® RT2855A/B 4A, 18V, 650kHz, ACOTTM Synchronous Step-Down Converter General Description The RT2855A/B are high-performance 650kHz 4A stepdown regulators with internal power switches and synchronous rectifiers. They feature quick transient response using their Advanced Constant On-Time (ACOT TM) 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 0.765V to 7V. avoid low-frequency interference while the RT2855A features a power-saving discontinuous operating mode at light loads. Features TM The proprietary ACOT control improves upon other fastresponse 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 RT2855A/B are stable with and optimized for ceramic output capacitors With internal 70mΩ switches and 30mΩ synchronous rectifiers, the RT2855A/B display excellent efficiency and good behavior across a range of applications, especially for low output voltages and low duty cycles. Cycle-bycycle current limit, input under-voltage lock-out, externallyadjustable soft-start, output under- and over-voltage protection, and thermal shutdown provide safe and smooth operation in all operating conditions. The RT2855A and RT2855B are each available in WQFN-16L 3x3 package, with exposed thermal pads. The RT2855B switches continuously even at light loads to Fast Transient Response Steady 650kHz Switching Frequency at all Load Current (RT2855B) Discontinuous operating mode at Light Load (RT2855A) 4A Output Current Advanced Constant On-Time (ACOTTM) Control Optimized for Ceramic Output Capacitors 4.5V to 18V Input Voltage Range Internal 70mΩ Ω Switch and 30mΩ Ω Synchronous Rectifier 0.765V to 7V Adjustable Output Voltage Externally-adjustable, Pre-biased Compatible SoftStart Cycle-by-Cycle Current Limit Optional Output Discharge Function Output Over- and Under-voltage Shut-down Latched (RT2855ALGQW/RT2855BLGQW Only) Hiccup Mode (RT2855AHGQW/RT2855BHGQW Only) Input Under Voltage Lockout Thermal Shutdown Simplified Application Circuit VIN RT2855A/B VIN SW VCC Power Good VREG5 Input Signal PGOOD September 2015 BOOT VS FB EN SS VREG5 GND PGND Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 VOUT is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT2855A/B 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 Ordering Information RT2855A/B Package Type QW : WQFN-16L 3x3 (W-Type) Lead Plating System G : Green (Halogen Free and Pb Free) H : Hiccup Mode OVP & UVP L : Latched OVP & UVP A : PSM B : PWM Marking Information RT2855AHGQW 4G= : Product Code 4G=YM DNN YMDNN : Date Code Note : 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. RT2855ALGQW 4F= : Product Code YMDNN : Date Code Pin Configurations (TOP VIEW) VS VCC VIN VIN 4F=YM DNN 16 15 14 13 RT2855BHGQW 4E= : Product Code YMDNN : Date Code 4D= : Product Code YMDNN : Date Code Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 12 2 11 GND 3 10 17 4 5 RT2855BLGQW 4D=YM DNN 1 6 7 9 BOOT SW SW SW 8 PGOOD EN PGND PGND 4E=YM DNN FB VREG5 SS GND WQFN-16L 3x3 is a registered trademark of Richtek Technology Corporation. DS2855A/B-01 September 2015 RT2855A/B Functional Pin Description Pin No. Pin Name Pin Function 1 FB Feedback Input Voltage. Connect FB to the midpoint of the external feedback resistive divider to sense the output voltage. Place the resistive divider within 5mm from the FB pin. The IC regulates VFB at 0.765V (typical). 2 VREG5 Internal Regulator Output. Connect a 1F capacitor to GND to stabilize output voltage. 3 SS Soft-Start Control. Connect an external capacitor between this pin and GND to set the soft-start time. 4 GND Ground. 5 PGOOD Open Drain Power-good Output. PGOOD connects to PGND whenever VFB is less than 90% of its regulation threshold (typical). 6 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 10A. PGND Power Ground. PGND connects to the source of the internal N-channel MOSFET synchronous rectifier and to other power ground nodes of the IC. The exposed pad and the 2 PGND pins should be well soldered to the input and output capacitors and to a large PCB area for good power dissipation. SW Switching Node. SW is the source of the internal N-channel MOSFET switch and the drain of the internal N-channel MOSFET synchronous rectifier. Connect SW to the inductor with a wide short PCB trace and minimize its area to reduce EMI. BOOT Bootstrap Supply for High Side Gate Driver. Connect a 0.1F capacitor between BOOT and SW to power the internal gate driver. 13, 14 VIN Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a suitably large (10F x 2) ceramic capacitors at this pin. 15 VCC Internal Linear Regulator Supply Input. VCC supplies power for the internal linear regulator that powers the IC. Connect VIN to the input voltage and bypass to ground with a 0.1F ceramic capacitor. 