® RT6234A/B 3A, 18V, 500kHz, ACOTTM Step-Down Converter General Description Features The RT6234A/B is a high-efficiency, monolithic synchronous step-down DC/DC converter that can deliver up to 3A output current from a 4.5V to 18V input supply. The RT6234A/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. Ω MOSFETs Integrated 130mΩ Ω / 70mΩ 4.5V to 18V Supply Voltage Range 500kHz Switching Frequency ACOT Control 0.8V ± 2% Voltage Reference Output Adjustable from 0.8V to 6.5V Monotonic Start-Up into Pre-Biased Outputs Applications Ordering Information RT6234A/B Package Type QW : WDFN-8L 2x3 (W-Type) Lead Plating System G : Green (Halogen Free and Pb Free) Set Top Box Portable TV Access Point Router DSL Modem LCD TV Pin Configurations BOOT GND SW VIN A : PSM Mode B : PWM Mode 1 2 3 4 GND (TOP VIEW) UVP Option H : Hiccup 9 8 7 6 5 SS PGOOD EN FB Note : Richtek products are : WDFN-8L 2x3 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 RT6234A/B VIN VIN CIN Enable VPGOOD RPGOOD BOOT CBOOT L VOUT SW EN PGOOD R1 CFF COUT FB R2 SS GND CSS Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT6234A/B Marking Information RT6234AHGQW RT6234BHGQW 11 : Product Code 10 : Product Code W : Date Code 11W 10W W : Date Code Function Pin Description Pin No. Pin Name Pin Function BOOT Bootstrap, Supply for High-Side Gate Driver. Connect a 0.1F or greater ceramic capacitor between the BOOT pin and SW pin to power the high-side switch. GND System Ground. Provides the ground return path for the control circuitry and low-side power MOSFET. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 3 SW Switch Node. SW is the switching node that supplies power to the output and connect the output LC filter from SW pin to the output load. 4 VIN Power Input. Supplies the power switches of the device. 5 FB Feedback Voltage Input. This pin is used to set the desired output voltage via an external resistive divider. 6 EN Enable Control Input. Floating this pin or connecting this pin to ground can disable the device and connecting this pin to logic high can enable the device. 7 PGOOD Power Good Indicator. This pin is an open-drain logic output that is pulled to ground when the output voltage is lower or higher than its specified threshold under the conditions of OTP, EN shutdown, or during soft-start. 8 SS Soft-Start Control Input. Connect a capacitor between the SS pin and ground to set the soft-start period. 1 2, 9 (Exposed Pad) Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B Function Block Diagram BOOT VIN VIN Reg PVCC VCC VIBIAS UV VREF Protection PVCC UGATE OC VCC SS FB Ripple Gen. SW Driver LGATE Comparator GND PGOOD SW + + - 2µA Control FB EN 0.9 VREF + - Comparator EN Operation The RT6234A/B is a synchronous step-down converter with advanced constant on-time control mode. Using the ACOTTM control mode can reduce the output capacitance and provide 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 The inductor current is monitored via the internal switches cycle-by-cycle. Once the output voltage drops under UV threshold, the RT6234A/B will enter hiccup mode. Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 When the junction temperature exceeds the OTP 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 RT6234A/B Absolute Maximum Ratings (Note 1) Supply Input Voltage, VIN ---------------------------------------------------------------------------------- −0.3V to 20V Switch Voltage, SW ----------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) <10ns ----------------------------------------------------------------------------------------------------------- −5V to 25V BOOT Voltage, VBOOT ---------------------------------------------------------------------------------------------------------------------------- (VSW − 0.3V) to (VIN + 6.3V) All Other Pins ------------------------------------------------------------------------------------------------- −0.3V