® RT2875A/B 3A, 36V, Synchronous Step-Down Converter General Description Features The RT2875A/B is a high efficiency, current-mode synchronous DC/DC step-down converter that can deliver up to 3A output current over a wide input voltage range from 4.5V to 36V. The device integrates 95mΩ high-side and 70mΩ low-side MOSFETs to achieve high conversion efficiency. The current-mode control architecture supports fast transient response and simple external compensation. A cycle-by-cycle current limit function provides protection 3A Output Current Internal N-MOSFETs Current Mode Control Adjustable Switching Frequency : 300kHz to 2.1MHz Adjustable Current Limit : 1.5A to 6A Synchronous to External Clock : 300kHz to 2.1MHz Adjustable Output Voltage from 0.6V to 24V High Efficiency Up to 95% Stable with Low ESR Ceramic Output Capacitors Cycle-by-Cycle Current Limit Input Under-Voltage Lockout Output Under-Voltage Protection Thermal Shutdown AEC-Q100 Grade 2 Certification RoHS Compliant and Halogen Free against shorted output and an external soft-start eliminates input current surge during start-up. The RT2875A/B provides complete protection functions such as input under-voltage lockout, output under-voltage protection, over-current protection and thermal shutdown. The RT2875A/B is available in the thermal enhanced TSSOP-14 (Exposed Pad) package. Applications Pin Configurations (TOP VIEW) SW SW PGND RT/SYNC AGND RLIM FB BOOT VIN VIN PGOOD EN SS COMP 14 2 13 3 4 12 PGND 5 11 10 6 15 7 9 8 Point of Load Regulator in Distributed Power Systems Digital Set Top Boxes Broadband Communications Vehicle Electronics TSSOP-14 (Exposed Pad) Simplified Application Circuit VIN VIN CIN BOOT CBOOT L RT2875A/B VOUT SW Enable PGOOD EN PGOOD RLIM RT/SYNC RLIM ROSC R1 COUT FB RCOMP CCOMP COMP R2 SS CSS AGND Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 PGND is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT2875A/B Ordering Information Marking Information RT2875A/B RT2875AQGCP Package Type CP: TSSOP-14 (Exposed Pad) Lead Plating System G : Green (Halogen Free and Pb Free) AQ : Latched UVP BQ : Hiccup Mode UVP Note : RT2875AQGCP : Product Number RT2875AQ GCPYMDNN YMDNN : Date Code RT2875BQGCP RT2875BQGCP : Product Number RT2875BQ GCPYMDNN YMDNN : Date Code 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. Functional Pin Description Pin No. 1, 2 Pin Name SW 3, PGND 15 (Exposed Pad) Pin Function Switch Node. Connect to external L-C filter. Power Ground. The exposed pad must be soldered to a large PCB and connected to PGND for maximum power dissipation. 4 RT/SYNC Oscillator Resistor and External Frequency Synchronization Input. Must connect a resistor from this pin to GND to set the switching frequency. If SYNC clock is requested, connect an external clock to change the switching frequency. 5 AGND Analog Ground. 6 RLIM Current Limit Setting. Connect a resistor from this pin to GND to set the current limit value. 7 FB Feedback Voltage Input. The pin is used to set the output voltage of the converter to regulate to the desired via a resistive divider. Feedback reference = 0.6V. 8 COMP Compensation Node. COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND. In some cases, an additional capacitor from COMP to GND is required. 9 SS Soft-Start Time Setting. Connect a capacitor from SS to GND to set the soft-start period. 10 EN Enable Control Input. High = Enable. 11 PGOOD Power Good Indicator Output. VIN Power Input. Support 4.5V to 36V input voltage. Must bypass with a suitable large ceramic capacitor at this pin. BOOT Bootstrap Supply for High-Side Gate Driver. Connect a 0.1F ceramic capacitor between the BOOT and SW pins. 12, 13 14 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Function Block Diagram PGOOD 6k - EN VIN Internal Regulator UVLO + Shutdown Comparator 1.5V 3.8V Current Sense BOOT UVLO + 0.55V PGOOD Logic & Comparator Protection Control 0.3V + BOOT Power Stage & Deadtime Control UV Comparator FB 0.6V HS Switch Current Comparator +EA + AGND 6µA SS Oscillator SW LS Switch Current Comparator Current Sense PGND Slop Compensation COMP RT/SYNC RLIM Operation The RT2875A/B is current-mode synchronous step-down converter. In normal operation, the high-side N-MOSFET is turned on when the S-R latch is set by the oscillator and is turned off when the current comparator resets the S-R latch. While the high-side N-MOSFET is turned off, the low-side N-MOSFET is turned on to conduct