN = 5V -- 130 -- Current Limit ILIM Vally Current 1.7 2.2 2.8 A Oscillator Frequency f SW -- 500 -- kHz Maximum Duty Cycle DMAX -- 90 -- % Minimum On-Time tON -- 60 -- ns Feedback Threshold Voltage VFB 591 600 609 mV 1.5 -- -- VEN_L -- -- 0.4 VUVLO 3.55 3.9 4.25 Switch-On Resistance Enable Input Voltage Logic-High VEN_H Logic-Low VIN Under-Voltage Lockout Threshold-Rising Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 m V V is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 Parameter Symbol Test Conditions Min Typ Max Unit VIN Under-Voltage Lockout Threshold-Hysteresis VUVLO -- 340 -- mV Soft-Start Period tSS -- 800 -- s Thermal Shutdown Threshold TSD -- 160 -- C Thermal Shutdown Hysteresis TSD -- 20 -- C VOUT Discharge Resistance RDischarge EN = 0V, VOUT = 0.5V -- 50 100 VFB rising (Good) 87 92 97 VFB falling (Fault) -- 80 -- VSS = 0V -- 4 -- Power Good Threshold Soft-Start Charge Current ISS % A 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. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. The measurement case position of JC is on the lead of the package. Note 3. Devices are ESD sensitive. Handling precaution recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT7255 Typical Application Circuit 7 VIN 4.3V to 18V CIN 10μF 5V RPG 100k SW BOOT 5 6 Enable VIN RT7255 PGOOD EN 3 SS CSS 10nF FB L 3.6μH 8 VOUT 1.2V CBOOT 1μF 1 CFF R1 10k COUT 22μF 4 R2 10k GND 2 Table 1. Suggested Component Values VOUT (V) R1 (k) R2 (k) L (H) COUT (F) CFF (pF) 5 110 15 10 22 39 3.3 115 25.5 6.8 22 33 2.5 25.5 8.06 4.7 22 NC 1.2 10 10 3.6 22 NC Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 Typical Operating Characteristics Efficiency vs. Load Current Efficiency vs. Load Current 100 100 PWM 90 80 80 70 VIN = 5V 60 VIN = 9V 50 VIN = 12V Efficiency (%) Efficiency (%) PSM 90 VIN = 17V 40 30 70 60 VIN = 5V 50 VIN = 9V 40 VIN = 12V 30 VIN = 17V 20 20 10 10 VOUT = 1.2V 0 0 0.3 0.6 0.9 1.2 VOUT = 1.2V 0 0.01 1.5 0.1 1 10 Load Current (A) Load Current (A) Referecnce Voltage vs. Input Voltage Reference vs. Temperature 0.610 0.610 PWM PWM Reference Voltage (V) Referecnec Voltage(V) 0.608 0.605 0.603 0.600 0.598 0.595 0.605 VIN = 18V VIN = 12V 0.600 VIN = 9V VIN = 4.5V 0.595 0.593 VIN = 4.5V to 18V, V OUT = 1.2V, IOUT = 0A VIN = 4.5V to 18V, V OUT = 1.2V, IOUT = 0A 0.590 0.590 4.5 6.5 8.5 10.5 12.5 14.5 16.5 -50 18.5 -25 25 50 Output Voltage vs. Load Current 100 125 Output Voltage vs. Load Current 1.230 1.250 PWM PSM 1.222 Output Voltage (V) 1.234 VIN = 17V VIN = 12V 1.214 1.206 VIN = 9V VIN = 5V 1.198 1.218 1.202 VIN = 17V VIN = 12V 1.186 VIN = 4.5V VIN = 5V VOUT = 1.2V VOUT = 1.2V 1.170 1.190 0 0.3 0.6 0.9 1.2 Load Current (A) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 75 Temperature (°C) Input Voltage(V) Output Voltage (V) 0 June 2015 1.5 0 0.3 0.6 0.9 1.2 1.5 Load Current (A) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT7255 Switching Frequency. vs. Input Voltage Switching Frequency vs. Temperature 600 650 PWM Switching Frequency (kHz)1 Switcing Frequency (kHz)1 PWM 590 580 570 560 550 540 530 520 510 VOUT = 1.2V, IOUT = 0A 630 610 VIN = 18V 590 VIN = 12V VIN = 6V 570 VIN = 4.5V 550 530 510 490 470 VOUT = 1.2V 450 500 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 -50 -25 0 25 50 75 100 125 Temperature (°C) Input Voltage (V) Current Limit vs. Temperature 3.0 2.8 Current Limit (A) 2.6 2.4 VIN = 18V 2.2 VIN = 12V 2.0 VIN = 6V 1.8 1.6 1.4 1.2 1.0 -50 -25 0 25 50 75 100 125 Temperature (°C) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT7255 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 Application Information Inductor Selection The ripple current was selected at 0.6A and, as long as Selecting an inductor involves specifying its inductance and also its required peak current. The exact inductor value is generally flexible and is ultimately chosen to obtain the best mix of cost, physical size, and circuit efficiency. Lower inductor values benefit from reduced we use the calculated 3.6H inductance, that should be the actual ripple current amount. The ripple current and required peak current as below : size and cost and they can improve the circuit's transient response, but they increase the inductor ripple current and output voltage ripple and reduce the efficiency due to the resulting higher peak currents. Conversely, higher inductor values increase efficiency, but the inductor will either be physically larger or have higher resistance since more turns of wire are required and transient response will be slower since more time is required to change current (up or down) in the inductor. A good compromise between size, efficiency, and transient response is to use a ripple current (IL) about 20% to 40% of the desired full output load current. Calculate