16 VS Output Voltage Sense Input Pin. 7, 8, 17 (Exposed pad) 9, 10, 11 12 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 September 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT2855A/B Function Block Diagram BOOT VCC VREG5 POR & Reg EN VBIAS Min. Off-Time OC UV & OV VREG5 6µA SS VIN FB VS VREG5 VIN VREF Control Driver SW PGND SW ZC Ripple Gen. FB + Comparator GND FB 0.9 x VREF Comparator - PGOOD + On-Time Detailed Description The RT2855A/B are high-performance 650kHz 4A stepdown regulators with internal power switches and synchronous rectifiers. They feature an Advanced Constant On-Time (ACOTTM) control architecture that provides stable operation with ceramic output capacitors without complicated external compensation, among other benefits. The input voltage range is from 4.5V to 18V and the output is adjustable from 0.765V to 7V. 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 RT2855A/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 oneshot. 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 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 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. DS2855A/B-01 September 2015 RT2855A/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 © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 September 2015 rise to the input voltage. This increases the effective ontime 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 (at SW) and modifying the ontime 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 RT2855A/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 the on-time one-shot is triggered, as long as the minimum off-time 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 ontime, 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 (260ns typical) so that rapidly-repeated ontimes can raise the inductor current quickly when needed. Discontinuous Operating Mode (RT2855A Only) After soft start, the RT2855B operates in fixed frequency mode to minimize interference and noise problems. The RT2855A 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. www.richtek.com 5 RT2855A/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 RT2855A/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 upper current limit, the on-time one-shot is inhibited until the inductor current ramps down below the upper current limit plus a wide hysteresis band of about 1A until it drops below the lower current limit level. Thus, only when the inductor current is well below the upper current limit is another on-time permitted. This arrangement prevents the average output current from greatly exceeding the guaranteed upper 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 (see next section) the IC will stop switching to avoid excessive heat. The RT2855B also includes a negative current limit to protect the IC against sinking excessive current and possibly damaging the IC. If the voltage across the synchronous rectifier indicates the negative current is too high, the synchronous rectifier turns off until after the next high-side on-time. The RT2855A does not sink current and therefore does not need a negative current limit. Hiccup Mode The RT2855AHGQW/ RT2855BHGQW, use hiccup mode OVP and 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 the overload or short circuit is removed. During hiccup mode, the shutdown time is determined by the capacitor at SS. A 0.5μA current source discharges VSS from its starting voltage (normally VREG5). The IC remains shut down until VSS reaches 0.2V, about 38ms for a 3.9nF capacitor. At that point the IC begins to charge the SS capacitor by 6μA, and a normal start-up occurs. If the fault remains, OVP and 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 16ms for a 3.9nF SS capacitor. Latch-Off Mode The RT2855ALGQW/ RT2855BLGQW, use latch-off mode OVP and UVP. When the protection function is triggered the IC will shut down. 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. Input Under-voltage Lock-out In addition to the enable function, the RT2855A/B feature an under-voltage lock-out (UVLO) function that monitors the internal linear regulator output (VREG5). To prevent operation without fully-enhanced internal MOSFET switches, this function inhibits switching when VREG5 drops below the UVLO-falling threshold. The IC resumes switching when VREG5 exceeds the UVLO-rising threshold. 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. EN is a high voltage input that can be safely connected to VIN (up to 18V) for automatic start-up. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS2855A/B-01 September 2015 RT2855A/B Soft-Start (SS) Over Temperature Protection The RT2855A/B soft-start uses an external pin (SS) to clamp the output voltage and allow it to slowly rise. After VEN is high and VREG5 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. The available capacitance range is from 2.7nF to 220nF. Do not leave SS unconnected. The RT2855A/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. During start-up, while the SS capacitor charges, the RT2855A/B operate in discontinuous 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 pre-biased level to its regulated level. After VSS rises above about 2.2V output over-and under-voltage protections are enabled and the RT2855B begins continuous-switching operation. An internal linear regulator (VREG5) produces a 5.1V supply from VIN that powers the internal gate drivers, PWM logic, reference, analog circuitry, and other blocks. If VIN is 6V or greater, VREG5 is guaranteed to provide significant power for external loads. PGOOD Comparator Output Discharge Control When EN pin is low, the RT2855A/B will discharge the output with an internal 50Ω MOSFET connected between VOUT to GND pin. OVP/UVP Protection The RT2855A/B detects over and under voltage conditions by monitoring the feedback voltage on FB pin. The two functions are enabled after approximately 1.7 times the soft-start time. When the feedback voltage becomes higher than 120% of the target voltage, the OVP comparator will go high to turn off both internal high side and low side MOSFETs. When the feedback voltage is lower than 70% of the target voltage for 250μs, the UVP comparator will go high to turn off both internal high side and low side MOSFETs. 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 PGND. External Bootstrap Capacitor 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. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 September 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT2855A/B Absolute Maximum Ratings (Note 1) Supply Voltage, VIN, VCC ---------------------------------------------------------------------------------------Switch Voltage, SW -----------------------------------------------------------------------------------------------< 10ns ----------------------------------------------------------------------------------------------------------------BOOT to SW --------------------------------------------------------------------------------------------------------VREG5 to VIN or VCC --------------------------------------------------------------------------------------------EN, VS Pin ----------------------------------------------------------------------------------------------------------Other Pins Voltage -------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C WQFN-16L 3x3 -----------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WQFN-16L 3x3, θJA ------------------------------------------------------------------------------------------------WQFN-16L 3x3, θJC -----------------------------------------------------------------------------------------------Junction Temperature Range -------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------- Recommended Operating Conditions −0.3V to 20V −0.3V to (VIN + 0.3V) −5V to 25V −0.3V to 6V −17V to 0.3V −0.3V to 20V −0.3V to 6V 2.1W 47.4°C/W 7.5°C/W 150°C 260°C −65°C to 150°C (Note 3) Supply Voltage, VIN ------------------------------------------------------------------------------------------------ 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 = −40°C to 85°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Supply Current Shutdown Current ISHDN TA = 25C, VEN = 0V -- 1 10 A Quiescent Current IQ TA = 25C, VEN = 5V, VFB = 0.8V -- 1 1.3 mA Logic-High 2 -- 18 Logic-Low -- -- 0.4 Logic Threshold EN Voltage V VFB Voltage and Discharge Resistance TA = 25C 0.757 TA = 40C to 85C 0.755 -- 0.775 VFB = 0.8V, TA = 25C -- 0.01 0.1 A RDIS VEN = 0V, VS = 0.5V -- 50 100 VREG5 TA = 25C, 6V VIN 18V, 0 < IVREG5 5mA 4.8 5.1 5.4 V Feedback Threshold Voltage VFB Feedback Input Current IFB VOUT Discharge Resistance 0.765 0.773 V VREG5 Output VREG5 Output Voltage Line Regulation 6V VIN 18V, IVREG5 = 5mA -- -- 20 mV Load Regulation 0 IVREG5 5mA -- -- 100 mV VIN = 6V, VREG5 = 4V, TA = 25C 70 85 -- mA Output Current IVREG5 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 is a registered trademark of Richtek Technology Corporation. DS2855A/B-01 September 2015 RT2855A/B Parameter Symbol Test Conditions Min Typ Max Unit RDS(ON) Switch On Resistance High Side