RT6236BHGQUF

® RT6236A/B 6A, 18V, 650kHz, ACOTTM Synchronous Step-Down Converter General Description Features The RT6236A/B is a high-performance 650kHz, 6A 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 RT6236A/B is stable with and optimized for ceramic output capacitors. With internal 51mΩ switches and 18mΩ synchronous rectifiers, the RT6236A/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 lock-out, externally-adjustable soft-start, output under- and over-voltage protection, and thermal shutdown provide safe and smooth operation in all operating conditions. The RT6236A/B is available in the UQFN-13JL 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 6A Output Current 51mΩ Ω Internal High-Side N-MOSFET and 18mΩ Ω Internal Low-Side N-MOSFET Steady 650kHz 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 Externally-Adjustable, Pre-Biased Compatible SoftStart 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 RT6236A/B SW VIN VIN VPVCC Enable CSS BOOT CFF R1 COUT PGOOD EN FB SS PVCC GND Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS6236A/B-00 November 2014 VOUT CBOOT CIN RPG L VPVCC CPVCC R2 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT6236A/B Ordering Information Pin Configurations RT6236A/B BOOT 12 11 1 10 VIN EN 2 9 VIN FB 3 8 PGOOD NC 4 7 NC A : PSM B : PWM Note : Richtek products are : 5 6 PVCC GND H : Hiccup Mode UVP L : Latched OVP & UVP  13 SS Lead Plating System G : Green (Halogen Free and Pb Free) SW Package Type QUF : UQFN-13JL 2x3 (U-Type) (FC) GND (TOP VIEW) UQFN-13JL 2x3 (FC) RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.  Suitable for use in SnPb or Pb-free soldering processes. Marking Information RT6236AHGQUF RT6236BHGQUF 0Z : Product Code W : Date Code 0ZW 0X : Product Code 0XW RT6236ALGQUF RT6236BLGQUF 0Y : Product Code W : Date Code 0YW W : Date Code 0W : Product Code 0WW W : Date Code Functional Pin Description Pin No. 1, 13 Pin Name Pin Function GND Ground. 2 EN Enable Control Input. 3 FB Converter Feedback Input. Connect to output voltage feedback resistor divider. 4, 7 NC No Internal Connection. 5 SS Soft-Start Control. A external capacitor should be connected to GND. 6 PVCC 5V Power Supply Output. A capacitor (typical 1F) should be connected to GND. 8 PGOOD Open Drain Power Good Output. VIN Power Input and Connected to High-Side MOSFET Drain. 11 BOOT Bootstrap. This capacitor is needed to drive the power switch's gate above the supply voltage. It is connected between SW and BOOT pins to form a floating supply across the power switch driver. A 0.1F capacitor is recommended for use. 12 SW Switch Output. 9, 10 Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS6236A/B-00 November 2014 RT6236A/B 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 Detailed Description The RT6236A/B is a high-performance 650kHz 4A 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 provide 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 RT6236A/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 Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS6236A/B-00 November 2014 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 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 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT6236A/B 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 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 Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 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 RT6236A/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 is kept short (230ns typical) so that rapidly-repeated ontimes can raise the inductor current quickly when needed. Discontinuous Operating Mode (RT6236A Only) After soft-start, the RT6236A operates in fixed frequency mode to minimize interference and noise problems. The RT6236A uses variable-frequency discontinuous switching at light loads to improve efficiency. During discontinuous switching, the on-time is immediately increased to add is a registered trademark of Richtek Technology Corporation. DS6236A/B-00 November 2014 RT6236A/B “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 650kHz switching and encouraging the circuit to remain in continuous conduction, preventing repetitive mode transitions between continuous switching and discontinuous switching. Current Limit The RT6236A/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 naturally remains off and the synchronous rectifier turns on. For RT6236B, 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 still remains high, then IC's switches remain that the synchronous rectifier turns on and high-side MOS keeps off to operate at typical 500kHz switching protection, again if inductor current reaches low side current, the synchronous rectifier will turn off until next protection Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS6236A/B-00 November 2014 clock. If the output voltage exceeds the OVP trip threshold (1.25 times regulation level) for longer than 10μs (typical), then IC's output Over-Voltage Protection (OVP) is triggered. RT6236BL chip enters latch mode. For RT6236A If the output voltage Vo rises above the regulation level and lower 1.2 times 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 IC's switches remain off. If the output voltage exceeds the OVP trip threshold (1.25 times regulation level) for longer than 10μs (typical), the IC's OVP is triggered. RT6236AL chip enters latch mode. The RT6236A/B include output Under-Voltage Protection (UVP). If the output voltage drops below the UVP trip threshold for longer than 250μs (typical) then IC's UVP is triggered. Chip into latch or hiccup mode. (see next section). Hiccup Mode The RT6236AH/BH, use 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 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 RT6236AL/BL, use 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. is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT6236A/B 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 Lock-Out In addition to the enable function, the RT6236A/B feature an Under-Voltage Lock-Out (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 RT6236A/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. T= COUT  VOUT  0.6  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 RT6236A/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 preCopyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 biased level to its regulated level. After VSS rises above about 2.2V output over- and under-voltage protections are enabled and the RT6236A/B begins continuous-switching operation. 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 or OVP event is cleared, 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 or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT6236A/B. Note that the external boot voltage must be lower than 5.5V Over-Temperature Protection The RT6236A/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. is a registered trademark of Richtek Technology Corporation. DS6236A/B-00 November 2014 RT6236A/B Absolute Maximum Ratings             (Note 1) Supply Voltage, VIN -----------------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------Switch Voltage, 3.3V) transient response is improved by adding a is a registered trademark of Richtek Technology Corporation. www.richtek.com 17 RT6236A/B 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. VOUT Cff FB RT6236A/B For automatic start-up the low-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). BW R1 Enable Operation (EN) EN VIN Figure 1. Cff Capacitor Setting 1 Cff  2  3.1412  R1 BW  0.8 GND Figure 2. External Timing Control VIN REN 100k Soft-Start (SS) CSS  nF   0.7V 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 EN Q1 Enable The RT6236A/B soft-start uses an external capacitor at SS to adjust the soft-start timing according to the following equation : RT6236A/B CEN Cff can be calculated base on below equation : t  ms   EN R2 GND  REN RT6236A/B GND Figure 3. Digital Enable Control Circuit VIN REN1 REN2 EN RT6236A/B GND Figure 4. Resistor Divider for Lockout Threshold Setting Do not leave SS unconnected. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 18 is a registered trademark of Richtek Technology Corporation. DS6236A/B-00 November 2014 RT6236A/B Output Voltage Setting External BOOT Capacitor Series Resistance 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 : 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 (