the approximate inductor value by selecting the input and output voltages, the switching frequency (f SW ), the maximum output current (IOUT(MAX)) and estimating a IL as some percentage of that current. L= VOUT VIN VOUT VIN fSW IL Once an inductor value is chosen, the ripple current (IL) is calculated to determine the required peak inductor current. VOUT VIN VOUT VIN fSW L I IL(PEAK) = IOUT(MAX) L 2 IL IL(VALLY) = IOUT(MAX) 2 IL = Considering the Typical Operating Circuit for 1.2V output at 1.5A and an input voltage of 12V, using an inductor ripple of 0.6A (40%), the calculated inductance value is : 1.2 12 1.2 L= = 3.6μH 12 500kHz 0.6 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 1.2 12 1.2 = 0.6A 12 500kHz 3.6μH and IL(PEAK) = 1.5A 0.6 = 1.8A 2 IL = Inductor's saturation current should be chosen over IC's current limit. Input Capacitor Selection The input filter capacitors are needed to smooth out the switched current drawn from the input power source and to reduce voltage ripple on the input. The actual capacitance value is less important than the RMS current rating (and voltage rating, of course). The RMS input ripple current (IRMS) is a function of the input voltage, output voltage, and load current : IRMS = IOUT(MAX) VOUT VIN VIN 1 VOUT Ceramic capacitors are most often used because of their low cost, small size, high RMS current ratings, and robust surge current capabilities. However, take care when these capacitors are used at the input of circuits supplied by a wall adapter or other supply connected through long, thin wires. Current surges through the inductive wires can induce ringing at the RT7255 input which could potentially cause large, damaging voltage spikes at VIN. If this phenomenon is observed, some bulk input capacitance may be required. Ceramic capacitors (to meet the RMS current requirement) can be placed in parallel with other types such as tantalum, electrolytic, or polymer (to reduce ringing and overshoot). Choose capacitors rated at higher temperatures than required. Several ceramic capacitors may be paralleled to meet the RMS current, size, and height requirements of the application. The typical operating circuit use 10F and one 0.1F low ESR ceramic capacitors on the input. is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT7255 Output Capacitor Selection The RT7255 is optimized for ceramic output capacitors and best performance will be obtained using them. The total output capacitance value is usually determined by the desired output voltage ripple level and transient response requirements for sag (undershoot on positive load steps) and soar (overshoot on negative load steps). Output Ripple Output ripple 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. VRIPPLE = VRIPPLE(ESR) VRIPPLE(C) VRIPPLE(ESR) = IL RESR VRIPPLE(C) = IL 8 COUT fSW For the Typical Operating Circuit for 1.2V output and an inductor ripple of 0.46A, with 1 x 22F output capacitance each with about 5m ESR including PCB trace resistance, the output voltage ripple components are : VRIPPLE(ESR) = 0.46A 5m = 2.3mV 0.46A = 5.227mV 8 22μF 500kHz = 2.3mV 5.227mV = 7.527mV VRIPPLE(C) = VRIPPLE Output Transient Undershoot and Overshoot In addition to voltage ripple 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 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 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 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 : VESR _STEP = IOUT x RESR 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 ACOTTM control scheme will ramp the current using on-times spaced apart with minimum off-times, which is as fast as allowed. Calculate the approximate on-time (neglecting parasitics) and maximum duty cycle for a given input and output voltage as : tON = VOUT tON and DMAX = VIN fSW tON tOFF(MIN) 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 : VSAG = L (IOUT )2 2 COUT VIN(MIN) DMAX VOUT is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 The amplitude of the capacitive soar is a function of the load step, the output capacitor value, the inductor value and the output voltage : steady-state stability of the circuit. Choose a suitable capacitor value that following below step. VSOAR = Get the BW the quickest method to do transient response form no load to full load. Confirm the damping frequency. The damping frequency is BW. L (IOUT ) 2 COUT VOUT 2 Output Capacitors Stability Criteria The RT7255's ACOTTM control architecture uses an internal virtual inductor current ramp and other compensation that ensures stability with any reasonable output capacitor. The internal ramp allows the IC to operate with very low ESR capacitors and the IC is stable with very small capacitances. Therefore, output capacitor