the inductor current until next cycle begins. Error Amplifier The error amplifier adjusts its output voltage by comparing the feedback signal (VFB) with the internal 0.6V reference. When the load current increases, it causes a drop in the feedback voltage relative to the reference, and then the error amplifier's output voltage rises to allow higher inductor current to match the load current. Internal Regulator The regulator provides low voltage power to supply the internal control circuits and the bootstrap power for highside gate driver. Enable The converter is turned on when the EN pin is higher than 1.6V. When the EN pin is lower than 0.4V, the converter will enter shutdown mode and reduce the supply current lower than 10μA. Soft-Start (SS) In order to prevent the converter output voltage from overshooting during the startup period, the soft-start function is necessary. The soft-start time is adjustable by an external capacitor. Switching Frequency The switching frequency can be set by using extra resister RT or external clock. Switching frequency range is from 300kHz to 2.1MHz. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT2875A/B UV Comparator If the feedback voltage is lower than 0.3V, the UV Comparator will go high to turn off the high-side MOSFET. The output under voltage protection is designed to operate in Hiccup mode. When the UV condition is removed, the converter will resume switching. Current Setting The current limit of high side MOSFET is adjustable by an external resistor connected to the RLIM pin. The current limit range is from 1.5A to 6A. Thermal Shutdown The over-temperature protection function will shut down the switching operation when the junction temperature exceeds 180°C. Once the junction temperature cools down by approximately 15°C, the converter will automatically resume switching. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Absolute Maximum Ratings (Note 1) Supply Voltage, VIN -----------------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------BOOT to SW --------------------------------------------------------------------------------------------------------Power Good Voltage, PGOOD -----------------------------------------------------------------------------------Other Pins ------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C TSSOP-14 (Exposed Pad) ---------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) TSSOP-14 (Exposed Pad), θJA ---------------------------------------------------------------------------------TSSOP-14 (Exposed Pad), θJC ---------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------- Recommended Operating Conditions −0.3V to 40V −0.3V to (VIN + 0.3V) −0.3V to 6V −0.3V to 40V −0.3V to 6V 4.464W 28°C/W 4.3°C/W 260°C 150°C −65°C to 150°C 2kV (Note 4) Supply Input Voltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 36V Junction Temperature Range -------------------------------------------------------------------------------------- −40°C to 150°C Ambient Temperature Range -------------------------------------------------------------------------------------- −40°C to 105°C Electrical Characteristics (VIN = 12V, TA = −40°C to 105°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Shutdown Supply Current VEN = 0V -- ‐‐ 10 A Switching quiescent current with no load at DCDC output VEN = 2V, VFB = 0.64V, RLIM = 91k, ROSC = 169k -- -- 1.3 mA 0.588 0.6 0.612 V -- 950 -- A/V Feedback Voltage VFB Error Amplifier Trans-conductance G EA 4.5V VIN 36V IC = ±10A High-Side R DS(ON)1 -- 95 -- Low-Side High-Side Switch Leakage Current Current Limit Setting Rage R DS(ON)2 -- 70 -- -- 1 -- A (Note 5) 1.5 -- 6 A High-Side Switch Current Limit 1 H OC1 RLIM = 100k 1.79 2.1 2.41 A High-Side Switch Current Limit 2 H OC2 RLIM = 47k 3.52 4 4.48 A High-Side Switch Current Limit 3 H OC3 RLIM = 33k 4.84 5.5 6.16 A -- 2 -- A -- 5.2 -- A/V 300 -- 2100 kHz Switch OnResistance VEN = 0V, VSW = 0V Low-Side Switch Current Limit COMP to Current Sense Transconductance Switching Frequency Range From Drain to Source G CS Include Sync mode and RT mode set point Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 m is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT2875A/B Parameter Symbol Test Conditions Min Typ Max Unit Switching Frequency1 fOSC1 Rt = 169k 275 305 335 kHz Switching Frequency2 fOSC2 Rt = 51k 0.83 0.98 1.13 MHz Switching Frequency3 fOSC3 Rt = 23k 1.89 2.1 2.31 MHz -- 31.25 -- kHz -- 20 -- ns