RDS(ON)_H TA = 25C (VBOOT VSW ) = 5.5V -- 70 -- Low Side RDS(ON)_L TA = 25C -- 30 -- 5 6 7 -- 150 -- -- 20 -- m Current Limit Current Limit ILIM A Thermal Shutdown Thermal Shutdown Threshold TSD Shutdown Temperature Thermal Shutdown Hysteresis TSD C On-Time Timer Control On-Time tON VIN = 12V, VOUT = 1.05V -- 135 -- ns Minimum Off-Time tOFF(MIN) VFB = 0.7V, TA = 25C -- 260 310 ns VSS = 0V 5 6 7.5 A VSS = 0.5V (Latch Mode) 0.1 0.2 -- mA VSS = 0.5V (Hiccup Mode) -- 0.5 -- A 3.6 3.85 4.1 0.18 0.35 0.47 VFB Rising 85 90 95 VFB Falling -- 85 -- 2.5 5 -- mA 114 120 126 % OVP Prop Delay -- 5 -- s UVP Trip Threshold 65 70 75 UVP Hysteresis -- 10 -- UVP Prop Delay -- 250 -- s -- tSS x 1.7 -- ms Soft-Start SS Charge Current SS Discharge Current UVLO UVLO Threshold Wake Up VREG5 Hysteresis V Power Good PGOOD Threshold PGOOD Sink Current PGOOD = 0.5V % Output Under Voltage and Over Voltage Protection OVP Trip Threshold UVP Enable Delay OVP Detect tUVPEN Relative to Soft-Start Time % 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 © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 September 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT2855A/B Typical Application Circuit RT2855A/B VIN 13, 14 C1 10µF x 2 C2 0.1µF 15 VIN VCC Output Signal VREG5 SW BOOT VS R3 100k 5 PGOOD 6 EN Input Signal C5 3.3nF L1 1.4µH 9, 10, 11 C6 0.1µF 12 VOUT 1.05V/4A C3 R1 8.25k 16 C7 22µF x 2 FB 1 R2 22.1k VREG5 2 VREG5 C4 3 SS 1µF GND PGND 4 7, 8, 17 (Exposed Pad) Table 1. Suggested Component Values (VIN = 12V) VOUT (V) R1 (k) R2 (k) C3 (pF) L1 (H) C7 (F) 1 6.81 22.1 -- 1.4 22 to 68 1.05 8.25 22.1 -- 1.4 22 to 68 1.2 12.7 22.1 -- 1.4 22 to 68 1.8 30.1 22.1 5 to 22 2 22 to 68 2.5 49.9 22.1 5 to 22 2 22 to 68 3.3 73.2 22.1 5 to 22 2 22 to 68 5 124 22.1 5 to 22 3.3 22 to 68 7 180 22.1 5 to 22 3.3 22 to 68 Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS2855A/B-01 September 2015 RT2855A/B Typical Operating Characteristics Output Voltage vs. Output Current Efficiency vs. Output Current 1.10 90 1.09 80 1.08 Efficiency (%) 70 Output Voltage (V) 100 VIN = 5V VIN = 12V VIN = 18V 60 50 40 30 1.07 1.06 1.05 VIN = 18V VIN = 12V VIN = 5V 1.04 1.03 1.02 20 10 1.01 RT2855A, VOUT = 1.05V 0 0.001 0.01 0.1 1 VOUT = 1.05V 1.00 0.0 10 0.4 0.8 1.2 Output Current (A) Feedback Voltage (V) Feedback Voltage (V) 2.8 3.2 3.6 4.0 VIN = 12V, VOUT = 0.765V, IOUT = 0.6A 0.775 0.78 0.77 0.76 0.75 0.74 0.73 0.72 0.770 0.765 0.760 0.755 0.750 0.745 0.71 0.740 0.70 -50 -25 0 25 50 75 100 4 125 6 8 Shutdown Current vs. Temperature 30 10 12 14 16 18 Input Voltage (V) Temperature (°C) Quiescent Current vs. Temperature 1000 VIN = 12V, VOUT = 1.05V, IOUT = 0A VIN = 12V, VOUT = 1.05V, IOUT = 0A 950 25 Quiescent Current (µA) Shutdown Current (µA)1 2.4 Feedback Voltage vs. Input Voltage 0.780 VIN = 12V, VOUT = 0.765V, IOUT = 0.6A 0.79 2.0 Output Current (A) Feedback Voltage vs. Temperature 0.80 1.6 20 15 10 5 900 850 800 750 700 650 0 600 -50 -25 0 25 50 75 100 Temperature (°C) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 September 2015 125 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT2855A/B Current Limit vs. Input Voltage 7.0 Output Ripple Voltage VIN = 12V, VOUT = 1.05V Current Limit (A) 6.5 VIN = 12V, VOUT = 1.05V, IOUT = 4A VOUT (10mV/Div) 6.0 Upper Threshold 5.5 5.0 Lower Threshold 4.5 4.0 VSW (5V/Div) 3.5 3.0 4 6 8 10 12 14 16 18 Time (500ns/Div) Input Voltage (V) Load Transient Response Load Transient Response RT2855A RT2855B VOUT (50mV/Div) VOUT (50mV/Div) IOUT (1A/Div) IOUT (1A/Div) VIN = 12V, VOUT = 1.05V, IOUT = 0A to 3A Time (100μs/Div) VIN = 12V, VOUT = 1.05V, IOUT = 0A to 3A Time (100μs/Div) Pre Short RT2855A/B PGOOD (5V/Div) VOUT (400mV/Div) ILX (2A/Div) Time (200μs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS2855A/B-01 September 2015 RT2855A/B Applications Information REN 100k Soft-Start (SS) VIN The RT2855A/B soft-start uses an external capacitor at SS to adjust the soft-start timing according to the following equation : C (nF) 0.765V tSS (ms) = SS ISS (μA) The soft-start timing is the output voltage rising time from 0V to settled level and can be programmed by the external capacitor between the SS and GND pins. The available capacitance range is from 2.7nF to 220nF. If a 3.9nF capacitor is used, the typical soft-start will be 0.5ms. Do not leave SS unconnected. 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 1). Calculate the delay time using EN's internal threshold where switching operation begins (1.4V, typical). An external MOSFET can be added to implement digital control of EN when no system voltage above 2V is available (Figure 2). 