selection is nearly always a matter of meeting output voltage ripple and transient response requirements, as discussed in the previous sections. For the sake of the unusual application where ripple voltage is unimportant. Any ESR in the output capacitor lowers the required minimum output capacitance, sometimes considerably. For the rare application where that is needed and useful, the equation including ESR is given here : VOUT COUT 2 fSW VIN (RESR 13647 L VOUT ) As can be seen, setting RESR to zero and simplifying the equation yields the previous simpler equation. To allow for the capacitor's temperature and bias voltage coefficients, use at least double the calculated capacitance and use a good quality dielectric such as X5R or X7R with an adequate voltage rating since ceramic capacitors exhibit considerable capacitance reduction as their bias voltage increases. Feed-forward Capacitor (Cff) The RT7255 is optimized for ceramic output capacitors and for low duty cycle applications. However for high-output voltages, with high feedback attenuation, the circuit's response becomes over-damped and transient response can be slowed. In high-output voltage circuits (VOUT > 3.3V) transient response is improved by adding a small “feed-forward” capacitor (Cff) across the upper FB divider resistor (Figure 1), to increase the circuit's Q and reduce damping to speed up the transient response without affecting the Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 BW VOUT R1 Cff FB RT7255 R2 GND Figure 1. Cff Capacitor Setting Cff can be calculated base on below equation : Cff 1 2 3.1412 R1 BW 0.8 External Soft-Start (SS) The RT7255 provides soft-start function. The soft-start function is used to prevent large inrush current while converter is being powered-up. The soft-start timing can be programmed by the external capacitor CSS between SS and GND. An internal current source ISS (6A) charges an external capacitor to build a soft-start ramp voltage. The VFB voltage will track the internal ramp voltage during soft-start interval. The typical soft start time is calculated as follows : Soft-Start time tSS = CSS x 0.6 / 6A The available capacitance range is from 2.7nF to 120nF. Do not leave SS unconnected Enable Operation (EN) For automatic start-up the high-voltage EN pin can be connected to VIN, through a 100k resistor. Its large hysteresis band makes EN useful for simple delay and timing circuits. EN can be externally pulled to VIN by is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT7255 adding a resistor-capacitor delay (REN and CEN in Figure 2). Calculate the delay time using EN's internal threshold where switching operation begins (1.4V, typical). An external MOSFET can be added to implement digital control of EN when no system voltage above 2V is available (Figure 3). In this case, a 100kpull-up resistor, REN, is connected between VIN and the EN pin. MOSFET Q1 will be under logic control to pull down the EN pin. To prevent enabling circuit when VIN is smaller than the VOUT target value or some other desired voltage level, a resistive voltage divider can be placed between the input voltage and ground and Output Voltage Setting Set the desired output voltage using a resistive divider from the output to ground with the midpoint connected to FB. The output voltage is set according to the following equation : VOUT = 0.6 x (1 + R1 / R2) VOUT R1 FB RT7255 GND connected to EN to create an additional input under voltage lockout threshold (Figure 4). Figure 5. Output Voltage Setting Place the FB resistors within 5mm of the FB pin. Choose R2 between 10k and 100k to minimize EN VIN REN EN RT7255 CEN power consumption without excessive noise pick-up and calculate R1 as follows : GND R1 Figure 2. External Timing Control VIN REN 100k R2 (VOUT 0.6) 0.6 For output voltage accuracy, use divider resistors with 1% or better tolerance. EN RT7255 Q1 Enable GND Figure 3. Digital Enable Control Circuit VIN R2 REN1 EN REN2 RT7255 GND Figure 4. Resistor Divider for Lockout Threshold Setting External BOOT Bootstrap Diode When the input voltage is lower than 5.5V it is recommended to add an external bootstrap diode between VIN (or VINR) and the BOOT pin to improve enhancement of the internal MOSFET switch and improve efficiency. The bootstrap diode can be a low cost one such as 1N4148 or BAT54. External BOOT Capacitor Series Resistance The internal power MOSFET switch gate driver is optimized to turn the switch on fast enough for low power loss and good efficiency, but also slow enough to reduce EMI. Switch turn-on is when most EMI occurs since VSW rises rapidly. During switch turn-off, SW is discharged relatively slowly by the inductor current during the deadtime between high-side and low-side switch on-times. In