RT6238AHGQUF

® RT6238A/B 8A, 18V, 500kHz, ACOTTM Synchronous Step-Down Converter General Description Features The RT6238A/B is a high-performance 500kHz, 8A stepdown regulator with internal power switches and synchronous rectifiers. It features quick transient response using its Advanced Constant On-Time (ACOTTM) control  architecture that provides stable operation with small ceramic output capacitors and without complicated external compensation, among other benefits. The input voltage range is from 4.5V to 18V and the output is adjustable from 0.7V to 8V. The proprietary ACOTTM control improves upon other fast response constant on-time architectures, achieving nearly constant switching frequency over line, load, and output voltage ranges. Since there is no internal clock, response to transients is nearly instantaneous and inductor current can ramp quickly to maintain output regulation without large bulk output capacitance. The RT6238A/B is stable with and optimized for ceramic output capacitors. With internal 35mΩ switches and 14mΩ synchronous rectifiers, the RT6238A/B displays excellent efficiency and good behavior across a range of applications, especially for low output voltages and low duty cycles. Cycle-by-cycle current limit provides protection against shorted outputs, input under-voltage lockout, externally-adjustable soft-start, output under- and over-voltage protection, and thermal shutdown provide safe and smooth operation in all operating conditions. The RT6238A/B is available in the UQFN-14L 2x3 (FC) package, with exposed thermal pad.               Fast Transient Response Advanced Constant On-Time (ACOTTM) Control 4.5V to 18V Input Voltage Range Adjustable Output Voltage from 0.7V to 8V 8A Output Current 35mΩ Ω Internal High-Side N-MOSFET and 14mΩ Ω Internal Low-Side N-MOSFET Steady 500kHz Switching Frequency Up to 95% Efficiency Optimized for All Ceramic Capacitors Externally-Adjustable, Pre-Biased Compatible SoftStart Cycle-by-Cycle Current Limit Input Under-Voltage Lockout Output Over- and Under-Voltage Protection Power Good Output Thermal Shutdown Applications      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 Simplified Application Circuit VIN EN Signal Power Good RT6238A/B VIN SW EN VOUT BOOT FB PGOOD PVCC SS GND Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS6238A/B-00 April 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT6238A/B Ordering Information Marking Information RT6238A/B RT6238ALGQUF 0K : Product Code Package Type QUF : UQFN-14L 2x3 (U-Type) (FC) 0KW W : Date Code Lead Plating System G : Green (Halogen Free and Pb Free) UVP Option H : Hiccup Mode UVP L : Latched OVP & UVP A : PSM B : PWM RT6238BLGQUF 0H : Product Code 0HW W : Date Code Note : Richtek products are :  RoHS compliant and compatible with the current require- 0LW ments of IPC/JEDEC J-STD-020.  RT6238AHGQUF 0L : Product Code W : Date Code Suitable for use in SnPb or Pb-free soldering processes. RT6238BHGQUF 0J : Product Code Pin Configurations SS EN GND (TOP VIEW) 14 13 12 0JW GND 2 10 GND PVCC 3 9 GND PGOOD 4 8 VIN 5 6 7 VIN 11 FB SW 1 BOOT AGND W : Date Code UQFN-14L 2x3 (FC) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS6238A/B-00 April 2015 RT6238A/B Functional Pin Description Pin No. Pin Name Pin Function 1 AGND Analog GND. 2 FB Feedback Voltage Input. It is used to regulate the output of the converter to a set value via an external resistive voltage divider. The feedback reference voltage is 0.7V typically. 3 PVCC Internal Regulator Output. Connect a 1F capacitor to GND to stabilize output voltage. 4 PGOOD Power Good Indicator Open-Drain Output. 5 BOOT Bootstrap Supply for High-Side Gate Driver. This capacitor is needed to drive the power switch's gate above the supply voltage. It is connected between the SW and BOOT pins to form a floating supply across the power switch driver. A 0.1F capacitor is recommended for use. 6 SW Switch Node. Connect this pin to an external L-C filter. 7, 8 VIN Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a suitably large (10F x 2) ceramic capacitor. 9, 10, 11, 12 GND Ground. 13 EN Enable Control Input. A logic-high enables the converter; a logic-low forces the IC into shutdown mode reducing the supply current to less than 10A. Attach this pin to PVCC with a 100k pull-up resistor for automatic start-up. 14 SS Soft-Start Time Setting. An external capacitor should be connected between this pin and GND. Function Block Diagram BOOT PVCC VIN PVCC Reg Min. Off VIBIAS PVCC VIN VREF UGATE Control OC Driver SW LGATE UV & OV PVCC SW 6µA Ripple Gen. SS FB VIN SW GND GND SW + Comparator On-Time Comparator 0.9 VREF FB PGOOD + - EN EN Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS6238A/B-00 April 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT6238A/B Detailed Description The RT6238A/B is a high-performance 500kHz 8A stepdown regulators with internal power switches and synchronous rectifiers. It features an Advanced Constant On-Time (ACOTTM) control architecture that provides stable operation with ceramic output capacitors without complicated external compensation, among other benefits. The ACOTTM control mode also provides fast transient response, especially for low output voltages and low duty cycles. The input voltage range is from 4.5V to 18V and the output is adjustable from 0.7V to 8V. The proprietary ACOTTM control