to 6V Power Dissipation, PD @ TA = 25°C WDFN-8L 2x3 ------------------------------------------------------------------------------------------------- 3.17W Package Thermal Resistance (Note 2) WDFN-8L 2x3, θJA -------------------------------------------------------------------------------------------- 31.5°C/W WDFN-8L 2x3, θJC -------------------------------------------------------------------------------------------- 7.5°C/W Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------ 260°C Junction Temperature Range ------------------------------------------------------------------------------- 150°C Storage Temperature Range ------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 3) HBM (Human Body Model) --------------------------------------------------------------------------------- 2kV Recommended Operating Conditions (Note 4) Supply Voltage ------------------------------------------------------------------------------------------------ 4.5V to 18V Junction Temperature Range ------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range ------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VIN = 12V, TA = 25°C unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit 3.6 3.9 4.2 V -- 340 -- mV Supply Voltage VIN Under-Voltage Lockout Threshold-Rising VUVLO Vin Under-Voltage Lockout Threshold-Hysteresis VUVLO VIN Rising Supply Current Supply Current (Shutdown) ISHDN VEN = 0V -- -- 6 A Supply Current (Quiescent) IQ VEN = 2V, VFB = 1V -- 0.8 -- mA -- 2 -- A Soft-Start Soft-Start Current ISS Enable Voltage EN Rising Threshold VEN_H 1.38 1.5 1.62 V EN Falling Threshold VEN_L 1.16 1.28 1.4 V VFB 784 800 816 mV Feedback Voltage Feedback Voltage Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B Parameter Symbol Test Conditions Min Typ Max Unit -- 130 -- m Internal MOSFET VBOOT VSW = 4.8V High-Side Switch-On Resistance RDS(ON)_H Low-Side Switch-On Resistance RDS(ON)_L -- 70 -- m ILIM 4 4.5 -- A f SW -- 500 -- kHz Maximum Duty Cycle DMAX -- 90 -- % Minimum On Time tON(MIN) -- 60 -- ns Thermal Shutdown TSD -- 150 -- C Thermal Hysteresis TSD -- 20 -- C UVP detect 45 50 55 % Hysteresis -- 10 -- % Current Limit Current Limit Switching Frequency Oscillator Frequency On-Time Timer Control Thermal Shutdown Output Under Voltage Protections Output Under Voltage Trip Threshold 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. The device is not guaranteed to function outside its operating conditions. Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT6234A/B Typical Application Circuit RT6234A/B VIN 4.5V to 18V 4 CIN 22µF BOOT SW 1 CBOOT 0.1µF 3 L 5 R T* VOUT 6 EN Enable VPGOOD VIN RPGOOD 100k 7 8 CSS 10nF PGOOD SS FB GND R1 2, 9 (Exposed Pad) CFF Option COUT 22µF R2 *Note : When CFF is added, it is necessary to add RT = 10k between feedback network and chip FB pin. Table 1. Suggested Component Values (VIN = 12V) VOUT (V) R1 (k) R2 (k) L (H) COUT (F) CFF (pF) 1.05 10 32.4 2.2 44 -- 1.2 20.5 41.2 2.2 44 -- 1.8 40.2 32.4 3.3 44 -- 2.5 40.2 19.1 3.3 44 22 to 68 3.3 40.2 13 4.7 44 22 to 68 5 40.2 7.68 4.7 44 22 to 68 Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B Typical Operating Characteristics Efficiency vs. Output Current 100 Output Voltage vs. Output Current 1.082 RT6234AH 90 1.072 70 VOUT = VOUT = VOUT = VOUT = 60 50 Output Voltage (V) 80 Efficiency (%) RT6234AH 1.05V 1.8V 3.3V 5V 40 30 20 1.061 1.051 1.040 1.030 10 VIN = 12V 0 0.001 VIN = 12V, VOUT = 1.05V 1.019 0.01 0.1 1 10 0 0.5 Output Current (A) RT6234AH 2.5 3 RT6234AH 3.45 Output Voltage (V) 1.818 1.800 1.782 1.764 3.40 3.35 3.30 3.25 3.20 3.15 VIN = 12V, VOUT = 1.8V VIN = 12V, VOUT = 3.3V 1.746 3.10 0 0.5 1 1.5 2 2.5 3 0 0.5 Output Current (A) 1.5 2 2.5 3 Output Voltage vs. Input Voltage 1.082 RT6234AH 5.15 1 Output Current (A) Output Voltage vs. Output Current 5.20 RT6234AH 1.071 