RT6255AHGJ6F

Reference Design Sample & Buy ® RT6255A/B 5A, 18V, 500kHz, ACOTTM Step-Down Converter General Description Features The RT6255A/B is a high-performance, synchronous stepdown DC-DC converter that can deliver up to 5A output current from a 4.5V to 18V input supply. The device integrates low RDS(ON) power MOSFETs, accurate 0.6V reference and an integrated diode of bootstrap circuit to offer a very compact solution.  The RT6255A/B adopts Advanced Constant On-Time (ACOTTM) control architecture that provides ultrafast transient response and further reduces the externalcomponent count. In steady states, the ACOTTM operates in nearly constant switching frequency over line, load and output voltage ranges and makes the EMI filter design easier.         The device offers Independent enable control input pin and power good indicator (TSOT23-8 only) for easily sequence control. To control the inrush current during the startup, the device provides a fixed 1.5ms soft-start up. Fully protection features are also integrated in the device including the cycle-by-cycle current limit control, UVP, input UVLO and OTP. The RT6255A/B is available in the TSOT-23-6 (FC) and TSOT-23-8 (FC) package.      Input Supply Voltage Range : 4.5V to 18V Output Current : 5A Advanced Constant On-Time (ACOTTM) Control for Ultrafast Transient Response Steady Switching Frequency : 500kHz  Forced PWM Mode (RT6255B)  Pulse-Skipping Mode (PSM) at Light Load (RT6255A) Optimized for Low-ESR Ceramic Output Capacitors Programmable Output Voltage : 0.6V to 5V Internal 45mΩ Ω Switch and 23mΩ Ω Synchronous Rectifier Cycle-by-Cycle Current Limit Protection Internal Soft-Start (Typ 1.5ms) or Externally Adjustable, Pre-biased Compatible Soft-Start (Only for Option with SS Pin) Power Good Indicator (90%) (Only for Option with PGOOD Pin) Output Under-Voltage Protection (UVP) Over-Temperature Protection (OTP) Input Under-Voltage Lockout (UVLO) Available in TSOT-23-6 (FC) and TSOT-23-8 (FC) Packages Applications      Set Top Box Portable TV Access Point Router DSL Modem LCD TV Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS6255A/B-03 October 2018 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT6255A/B Ordering Information RT6255A/B Pin Configuration ( ) (TOP VIEW) Package Type J6F : TSOT-23-6 (FC) J8F : TSOT-23-8 (FC) BOOT LX 6 Lead Plating System G : Green (Halogen Free and Pb Free) FB Pin8 Definition Empty means PGOOD pin S : SS pin VIN 5 4 2 3 EN GND LX VIN 8 7 6 5 2 3 4 AGND GND PSM/PWM A : PSM/PWM B : Force-PWM BOOT UVP Option H : Hiccup PGOOD/SS TSOT-23-6 (FC) Note : RoHS compliant and compatible with the current require- FB  ments of IPC/JEDEC J-STD-020.  Suitable for use in SnPb or Pb-free soldering processes. EN Richtek products are : TSOT-23-8 (FC) Marking Information RT6255AHGJ6F RT6255AHGJ8F 27= : Product Code 27=DNN DNN : Date Code 1C= : Product Code 1C=DNN RT6255BHGJ8F RT6255BHGJ6F 1B= : Product Code 26= : Product Code 26=DNN DNN : Date Code 1B=DNN 1U= : Product Code DNN : Date Code Copyright © 2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 DNN : Date Code RT6255BHSGJ8F RT6255AHSGJ8F 1U=DNN DNN : Date Code 1T= : Product Code 1T=DNN DNN : Date Code is a registered trademark of Richtek Technology Corporation. DS6255A/B-03 October 2018 RT6255A/B Functional Pin Description Pin No. Pin Name Pin Function TSOT-23-6 (FC) TSOT-23-8 (FC) 1 1 FB Feedback voltage input. This pin is used to set the desired output voltage via an external resistive divider. -- 3 AGND Analog ground. This is the signal ground reference for the IC. 2 2 EN Enable control input. Connecting this pin to logic high can enable the device and connecting this pin to GND can disable the device. 3 4 GND System ground. This is the power return for the IC. 4 5 VIN Power input. Supplies the power switches of the device. 5 6 LX Switch node. LX is the switching node that supplies power to the output and connect the output LC filter from LX to the output load. 6 7 BOOT Bootstrap supply for high-side gate driver. Connect a 0.1F ceramic capacitor from LX to BOOT to power the high-side switch. -- 8 PGOOD Power good indicator. Open-drain output when the output voltage is within 90% to 120% of regulation point. -- 8 SS Soft-Start control input. Connect a capacitor from SS to GND to set the soft-start time. The capacitance CSS is suggested to be in the range from 2.8nF to 220nF. Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS6255A/B-03 October 2018 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT6255A/B Functional Block Diagram (RT6255AHGJ6F/RT6255BHGJ6F) BOOT VIN Reg VIN PVCC VCC VIBIAS Minoff PVCC VREF UGATE OC Control LX Driver LGATE UV GND GND LX VCC LX Ripple Gen. EN EN VIN + + - On-Time Comparator LX FB (RT6255AHGJ8F/RT6255BHGJ8F) BOOT VIN Reg PVCC VCC VIBIAS Minoff PVCC VREF UGATE OC Control LX Driver LGATE UV GND GND LX VCC LX Ripple Gen. EN - FB Copyright © 2018 Richtek Technology Corporation. All rights reserved. EN VIN + + On-Time Comparator www.richtek.com 4 VIN LX PGOOD AGND is a registered trademark of Richtek Technology Corporation. DS6255A/B-03 October 2018 RT6255A/B (RT6255AHSGJ8F/RT6255BHSGJ8F) BOOT VIN Reg PVCC VCC VIBIAS Minoff PVCC VREF UGATE OC Control LX Driver LGATE UV GND GND LX VCC LX Ripple Gen. SS EN - FB Copyright © 2018 Richtek Technology Corporation. All rights reserved. EN VIN + + On-Time Comparator DS6255A/B-03 October 2018 VIN LX AGND is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT6255A/B Operation The RT6255A/B is a high-efficiency, monolithic synchronous step-down DC-DC converter that can deliver up to 5A 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. operation as soon as the over-load or short-circuit condition is removed. VOUT Resume normal Abnormal case operation tHICCUP_ON = 2.5ms detected (UV) Fault condition tHICCUP_OFF = 5ms removed t 0.8ms Current Limit The RT6255A/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. Output Under-Voltage Protection and Hiccup Mode The RT6255A/B includes output under-voltage protection (UVP) against over-load or short-circuited condition by constantly monitoring the feedback voltage VFB. If VFB drops below the under-voltage protection trip threshold (typically 60% of the internal feedback reference voltage), the UV comparator will go high to turn off both the internal high-side and low-side MOSFET switches. If the output under-voltage condition continues for a period of time, the RT6255A/B will enter output under-voltage protection with hiccup mode. It is shown as Figure 1. During hiccup mode, the IC will shut down for tHICCUP_OFF (5ms), and then attempt to recover automatically for tHICCUP_ON (2.5ms). Upon completion of the soft-start sequence, if the fault condition is removed, the converter will resume normal operation; otherwise, such cycle for auto-recovery will be repeated until the fault condition is cleared. Hiccup mode allows the circuit to operate safely with low input current and power dissipation, and then resume normal Copyright © 2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 VLX t Figure 1. UVP and Hiccup Mode Shut-Down, Start-Up and Enable (EN) The enable input (EN) has a logic-low level. When VEN is below this level the IC enters shutdown mode. When VEN exceeds its logic-high level the IC is fully operational. VUVLO VIN VENH VEN VOUT VPGOOD VLX EN Delay ~0.5ms Soft Start time PG delay Pulse skipping mode ~1.5ms ~1.1ms (depending on loading) Figure 2. Power Up Sequence is a registered trademark of Richtek Technology Corporation. DS6255A/B-03 October 2018 RT6255A/B External Bootstrap Capacitor Connect a 0.1μF low ESR ceramic capacitor between BOOT and LX. This bootstrap capacitor provides the gate driver supply voltage for the high side N- MOSFET switch. comparator, which the feedback signal VFB is fed to. If VFB is above 90% of the internal reference voltage, the PGOOD pin will be in high impedance and VPGOOD will be held high. Otherwise, the PGOOD output will be pulled low. Over-Temperature Protection The RT6255A/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 RT6255A/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. Soft-Start (SS) (For RT6255AHSGJ8F/RT6255BHSGJ8F) The soft-start function is used to prevent large inrush currents while the converter is being powered up. The RT6255A/B provides a soft-start feature for inrush control. For some options, it provides an SS pin so that the softstart time can be programmed by selecting the value of the external capacitor CSS connected from the SS pin to GND. During the start-up sequence, the external capacitor is charged by an internal current source ISS (typically, 4μA) to generate a soft-start ramp voltage as a reference voltage to the PWM comparator. The device will initiate switching and the output voltage will smoothly ramp up to its targeted regulation voltage only after this ramp voltage is greater than the feedback voltage VFB to ensure the converters have a smooth start-up. For soft-start control, the SS pin should never be left unconnected. Power Good Indicator (For RT6255AHGJ8F/RT6255BHGJ8F) The PGOOD pin is an open-drain output and is connected to an external pull-up resistor. It is controlled by a Copyright © 2018 Richtek Technology Corporation. All rights reserved. DS6255A/B-03 October 2018 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT6255A/B Absolute Maximum Ratings (Note 1) Supply Input Voltage, VIN ------------------------------------------------------------------------------------------- −0.3V to 20V Enable Pin Voltage, EN ---------------------------------------------------------------------------------------------- −0.3V to 20V  Switch Node Voltage, LX --------------------------------------------------------------------------------------------- −0.3V to 20V