scheme improves upon other constant on-time architectures, achieving nearly constant switching frequency over line, load, and output voltage ranges. The RT6238A/B are optimized for ceramic output capacitors. Since there is no internal clock, response to transients is nearly instantaneous and inductor current can ramp quickly to maintain output regulation without large bulk output capacitance. Constant On-Time (COT) Control The heart of any COT architecture is the on-time one shot. Each on-time is a pre-determined “fixed” period that is triggered by a feedback comparator. This robust arrangement has high noise immunity and is ideal for low duty cycle applications. After the on-time one-shot period, there is a minimum off-time period before any further regulation decisions can be considered. This arrangement avoids the need to make any decisions during the noisy time periods just after switching events, when the switching node (SW) rises or falls. Because there is no fixed clock, the high-side switch can turn on almost immediately after load transients and further switching pulses can ramp the inductor current higher to meet load requirements with minimal delays. Traditional current mode or voltage mode control schemes typically must monitor the feedback voltage, current signals (also for current limit), and internal ramps and compensation signals, to determine when to turn off the high-side switch and turn on the synchronous rectifier. Weighing these small signals in a switching environment Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is difficult to do just after switching large currents, making those architectures problematic at low duty cycles and in less than ideal board layouts. Because no switching decisions are made during noisy time periods, COT architectures are preferable in low duty cycle and noisy applications. However, traditional COT control schemes suffer from some disadvantages that preclude their use in many cases. Many applications require a known switching frequency range to avoid interference with other sensitive circuitry. True constant on-time control, where the on-time is actually fixed, exhibits variable switching frequency. In a step-down converter, the duty factor is proportional to the output voltage and inversely proportional to the input voltage. Therefore, if the on-time is fixed, the off-time (and therefore the frequency) must change in response to changes in input or output voltage. Modern pseudo-fixed frequency COT architectures greatly improve COT by making the one-shot on-time proportional to VOUT and inversely proportional to VIN. In this way, an on-time is chosen as approximately what it would be for an ideal fixed-frequency PWM in similar input/output voltage conditions. The result is a big improvement but the switching frequency still varies considerably over line and load due to losses in the switches and inductor and other parasitic effects. Another problem with many COT architectures is their dependence on adequate ESR in the output capacitor, making it difficult to use highly-desirable, small, low-cost, but low-ESR ceramic capacitors. Most COT architectures use AC current information from the output capacitor, generated by the inductor current passing through the ESR, to function in a way like a current mode control system. With ceramic capacitors the inductor current information is too small to keep the control loop stable, like a current mode system with no current information. ACOTTM Control Architecture Making the on-time proportional to VOUT and inversely proportional to VIN is not sufficient to achieve good constant-frequency behavior for several reasons. First, voltage drops across the MOSFET switches and inductor is a registered trademark of Richtek Technology Corporation. DS6238A/B-00 April 2015 RT6238A/B cause the effective input voltage to be less than the measured input voltage and the effective output voltage to be greater than the measured output voltage. As the load changes, the switch voltage drops change causing a switching frequency variation with load current. Also, at light loads if the inductor current goes negative, the switch dead-time between the synchronous rectifier turn-off and the high-side switch turn-on allows the switching node to rise to the input voltage. This increases the effective on time and causes the switching frequency to drop noticeably. One way to reduce these effects is to measure the actual switching frequency and compare it to the desired range. This has the added benefit eliminating the need to sense the actual output voltage, potentially saving one pin connection. ACOTTM uses this method, measuring the actual switching frequency and modifying the on-time with a feedback loop to keep the average switching frequency in the desired range. To achieve good stability with low-ESR ceramic capacitors, ACOTTM uses a virtual inductor current ramp generated inside the IC. This internal ramp signal replaces the ESR ramp normally provided by the output capacitor's ESR. The ramp signal and other internal compensations are optimized for low-ESR ceramic output capacitors. ACOTTM One-Shot Operation The RT6238A/B control algorithm is simple to understand. The feedback voltage, with the virtual inductor current ramp added, is compared to the reference voltage. When the combined signal is less than