Output Voltage (V) 5.10 Output Voltage (V) 2 Output Voltage vs. Output Current 3.50 1.836 Output Voltage (V) 1.5 Output Current (A) Output Voltage vs. Output Current 1.854 1 5.05 5.00 4.95 4.90 4.85 4.80 1.061 1.050 1.040 IOUT = 1A IOUT = 2A IOUT = 3A 1.029 4.75 VIN = 6V to 18V, VOUT = 1.05V VIN = 12V, VOUT = 5V 4.70 1.019 0 0.5 1 1.5 2 2.5 Output Current (A) Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 3 6 8 10 12 14 16 18 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT6234A/B Output Voltage vs. Input Voltage 1.854 Output Voltage vs. Input Voltage 3.40 RT6234AH IOUT = 1A IOUT = 2A IOUT = 3A 3.35 1.818 Output Voltage (V) Output Voltage (V) 1.836 RT6234AH IOUT = 1A IOUT = 2A IOUT = 3A 1.800 1.782 1.764 3.30 3.25 3.20 3.15 VIN = 6V to 18V, VOUT = 1.8V VIN = 6V to 18V, VOUT = 3.3V 1.746 3.10 0 5 10 15 20 6 8 10 Input Voltage (V) 5.05 5.00 4.95 4.90 4.85 4.80 RT6234BH 70 VOUT = VOUT = VOUT = VOUT = 60 50 1.05V 1.8V 3.3V 5V 40 30 20 4.75 10 VIN = 7V to 18V, VOUT = 5V 4.70 6 8 10 12 14 16 VIN = 12V 0 0.001 18 0.01 Input Voltage (V) 0.1 1 10 Output Current (A) Output Voltage vs. Output Current 1.082 Output Voltage vs. Output Current 1.854 RT6234BH RT6234BH 1.836 Output Voltage (V) 1.072 Output Voltage (V) 18 80 Efficiency (%) Output Voltage (V) 90 IOUT = 1A IOUT = 2A IOUT = 3A 5.10 16 Efficiency vs. Output Current 100 RT6234AH 5.15 14 Input Voltage (V) Output Voltage vs. Input Voltage 5.20 12 1.061 1.051 1.040 1.030 1.818 1.800 1.782 1.764 VIN = 12V, VOUT = 1.8V VIN = 12V, VOUT = 1.05V 1.019 1.746 0 0.5 1 1.5 2 2.5 Output Current (A) Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 3 0 0.5 1 1.5 2 2.5 3 Output Current (A) is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B Output Voltage vs. Output Current 3.399 Output Voltage vs. Output Current 5.15 RT6234BH 5.10 Output Voltage (V) Output Voltage (V) 3.366 RT6234BH 3.333 3.300 3.267 3.234 5.05 5.00 4.95 4.90 VIN = 12V, VOUT = 3.3V VIN = 12V, VOUT = 5V 3.201 4.85 0 0.5 1 1.5 2 2.5 3 0 0.5 Output Current (A) RT6234BH 2.5 3 RT6234BH 1.836 Output Voltage (V) Output Voltage (V) 2 Output Voltage vs. Input Voltage 1.854 1.071 1.061 1.050 IOUT = 1A IOUT = 2A IOUT = 3A 1.040 IOUT = 1A IOUT = 2A IOUT = 3A 1.818 1.800 1.782 1.029 1.764 VIN = 6V to 18V, VOUT = 1.05V VIN = 6V to 18V, VOUT = 1.8V 1.019 1.746 6 8 10 12 14 16 18 6 8 Input Voltage (V) 10 12 14 16 18 Input Voltage (V) Output Voltage vs. Input Voltage 3.399 Output Voltage vs. Input Voltage 5.20 RT6234BH RT6234BH 5.15 Output Voltage (V) 3.366 Output Voltage (V) 1.5 Output Current (A) Output Voltage vs. Input Voltage 1.082 1 IOUT = 1A IOUT = 2A IOUT = 3A 3.333 3.300 3.267 3.234 IOUT = 1A IOUT = 2A IOUT = 3A 5.10 5.05 5.00 4.95 4.90 4.85 VIN = 7V to 18V, VOUT = 5V VIN = 6V to 18V, VOUT = 3.3V 3.201 4.80 6 8 10 12 14 16 Input Voltage (V) Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 18 6 8 10 12 14 16 18 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT6234A/B Enable Voltage vs. Temperature 1.80 4.8 1.72 4.6 1.64 Enable Voltage (V) UVLO Voltage (V) UVLO vs. Temperature 5.0 4.4 4.2 Turn On 4.0 3.8 3.6 Turn Off 3.4 1.56 Enable On 1.48 1.40 1.32 Enable Off 1.24 1.16 3.2 1.08 VOUT = 1.05V, IOUT = 1A 3.0 VOUT = 1.05V, IOUT = 0A 1.00 -50 -25 0 25 50 75 100 125 -50 -25 0 Temperature (°C) 25 50 75 100 125 Temperature (°C) Output Voltage vs. Temperature Reference Voltage vs. Temperature 1.050 0.820 1.044 Output Voltage (V) 1.032 Reference Voltage (V) VIN = 4.5V VIN = 12V VIN = 18V 1.038 1.026 1.020 1.014 1.008 1.002 0.996 0.812 0.804 0.796 VIN = 4.3V VIN = 12V VIN = 18V 0.788 IOUT = 1A VOUT = 1.05V, IOUT = 1A 0.990 0.780 -50 -25 0 25 50 75 100 125 -50 -25 0 25 50 75 Temperature (°C) Temperature (°C) Load Transient Response Output Ripple Voltage 100 125 VOUT (20mV/Div) VOUT (20mV/Div) VSW (10V/Div) IOUT (1A/Div) I SW (2A/Div) VIN = 12V, VOUT = 1.05V, IOUT = 1.5A to 3A, L = 1.8μH Time (100μs/Div) Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 VIN = 12V, VOUT = 1.05V, IOUT = 3A, L = 1.8μH Time (2μs/Div) is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B Power On from VIN VIN = 12V, VOUT = 1.05V, IOUT = 3A, L = 1.8μH VIN (10V/Div) VOUT (1V/Div) Power Off from VIN VIN (10V/Div) VOUT (1V/Div) VSW (10V/Div) VSW (10V/Div) I SW (2A/Div) I SW (2A/Div) VIN = 12V, VOUT = 1.05V, IOUT = 3A, L = 1.8μH Time (4ms/Div) Time (10ms/Div) Power On from EN Power Off from EN EN (2V/Div) VIN = 12V, VOUT = 1.05V, IOUT = 3A, L = 1.8μH VOUT (1V/Div) EN (2V/Div) VIN = 12V, VOUT = 1.05V, IOUT = 3A, L = 1.8μH VOUT (1V/Div) VSW (10V/Div) VSW (10V/Div) I SW (2A/Div) I SW (2A/Div) Time (4ms/Div) Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 Time (4ms/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT6234A/B Application Information Inductor Selection 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 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 (ΔIL) about 20% to 50% of the desired full output load current. Calculate the approximate inductor value by selecting the input and output voltages, the switching frequency (fSW), the maximum output current (IOUT(MAX)) and estimating a ΔIL as some percentage of that current. L= VOUT VIN VOUT VIN fSW IL Once an inductor value is chosen, the ripple current (ΔIL) is calculated to determine the required peak inductor current. VOUT VIN VOUT VIN fSW L I IL(PEAK) = IOUT(MAX) L 2 IL = To guarantee the required output current, the inductor needs a saturation current rating and a thermal rating that exceeds IL(PEAK). These are minimum requirements. To maintain control of inductor current in overload and short circuit conditions, some applications may desire current ratings up to the current limit value. However, the IC's output under-voltage shutdown feature make this unnecessary for most applications. the output current) while ensuring that IL(PEAK) does not exceed the upper current limit level. For best efficiency, choose an inductor with a low DC resistance that meets the cost and size requirements. For low inductor core losses some type of ferrite core is usually best and a shielded core type, although possibly larger or more expensive, will probably give fewer EMI and other noise problems. Considering the Typical Operating Circuit for 1.2V output at 3A and an input voltage of 12V, using an inductor ripple of 0.9A (30%), the calculated inductance value is : L= 1.2 12 1.2 = 2.4μH 12 500kHz 0.9A The ripple current was selected at 0.9A and, as long as we use the calculated 2.4μH inductance, that should be the actual ripple current amount. The ripple current and required peak current as below : IL = 1.2 12 1.2 = 0.9A 12 500kHz 2.4μH and IL(PEAK) = 3A 0.9A = 3.45A 2 For the 2.4μH value, the inductor's saturation and thermal rating should exceed 3.45A. Since the actual value used was 2.4μH and the ripple current exactly 0.9A, the required peak current is 3.45A. 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 : V IRMS = IOUT(MAX) OUT VIN VIN 1 VOUT IL(PEAK) should not exceed the minimum value of IC's upper current limit level or the IC may not be able to meet the desired output current. If needed, reduce the inductor ripple current (ΔIL) to increase the average inductor current (and Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B 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 RT6234A/B 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 uses two 10μF and one 0.1μF low ESR ceramic capacitors on the input. Output Capacitor Selection The RT6234A/B are 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). For the Typical Operating Circuit for 1.2V output and an inductor ripple of 0.4A, with 2 x 22μF output capacitance each with about 5mΩ ESR including PCB trace resistance, the output voltage ripple components are : VRIPPLE(ESR) = 0.9A 5m = 4.5mV 0.9A = 5.11mV 8 44μF 500kHz = 4.5mV 5.11mV = 9.61mV VRIPPLE(C) = VRIPPLE Feed-forward Capacitor (Cff) The RT6234A/B are 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 steady-state stability of the circuit. Choose a suitable capacitor value that following below step. 