some cases it is desirable to reduce EMI further, at the expense of some additional power dissipation. The switch turn-on can be slowed by placing a small (<47) resistance between BOOT and the external bootstrap capacitor. This will slow the Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 high-side switch turn-on and VSW's rise. To remove the resistor from the capacitor charging path (avoiding poor enhancement due to undercharging the BOOT Thermal Considerations capacitor), use the external diode shown in Figure 6 to charge the BOOT capacitor and place the resistance between BOOT and the capacitor/diode connection. dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : 5V For continuous operation, do not exceed absolute maximum junction temperature. The maximum power PD(MAX) = (TJ(MAX) TA) / JA RT7255 0.1μF SW Figure 6. External Bootstrap Diode Over-Temperature Protection The RT7255 features an Over-Temperature Protection (OTP) circuitry to prevent from overheating due to excessive power dissipation. The OTP will shut down switching operation when junction temperature exceeds 150C. Once the junction temperature cools down by approximately 20C, the converter will resume operation. To maintain continuous operation, the maximum junction temperature should be lower than 125C. Under-Voltage Protection Hiccup Mode For the Hiccup, it provides Hiccup Mode Under Voltage Protection (UVP). When the VFB voltage drops below 0.4V, the UVP function will be triggered to shut down switching operation. If the UVP condition remains for a period, theRT7255 will retry automatically. When the UVP condition is removed, the converter will resume operation. The UVP is disabled during soft-start period. Clamp Mode For the Clamp, it provides Current limit protection, Under Voltage Protection (UVP) is disable, when the UV condition is removed, the converter will resume operation. where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and JA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125C. The junction to ambient thermal resistance, JA, is layout dependent. For SOT-23-8 package, the thermal resistance, JA, is 186.2C/W on a standard four-layer thermal test board. The maximum power dissipation at TA = 25C can be calculated by the following formula : PD(MAX) = (125C 25C) / (186.2C/W) = 0.53W for SOT-23-8 package The maximum power dissipation depends on the operating ambient temperature for fixed TJ(MAX) and thermal resistance, JA. The derating curve in Figure 7 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 0.6 Maximum Power Dissipation (W)1 BOOT Four-Layer PCB 0.5 0.4 0.3 0.2 0.1 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 7. Derating Curve of Maximum Power Dissipation Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT7255 Layout Considerations For best performance of the RT7255, the following SW node is with high frequency voltage swing and should be kept at small area. Keep analog components away from the SW node to prevent stray capacitive noise pick-up. Components near the RT7255 An example of PCB layout guide is shown in Figure for reference. layout guidelines must be strictly followed. Input capacitor must be placed as close to the IC as possible. Keep the traces of the main current paths as short and wide as possible. SW should be connected to inductor by Wide and short trace. Keep sensitive components away from this trace. Suggestion layout trace wider for thermal. COUT VOUT Keep sensitive components away from this trace. Suggestion layout trace wider for thermal. COUT SW CS* R S* VIN 6 EN 5 4 VOUT 3 R1 FB 2 CSSGND SS SW 7 BOOT 8 Suggestion layout trace wider for thermal. PGOOD GND CIN REN CIN VIN 5V Input capacitor must be placed as close to the IC as possible. Suggestion layout trace wider for thermal. RPG R2 The REN component must be connected to VIN. Suggestion layout trace wider for thermal. The feedback components must be connected as close to the device as possible. Figure 8. PCB Layout Guide Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS7255-00 June 2015 RT7255 Outline Dimension Dimensions In Millimeters Symbol Dimensions In Inches Min Max Min Max A 1.000 1.450 0.039 0.057 A1 0.000 0.150 0.000 0.006 B 1.500 1.700 0.059 0.067 b 0.220 0.500 0.009 0.020 C 2.600 3.000 0.102 0.118 D 2.800 3.000 0.110 0.118 e 0.585 0.715 0.023 0.028 H 0.100 0.220 0.004 0.009 L 0.300 0.600 0.012 0.024 SOT-23-8 Surface Mount Package 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 furnished 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 infringements 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 Richtek or its subsidiaries. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS7255-00 June 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 17