the reference and the ontime one-shot is triggered, as long as the minimum offtime one-shot is clear and the measured inductor current (through the synchronous rectifier) is below the current limit. The on-time one-shot turns on the high-side switch and the inductor current ramps up linearly. After the on time, the high-side switch is turned off and the synchronous rectifier is turned on and the inductor current ramps down linearly. At the same time, the minimum off-time one-shot is triggered to prevent another immediate on-time during the noisy switching time and allow the feedback voltage and current sense signals to settle. The minimum off-time Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS6238A/B-00 April 2015 is kept short (230ns typical) so that rapidly-repeated ontimes can raise the inductor current quickly when needed. Discontinuous Operating Mode (RT6238A Only) After soft-start, the RT6238A operates in fixed frequency mode to minimize interference and noise problems. The RT6238A uses variable-frequency discontinuous switching at light loads to improve efficiency. During discontinuous switching, the on-time is immediately increased to add “hysteresis” to discourage the IC from switching back to continuous switching unless the load increases substantially. The IC returns to continuous switching as soon as an ontime is generated before the inductor current reaches zero. The on-time is reduced back to the length needed for 500kHz switching and encouraging the circuit to remain in continuous conduction, preventing repetitive mode transitions between continuous switching and discontinuous switching. Current Limit The RT6238A/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. If the inductor current exceeds the current limit, the on-time one-shot is inhibited (Mask high side signal) until the inductor current ramps down below the current limit. Thus, only when the inductor current is well below the current limit is another on time permitted. This arrangement prevents the average output current from greatly exceeding the guaranteed current limit value, as typically occurs with other valley-type current limits. 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 the IC will stop switching (see next section). Output Over-Voltage Protection and Under-Voltage Protection If the output voltage VOUT rises above the regulation level and lower 1.2 times regulation level, the high-side switch is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT6238A/B naturally remains off and the synchronous rectifier turns on. For the RT6238B, if the output voltage remains high the synchronous rectifier remains on until the inductor current reaches the low-side current limit. If the output voltage remains high, then IC's switches remain that the synchronous rectifier turns on and high-side MOSFET keeps off to operate at typical 500kHz switching protection. If inductor current reaches low-side current limit, the synchronous rectifier will turn off until next clock. If the output voltage exceeds the OVP trip threshold (1.2 times regulation level) for longer than 10μs (typical), then IC's output Over-Voltage Protection (OVP) is triggered. For the RT6238BH, the chip enters into hiccup mode; for the RT6238BL, the chip enters into latch mode. For the RT6238A, if the output voltage VOUT rises above the regulation level or drops below 1.2 times the regulation level, the high-side switch naturally remains off and the synchronous rectifier turns on until the inductor current reaches zero current. If the output voltage remains high, then the IC's switches remain off. If the output voltage exceeds the OVP trip threshold (1.2 times regulation level) for longer than 10μs (typical), the IC's OVP is triggered. For the RT6238AH, the chip enters into hiccup mode; for the RT6238AL, the chip enters into latch mode. The RT6238A/B includes output Under-Voltage Protection (UVP). If the output voltage drops below the UVP trip threshold for longer than 270μs (typical) then the IC's UVP is triggered, and the chip enters into latch or hiccup mode. (see next section). Hiccup Mode The RT6238AH/BH uses hiccup mode for UVP. When the protection function is triggered, the IC will shut down for a period of time and then attempt to recover automatically. Hiccup mode allows the circuit to operate safely with low input current and power dissipation, and then resume normal operation as soon as UVP is removed. During hiccup mode, the shutdown time is determined by the capacitor at SS. A 2μA current source discharges VSS from its starting voltage (normally VPVCC). The IC remains shut down until VSS reaches 0.2V, about 10ms for a 3.9nF capacitor. At that point the IC begins to charge the SS Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 capacitor at 6μA, and a normal start-up occurs. If the fault remains, UVP protection will be enabled when VSS reaches 2.2V (typical). The IC will then shut down and discharge the SS capacitor from the 2.2V level, taking about 4ms for a 3.9nF SS capacitor. Latch-Off Mode The RT6238AL/BL uses latch-off mode OVP and UVP. When the protection function is triggered, the IC will shut down in Latch-Off Mode. The IC stops switching, leaving both switches open, and is latched off. To restart operation, toggle EN or power the IC off and then on again. Shut-Down, Start-Up and Enable (EN) The