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. 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 BW VOUT R1 Cff FB RT6234A/B R2 GND Figure 1. Cff Capacitor Setting Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT6234A/B Cff can be calculated base on below equation : 1 Cff 2 3.1412 R1 BW 0.8 Output Voltage Setting Enable Operation (EN) 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 0.8V (1 + R1 ) R2 For automatic start-up the high-voltage EN pin can be connected to VIN, through a 100kΩ resistor. Its large hysteresis band makes EN useful for simple delay and timing circuits. EN can be externally pulled to VIN by adding a resistor-capacitor delay (REN and CEN in Figure 2). Calculate the delay time using EN's internal threshold where switching operation begins. 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 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. 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 connected to EN to create an additional input under voltage lockout threshold (Figure 4). EN VIN REN EN RT6234A/B CEN GND Figure 2. External Timing Control VIN REN 100k EN Q1 Enable Figure 3. Digital Enable Control Circuit REN1 REN2 EN RT6234A/B GND Figure 4. Resistor Divider for Lockout Threshold Setting Copyright © 2016 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 R1 FB RT6234A/B R2 GND Figure 5. Output Voltage Setting Place the FB resistors within 5mm of the FB pin. Choose R2 between 10kΩ and 100kΩ to minimize power consumption without excessive noise pick-up and calculate R1 as follows : R1 R2 (VOUT VREF ) VREF For output voltage accuracy, use divider resistors with 1% or better tolerance. 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. RT6234A/B GND VIN VOUT 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 dead time 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 (<47Ω) is a registered trademark of Richtek Technology Corporation. DS6234A/B-00 January 2016 RT6234A/B 5V 3.5 Maximum Power Dissipation (W)1 resistance between BOOT and the external bootstrap capacitor. This will slow the 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 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. Four-Layer PCB 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 BOOT RT6234A/B 0.1µF SW 25 50 75 100 125 Ambient Temperature (°C) Figure 7. Derating Curve of Maximum Power Dissipation Figure 6. External Bootstrap Diode Thermal Considerations 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 : 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 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For WDFN-8L 2x3 package, the thermal resistance, θJA, is 31.5°C/W on a standard four-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formula : PD(MAX) = (125°C − 25°C) / (31.5°C/W) = 3.17W for WDFN-8L 2x3 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 7 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Copyright © 2016 Richtek Technology Corporation. All rights reserved. DS6234A/B-00 January 2016 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT6234A/B Outline Dimension D D2 L E E2 SEE DETAIL A 1 e b 2 A A1 1 2 1 A3 DETAIL A Pin #1 ID and Tie Bar Mark Options Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.700 0.800 0.028 0.031 A1 0.000 0.050 0.000 0.002 A3 0.175 0.250 0.007 0.010 b 0.200 0.300 0.008 0.012 D 1.900 2.100 0.075 0.083 D2 1.550 1.650 0.061 0.065 E 2.900 3.100 0.114 0.122 E2 1.650 1.750 0.065 0.069 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 8L DFN 2x3 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 DS6234A/B-00 January 2016