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 undervoltage lockout threshold (Figure 3). EN REN CEN EN RT2855A/B GND Figure 1. External Timing Control Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 Q1 Enable RT2855A/B GND Figure 2. Digital Enable Control Circuit REN1 VIN EN REN2 RT2855A/B GND Figure 3. Resistor Divider for Lockout Threshold Setting Enable Operation (EN) VIN EN September 2015 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 : R1 VOUT = 0.765 (1 ) R2 VOUT R1 FB RT2855A/B R2 GND Figure 4. 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 : R2 (VOUT 0.765V) 0.765V For output voltage accuracy, use divider resistors with 1% R1 = or better tolerance. Under-Voltage Lockout Protection The RT2855A/B feature an under-voltage lock-out (UVLO) function that monitors the internal linear regulator output (PVCC) and prevents operation if VPVCC is too low. In some is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT2855A/B 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. 5V BOOT RT2855A/B 0.1µF SW Figure 5. External Bootstrap Diode 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 deadtime 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 (<10Ω) 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 5 to charge the BOOT capacitor and place the resistance between BOOT and the capacitor/diode connection. 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 WQFN-16L 3x3 package, the thermal resistance, θJA, is 47.4°C/W on a standard JEDEC 51-7 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) / (47.4°C/W) = 2.1W for WQFN-16L 3x3 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. 2.5 Maximum Power Dissipation (W)1 multiple input voltage applications, it may be desirable to use a power input that is too low to allow VPVCC to exceed the UVLO threshold. Four-Layer PCB 2.0 1.5 1.0 0.5 0.0 0 PVCC Capacitor Selection Decouple PVCC to PGND with a 1μF ceramic capacitor. High grade dielectric (X7R, or X5R) ceramic capacitors are recommended for their stable temperature and bias voltage characteristics. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 25 50 75 100 125 Ambient Temperature (°C) Figure 6. Derating Curve of Maximum Power Dissipation is a registered trademark of Richtek Technology Corporation. DS2855A/B-01 September 2015 RT2855A/B Layout Considerations Connect the feedback network to the output capacitors rather than the inductor. Place the feedback components near the FB pin. The exposed pad, PGND, and GND should be connected to large copper areas for heat sinking and noise protection. Provide dedicated wide copper traces for the power path ground between the IC and the input and output capacitor grounds, rather than connecting each of these individually to an internal ground plane. Avoid using vias in the power path connections that have switched currents (from CIN to PGND and CIN to VIN) and the switching node (SW). Follow the PCB layout guidelines for optimal performance of the RT2855A/B. Keep the traces of the main current paths as short and wide as possible. Put the input capacitor as close as possible to the device pins (VIN and PGND). The high-frequency switching node (SW) has large voltage swings and fast edges and can easily radiate noise to adjacent components. Keep its area small to prevent excessive EMI, while providing wide copper traces to minimize parasitic resistance and inductance. Keep sensitive components away from the SW node or provide ground traces between for shielding, to prevent stray capacitive noise pickup. Place the feedback components as close Place the input capacitors as to the FB as possible for better regulation. close to the IC as possible. VS VCC VIN VIN VOUT R1 R2 PGND 16 15 14 13 FB VREG5 SS GND CSS 1 12 2 11 GND 3 10 17 4 5 6 7 9 BOOT SW SW SW CBOOT 8 PGOOD EN PGND PGND CREG5 CIN L COUT VOUT SW should be connected to inductor by wide and short trace. Keep sensitive components away from this trace. Place the output capacitors as close to the IC as possible. Figure 7. PCB Layout Guide Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS2855A/B-01 September 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT2855A/B Outline Dimension D SEE DETAIL A D2 L 1 E E2 e b A A1 1 1 2 2 DETAIL A Pin #1 ID and Tie Bar Mark Options A3 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 0.180 0.300 0.007 0.012 D 2.950 3.050 0.116 0.120 D2 1.300 1.750 0.051 0.069 E 2.950 3.050 0.116 0.120 E2 1.300 1.750 0.051 0.069 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 16L QFN 3x3 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 DS2855A/B-01 September 2015