enable input (EN) has a logic-low level of 0.4V. When VEN is below this level the IC enters shutdown mode and supply current drops to less than 10μA. When VEN exceeds its logic-high level of 2V the IC is fully operational. Between these 2 levels there are 2 thresholds (1.2V typical and 1.4V typical). When VEN exceeds the lower threshold the internal bias regulators begin to function and supply current increases above the shutdown current level. Switching operation begins when VEN exceeds the upper threshold. Unlike many competing devices, EN is a high voltage input that can be safely connected to VIN (up to 18V) for automatic start-up. Input Under-Voltage Lockout In addition to the enable function, the RT6238A/B feature an Under-Voltage Lockout (UVLO) function that monitors the internal linear regulator output (VIN). To prevent operation without fully-enhanced internal MOSFET switches, this function inhibits switching when VIN drops below the UVLO-falling threshold. The IC resumes switching when VIN exceeds the UVLO-rising threshold Soft-Start (SS) The RT6238A/B soft-start uses an external pin (SS) to clamp the output voltage and allow it to slowly rise. After VEN is high and VIN exceeds its UVLO threshold, the IC begins to source 6μA from the SS pin. An external capacitor at SS is used to adjust the soft-start timing. Following below equation to get the minimum capacitance range in order to avoid UV occur. is a registered trademark of Richtek Technology Corporation. DS6238A/B-00 April 2015 RT6238A/B T= COUT  VOUT  0.75  1.2 ILIM  Load Current   0.8 CSS  T  6μA VREF Do not leave SS unconnected. During start-up, while the SS capacitor charges, the RT6238A/B operates in discontinuous switching mode with very small pulses. This prevents negative inductor currents and keeps the circuit from sinking current. Therefore, the output voltage may be pre-biased to some positive level before start-up. Once the VSS ramp charges enough to raise the internal reference above the feedback voltage, switching will begin and the output voltage will smoothly rise from the prebiased level to its regulated level. After VSS rises above about 2.2V output over- and under-voltage protections are enabled and the RT6238A/B begins continuous-switching operation. or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT6238A/B. Note that the external boot voltage must be lower than 5.5V Over-Temperature Protection The RT6238A/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 20°C the IC will resume normal operation with a complete soft-start. For continuous operation, provide adequate cooling so that the junction temperature does not exceed 150°C. Internal Regulator (PVCC) An internal linear regulator (PVCC) produces a 5V supply from VIN. The 5V power supplies the internal control circuit, such as internal gate drivers, PWM logic, reference, analog circuitry, and other blocks. 1μF ceramic capacitor for decoupling and stability is required. PGOOD Comparator PGOOD is an open-drain output controlled by a comparator connected to the feedback signal. If FB exceeds 90% of the internal reference voltage, PGOOD will be high impedance. Otherwise, the PGOOD output is connected to GND. External Bootstrap Capacitor (CBOOT) Connect a 0.1μF low ESR ceramic capacitor between BOOT and SW. This bootstrap capacitor provides the gate driver supply voltage for the high-side N-channel MOSFET switch. Some of case, such like duty ratio is higher than 65% application or input voltage is lower than 5.5V which are recommended to add an external bootstrap diode between an external 5V and BOOT pin for efficiency improvement The bootstrap diode can be a low cost one such as IN4148 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS6238A/B-00 April 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT6238A/B Absolute Maximum Ratings             (Note 1) Supply Voltage, VIN -----------------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------Switch Voltage, 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. The RT6238A/B soft-start uses an external capacitor at SS to adjust the soft-start timing according to the following equation :  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. t  ms   CSS  nF   0.7 ISS μA  Following below equation to get the minimum capacitance range in order to avoid UV occur. COUT  VOUT  0.6  1.2 (ILIM  Load Current)  0.8 T  6μA CSS  VREF T Do not leave SS unconnected. Enable Operation (EN) For automatic start-up the high-voltage EN pin can be connected to VIN, either directly or 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 (1.4V, typical). BW VOUT R1 Cff FB RT6238A/B R2 GND Figure 1. Cff Capacitor Setting 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  Cff can be calculated base on below equation : 1 Cff  2  3.1412  R1 BW  0.8 VIN REN CEN EN RT6238A/B GND Figure 2. External Timing Control Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS6238A/B-00 April 2015 RT6238A/B VIN REN 100k External BOOT Bootstrap Diode EN Q1 Enable RT6238A/B GND Figure 3. Digital Enable Control Circuit VIN 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 REN1 EN REN2 RT6238A/B GND Figure 4. Resistor Divider for Lockout Threshold Setting 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 = 0.7 x (1 + R1 / R2) VOUT R1 FB RT6238A/B R2 GND 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 (