High-Level -- -- 2 Low- Level 0.8 -- -- tON -- 100 -- VIH 1.4 1.5 1.6 EN hysteresis voltage -- 0.2 -- VUVLO VIN Rising -- 4.1 -- V VUVLO Hysteresis -- 300 -- mV Rising -- 90 -- Falling -- 85 -- Power Good Output High Leakage Current VFB = VREF, VPGOOD = 5.5V -- 30 -- nA Power Good Output Low IPGOOD = 0.4mA -- -- 0.3 V -- 6 -- A -- 80 -- 160 180 200 C -- 15 -- C Short Circuit Oscillation Frequency VFB = 0V, ROSC = 100k, VIN = 12V Minimum SYNC Pulse width SYNC Input Voltage Minimum On-Time EN Input Voltage Logic-High Hysteresis Input Under-Voltage Lockout Threshold Power Good Threshold Soft-Start Charge Current ISS SW Discharge Resistance Thermal Shutdown TSD Thermal Shutdown Hysteresis TSD V ns V % 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. θJA is measured at TA = 25 °C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is measured at the exposed pad 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. Note 5. Guarantee by design. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Typical Application Circuit 12, 13 BOOT VIN CIN RT2875A/B 10µF x 2 SW 10 EN 11 PGOOD FB 6 RLIM 4 RT/SYNC COMP VIN Enable PGOOD RLIM ROSC SS 14 1, 2 CBOOT L VOUT R1 COUT 22µF x 2 7 8 RCOMP CCOMP R2 9 CSS AGND 5 PGND 3, 15 (Exposed Pad) For 500kHz Only VOUT R1 (k) R2 (k) ROSC (k) RCOMP (k) CCOMP (nF) L (H) 12 102 5.36 100 32 3.9 10 8 102 8.25 100 20 3.3 8.2 5 110 15 100 15 3.3 6.8 3.3 115 25.5 100 10 3.3 4.7 2.5 25.5 8.06 100 7.5 3.3 3.6 1.2 10 10 100 4.3 3.9 2.2 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT2875A/B Typical Operating Characteristics Efficiency vs. Load Current Output Voltage vs. Load Current 100 3.37 90 3.36 Output Voltage (V) Efficiency (%) 80 VIN = 5V VIN = 12V VIN = 23V VIN = 30V VIN = 36V 70 60 50 40 30 3.35 3.34 VIN = 12V VIN = 5V VIN = 24V VIN = 30V VIN = 36V 3.33 3.32 20 3.31 10 VOUT = 3.3V VOUT = 3.3V 0 3.30 0 0.5 1 1.5 2 2.5 3 0 0.5 1 Load Current (A) Referecnec Voltage vs. Input Voltage 2.5 3 Reference Voltage vs. Temperature 0.65 0.64 Reference Voltage (V) 0.608 Referecnec Voltage (V) 2 Load Current (A) 0.610 0.605 0.603 0.600 0.598 0.595 0.593 0.63 0.62 0.61 0.60 0.59 0.58 0.57 0.56 VIN = 4.5V to 36V, VOUT = 3.3V, IOUT = 0A 0.590 VIN = 12V, VOUT = 1.2V, IOUT = 0A 0.55 2 9.6 17.2 24.8 32.4 -50 40 -25 0 Switching Frequency vs. RT 50 75 100 125 Switching Frequency vs. Input Voltage 600 1800 590 Switching Frequency (kHz)1 2000 1600 1400 1200 1000 800 600 400 200 25 Temperature (°C) Input Voltage (V) Switching Frequency (kHz)1 1.5 VIN = 12V, VOUT = 3.3V, IOUT = 0A 0 580 570 560 550 540 530 520 510 VIN = 12V, VOUT = 3.3V, IOUT = 0A, RT = 100kΩ 500 20 40 60 80 100 120 140 160 180 RT(k Ω) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 200 4 8 12 16 20 24 28 32 36 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Current Limit vs. RLIM Switching Frequency vs. Temperature 7 580 6 560 VIN = 4.5V VIN = 12V VIN = 24V VIN = 36V 540 520 Current Limit (A) Switching Frequency (kHz)1 600 500 480 460 5 4 3 2 440 1 420 VOUT = 3.3V, IOUT = 0A 0 400 -50 -25 0 25 50 75 100 20 125 30 40 50 60 70 80 90 100 RLIM (kΩ) Ambient Temperature (°C) Current Limit vs. Temperature Enable Voltage vs. Temperature 8 2.0 1.8 6 5 4 1.4 1.2 Enable_Falling 1.0 0.8 0.6 0.4 3 VIN = 12V, VOUT = 3.3V, RLIM = 39kΩ 2 0.2 VIN = 12V, VOUT = 3.3V 0.0 -50 Input Voltage (V) Enable_Rising 1.6 Enable Voltage (V) Current limit (A) 7 -25 0 25 50 75 100 125 -50 -25 0 25 50 75 100 Temperature (°C) Temperature (°C) UVLO vs. Temperature Load Transient Response 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 125 VOUT (200mV/Div) Turn On Turn Off VIN = 12V, VOUT = 3.3V -50 -25 0 25 50 75 100 125 IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 0A to 3A Time (250μs/Div) Temperature (°C) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT2875A/B Load Transient Response Switching VOUT (5mV/Div) VOUT (200mV/Div) VSW (10V/Div) IOUT (2A/Div) VIN = 12V, VOUT = 1.2V, IOUT = 0A to 2.5A IOUT (1A/Div) Time (250μs/Div) Time (1μs/Div) Switching Power On from EN VOUT (5mV/Div) VEN (2V/Div) VSW (10V/Div) VOUT (2V/Div) IOUT (2A/Div) IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 3A Time (5ms/Div) Power Off from EN Power On from VIN VIN (5V/Div) VOUT (2V/Div) VOUT (2V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 3A Time (50μs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 VIN = 12V, VOUT = 3.3V, IOUT = 3A Time (1μs/Div) VEN (2V/Div) IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.5A IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 3A Time (10ms/Div) is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Power Off from VIN VIN (5V/Div) VOUT (2V/Div) IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, sIOUT = Time (5ms/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT2875A/B Application Information Output Voltage Setting Chip Enable Operation The resistive divider allows the FB pin to sense the output voltage as shown in Figure 1. The EN pin is the chip enable input. Pulling the EN pin low (<0.4V) will shutdown the device. During shutdown mode, the RT2875A/B quiescent current drops to lower than 10μA. Driving the EN pin high (>1.6V) will turn on the device again. For external timing control, the EN pin can also be externally pulled high by adding a REN resistor and CEN capacitor from the VIN pin (see Figure 3). VOUT R1 FB RT2875A/B R2 GND EN Figure 1. Output Voltage Setting VIN REN EN RT2875A/B CEN The output voltage is set by an external resistive voltage divider according to the following equation : VOUT = VREF 1 R1 R2 Where VREF is the reference voltage (0.6V typ.). External Bootstrap Diode Connect a 0.1μF low ESR ceramic capacitor between the BOOT and SW pins. This capacitor provides the gate driver voltage for the high side MOSFET. It is recommended to add an external bootstrap diode between an external 5V and BOOT pin for efficiency improvement when input voltage is lower than 5.5V or duty ratio is higher than 65% .The bootstrap diode can be a low cost one such as IN4148 or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT2875A/B. Note that the external boot voltage must be lower than 5.5V BOOT 100nF SW Figure 2. External Bootstrap Diode Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 Figure 3. Enable Timing Control An external MOSFET can be added to implement digital control on the EN pin when no system voltage above 2.5V is available, as shown in Figure 4. In this case, a 100kΩ pull-up resistor, REN, is connected between VIN and the EN pin. MOSFET Q1 will be under logic control to pull down the EN pin. VIN EN REN 100k EN Q1 RT2875A/B GND Figure 4. Digital Enable Control Circuit Under Voltage Protection Hiccup Mode 5V RT2875A/B GND The RT2875B provides Hiccup Mode Under Voltage Protection (UVP). When the VFB voltage drops below 0.3V, the UVP function will be triggered to shut down switching operation. If the UVP condition remains for a period, the RT2875B will retry automatically. When the UVP condition is removed, the converter will resume operation. The UVP is disabled during soft-start period. is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Latch Mode For the RT2875A it provides Latch-Off Mode Under Voltage Protection (UVP). When the VFB voltage drops below 0.3V, UVP will be triggered and the RT2875A will shut down in Latch-Off Mode. In shutdown condition, the RT2875A can be reset by EN pin or power input VIN. For the ripple current selection, the value of ΔIL = 0.24(IMAX) will be a reasonable starting point. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : VOUT VOUT L = 1 f I V L(MAX) IN(MAX) Hiccup Mode The inductor's current rating (caused a 40°C temperature rising from 25°C ambient) should be greater than the maximum load current and its saturation current should be greater than the short circuit peak current limit. Please see Table 2 for the inductor selection reference. VOUT (2V/Div) Table 2. Suggested Inductors for Typical Application Circuit ILX (2A/Div) IOUT = Short Time (50ms/Div) Figure 5. Hiccup Mode Under Voltage Protection Component Supplier Series Dimensions (mm) TDK VLF10045 10 x 9.7 x 4.5 TDK TAIYO YUDEN SLF12565 12.5 x 12.5 x 6.5 NR8040 8x8x4 Over Temperature Protection The RT2875A/B 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 180°C. Once the junction temperature cools down by approximately 15°C, the converter will resume operation. To maintain continuous operation, the maximum junction temperature should be lower than 150°C. Inductor Selection The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current ΔIL increases with higher VIN and decreases with higher inductance. V V IL = OUT 1 OUT f L VIN Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. High frequency with small ripple current can achieve the highest efficiency operation. However, it requires a large inductor to achieve this goal. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 CIN and COUT Selection The input capacitance, C IN, is needed to filter the trapezoidal current at the Source of the high side MOSFET. To prevent large ripple current, a low ESR input capacitor sized for the maximum RMS current should be used. The approximate RMS current equation is given : V IRMS = IOUT(MAX) OUT VIN VIN 1 VOUT This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT / 2. This simple worst case condition is commonly used for design because even significant deviations do not offer much relief. Choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. For the input capacitor, two 10μF low ESR ceramic capacitors are suggested. For the suggested capacitor, please refer to Table 3 for more details. The selection of COUT is determined by the required ESR to minimize voltage ripple. is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT2875A/B Moreover, the amount of bulk capacitance is also a key for COUT selection to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT , is determined by : 1 VOUT IL ESR 8fC OUT The output ripple will be the highest at the maximum input voltage since ΔIL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirement. Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. Switching Frequency Setting The switching frequency can be set by using extra resister RT or external clock. Switching frequency range is from 300kHz to 2.1MHz. Through extra resister RT connect to RT/SYNC pin to setting the switching frequency FS, below offer approximate formula equation : Current Setting The current limit of high side MOSFET is adjustable by an external resistor connected to the RLIM pin. The current limit range is from 1.5A to 6A. When the inductor current reaches the current limit threshold, the COMP voltage will be clamped to limit the inductor current. Inductor current ripple current also should be considered into current limit setting. Current limit minimum value should be set as below : Current limit minimum = (IO(max) + 1 / 2 inductor current ripple) x 1.2 Through extra resister RLIM connect to RLIM pin to setting the current limit value below offer approximate formula equation : ISET = current limit value (A) y = (ISET − 0.4206) / 167.79 RLIM (kΩ) = (1 / y) Soft-Start The RT2875A/B 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 (6μA) charges an external capacitor to build a soft-start ramp voltage. The VFB voltage will track the internal ramp voltage during softstart interval. The typical soft start time is calculated as follows : Soft-Start time tSS = CSS x 0.6 / 6μA Setting Frequency = FS (kHz) x = [FS − 31.379] / 47691 ROSC (kΩ) = (1 / x) The RT2875A/B can be synchronized with an external clock ranging from 300kHz to 2.1MHz applied to the RT/SYNC pin. The external clock duty cycle must be from 10% to 90%. The RT/SYNC pin is at logic-high level (>2V). If the EN pin is pulled to low-level for 10μs above, the IC will shut down. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS2875A/B-04 August 2015 RT2875A/B Thermal Considerations PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 150°C. The junction to ambient thermal resistance, θJA, is layout dependent. For TSSOP-14 (Exposed Pad) package, the thermal resistance, θJA, is 28°C/W on a standard JEDEC 51-7 Maximum Power Dissipation (W)1 For continuous operation, do not exceed absolute maximum junction temperature. The maximum power 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 : 5.0 Four-Layer PCB 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 25 50 75 100 125 150 Ambient Temperature (°C) Figure 6. Derating Curve of Maximum Power Dissipation four-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formula : PD(MAX) = (150°C − 25°C) / (28°C/W) = 4.464W for TSSOP-14 (Exposed Pad) package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curve in Figure 6 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2875A/B-04 August 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT2875A/B Outline Dimension Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A 1.000 1.200 0.039 0.047 A1 0.000 0.150 0.000 0.006 A2 0.800 1.050 0.031 0.041 b 0.190 0.300 0.007 0.012 D 4.900 5.100 0.193 0.201 e 0.650 0.026 E 6.300 6.500 0.248 0.256 E1 4.300 4.500 0.169 0.177 L 0.450 0.750 0.018 0.030 U 1.900 2.900 0.075 0.114 V 1.600 2.600 0.063 0.102 14-Lead TSSOP (Exposed Pad) Plastic 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. www.richtek.com 16 DS2875A/B-04 August 2015