RT9288A PWM Step-Up DC/DC Controller for White-LED Driver General Description Features The RT9288A is a wide input operating voltage range stepup controller. High voltage output and large output current z VIN Operating Range : 3V to 13.5V z are feasible by using an external N-MOSFET. The RT9288A input operating range is from 3V to 13.5V. Besides, it could support up to 60V output at 12V input. z Fixed PWM Frequency : 1MHz 200Hz to 200kHz PWM Dimming Frequency Flexible PWM/Analog Dimming Control Voltage Mode with External Compensation Soft Start Function RoHS Compliant and 100% Lead (Pb)-Free The RT9288A is an optimized design for WLED driver applications. Adjusting the output current of the RT9288A changes the brightness of the WLEDs. Chip Enable pin can be used as a digital input allowing WLED brightness control with a logic-level PWM signal. z z z z Applications z z Ordering Information RT9288A TFT LCD Panels LED Backlighting Pin Configurations (TOP VIEW) Package Type E : SOT-23-6 EXT GND COMP Lead Plating System P : Pb Free G : Green (Halogen Free and Pb Free) 6 5 4 2 3 Note : VDD EN Richtek products are : ` FB SOT-23-6 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 For marking information, contact our sales representative directly or through a Richtek distributor located in your area. Functional Pin Description Pin No. Pin Name Pin Function 1 VDD Supply Input Voltage Pin. Bypass an 1uF capacitor to GND to reduce the input noise. 2 EN Chip Enable (Active High). 3 FB Feedback to Error Amplifier Input. 4 COMP Output of Error Amplifier. Connect a capacitor between the COMP pin and GND for compensation. While shutdown, this pin is pulled down by an internal resistor. 5 GND Ground Pin. 6 EXT Output for External Transistor. DS9288A-02 April 2011 www.richtek.com 1 RT9288A Typical Application Circuit L1 10uH/1.5A VIN D1 VOUT 30V/25mA 5V C1 R1 C2 100nF 1 2 PWM Dimming 5 RT9288A VDD EXT COMP EN C4 10uF R2 10 6 4 C3 100nF FB 3 GND R3 Figure 1. LED Driver with PWM Brightness Control (5V J 30V) L1 10uH/1.5A VIN D1 VOUT 60V/25mA 12V C1 C2 100nF R1 1 2 PWM Dimming 5 VDD EXT COMP EN 6 4 C4 10uF R2 10 RT9288A C3 100nF FB 3 GND R3 Figure 2. LED Driver with PWM Brightness Control (12V J 60V) L1 10uH/1.5A VIN D1 VOUT 24V 12V C1 C2 100nF Chip Enable R1 RT9288A 1 2 VDD EXT EN COMP 5 GND 6 4 R2 10 R3 C4 10uF RC CC 30k 6.8nF FB 3 R4 Figure 3. Application for Constant Output Voltage www.richtek.com 2 DS9288A-02 April 2011 RT9288A Function Block Diagram VDD 1.8V + Soft Start/Short OC Circuit Protection POR Power On & UVLO PreRegulator COMP FB + Error Amplifier Bandgap Reference 1MHz OSC + + CMP PWM Logic PWM Dimming Timer EXT GND EN Operation Soft-Start and Short Circuit Protection While power-on, the RT9288A enters soft-start cycle to reduce the in-rush current and output voltage overshoot. The internal soft-start time is 10ms for the RT9288A. The RT9288A enters shutdown and can be re-enabled by turning off-on EN pin. In normal operation, if the output loading changes large enough to let error amplifier output larger than 1.8V, the short circuit timer is started. If the time duration of this condition is kept continuously to more than 10ms, the short circuit state is latched and the RT9288 enters shutdown and can be re-enabled by turning off-on EN pin. Dimming Control for LED Lighting EN is also used as a digital input allowing LED brightness control with a logic-level PWM signal applied directly to EN. The frequency range is from 200Hz to 200kHz, while 0% duty cycle corresponds to zero current and 100% duty cycle corresponds to full current. The error amplifier and compensation capacitor form a lowpass filter, so the PWM dimming results in DC current to the LEDs without any additional RC filters. The PWM signal must be applied after soft-start finished. Under-Voltage Lock-out The under voltage lock-out circuit is adopted as a voltage detector and always monitors the supply voltage (VDD) while EN at logic High. While power-on, the chip is kept in shutdown mode till the VDD rises to higher than 2.5V (MAX). While power-off, the chip does not leave operating mode till VDD falls to less than 2.2V(MIN). DS9288A-02 April 2011 www.richtek.com 3 RT9288A Absolute Maximum Ratings z z z z z z z z z (Note 1) Supply Input Voltage, VDD ------------------------------------------------------------------------------------------- −0.3V to 16V EN, EXT Pins ----------------------------------------------------------------------------------------------------------- −0.3V to VDD + 0.3V FB, COMP Pins ------------------------------------------------------------------------------------------------------- −0.3V to 7V Power Dissipation, PD @ TA = 25°C SOT-23-6 ---------------------------------------------------------------------------------------------------------------- 0.455W Package Thermal Resistance (Note 2) SOT-23-6, θJA ----------------------------------------------------------------------------------------------------------- 220°C/W Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260°C Junction Temperature ------------------------------------------------------------------------------------------------- 150°C Storage Temperature Range ---------------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 3) HBM (Human Body Mode) ------------------------------------------------------------------------------------------ 2kV MM (Machine Mode) -------------------------------------------------------------------------------------------------- 200V Recommended Operating Conditions z z z (Note 4) Supply Input Voltage, VDD ------------------------------------------------------------------------------------------- 3V to 13.5V Junction Temperature Range ---------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range ---------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VDD = 5V, TA = 25°C, unless otherwise specified) Parameter Power-On Reset Symbol Test Conditions Min Typ Max Unit Operating Supply Voltage Range VDD Normal operation 3 5 13.5 V Under Voltage Lock Out UVLO V DD Rising 2.2 -- 2.5 V Supply current in PWM Mode IPWM V FB = VREF + 0.1V -- 2 -- mA Shutdown Current ISHDN V EN = 0V -- 1 10 uA 0.8 1 1.2 MHz -- 2 10 % 85 90 95 % Sawtooth Generator Oscillation Frequency fOSC Frequency Stability V DD = 3V to 13.5V Maximum Duty Cycle Error Amplifier Trans-Conductance GM -- 60 -- uA/V Feedback Voltage VFB -- 0.5 -- V V DD = 3V to 13.5V -- 5 -- mV Feedback Line Regulation Maximum Output Voltage VFB_MAX V COMP = V FB = low -- 2.4 -- V Minimum Output Voltage VFB_MIN V COMP = V FB = high -- 0.05 -- V Output Source Current V COMP = 0.7V, VFB = low -- 20 -- uA Output Sink Current V COMP = 0.7V, VFB = high -- 20 -- uA 5 10 20 ms Soft Start & Short Circuit Unit Soft-Start Ramp Time To be continued www.richtek.com 4 DS9288A-02 April 2011 RT9288A Parameter Symbol Test Conditions Min Typ Max Unit Output driver On Resistance (P-MOSFET) RDS(ON)_P -- 30 60 Ω On Resistance (N-MOSFET) RDS(ON)_N -- 20 40 Ω -- 100 -- ns Output rising/falling time (Note 5) CL = 1000pF, V FB = Low Logic EN Pin Low Voltage VIL -- -- 0.5 V EN Pin High Voltage VIH 1.8 -- V DD V Note 1. Stresses listed as the above “Absolute Maximum Ratings” may cause permanent damage to the device. These are for stress ratings. 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 for extended periods may remain possibility to affect device reliability. Note 2. θJA is measured in the natural convection at T A = 25°C on a low effective thermal conductivity test board of JEDEC 51-3 thermal measurement standard. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Note 5. Guarantee by design. DS9288A-02 April 2011 www.richtek.com 5 RT9288A Typical Operating Characteristics Efficiency vs. Output Current 100 90 90 80 80 70 70 Efficiency (%) Efficiency (%) Efficiency vs. Output Current 100 60 50 40 30 60 50 40 30 20 20 10 10 VIN = 12V, VOUT = 15V, COUT = 10uF, L = 10uH VIN = 12V, VOUT = 30V, COUT = 10uF, L = 10uH 0 0 0 200 400 600 800 0 1000 100 200 300 400 500 600 700 Output Current (mA) Output Current (mA) Output Voltage vs. Output Current Output Voltage vs. Output Current 15.10 30.40 30.38 30.36 Output Voltage (V) Output Voltage (V) 15.07 15.04 15.01 14.98 30.34 30.32 30.30 30.28 30.26 30.24 VIN = 12V, VOUT = 15V, COUT = 10uF, L = 10uH 14.95 30.22 VIN = 12V, VOUT = 30V, COUT = 10uF, L = 10uH 30.20 0 100 200 300 400 500 600 700 800 900 1000 100 0 0 100 300 400 500 600 700 Output Current (mA) Output Current (mA) Supply Current vs. Input Voltage Output Voltage vs. Input Voltage 16.15 1.8 16.10 1.6 Supply Current (mA) 16.05 Output Voltage (V) 200 16.00 15.95 15.90 15.85 15.80 15.75 15.70 1.4 1.2 1 0.8 0.6 0.4 0.2 15.65 VOUT = 15.9V, IOUT = 1mA, COUT = 10uF, L = 10uH Duty = 50%, f = MHz 0 15.60 3 5.1 7.2 9.3 Input Voltage (V) www.richtek.com 6 11.4 13.5 3 5.1 7.2 9.3 11.4 13.5 Input Voltage (V) DS9288A-02 April 2011 RT9288A Frequency vs. Input Voltage Supply Current vs. Temperature 1100 2.5 1060 2 Frequency (kHz) Supply Current (mA) 1080 1.5 1 1040 1020 1000 980 960 940 0.5 920 VIN = 5V, Duty = 50%, f = MHz COUT = 10uF, L = 10uH, TA = 25°C 900 0 -50 -25 0 25 50 75 100 3 125 5.1 7.2 9.3 11.4 13.5 Input Voltage (V) Temperature (°C) Frequency vs. Temperature Maximum Duty vs. Temperature 95 1300 94 93 Maximum Duty (%) Frequency (kHz) 1200 1100 1000 900 92 91 90 89 88 87 800 86 VIN = 5V, COUT = 10uF, L = 10uH VIN = 5V 85 700 -50 -25 0 25 50 75 100 -50 125 -25 VFB vs. Temperature 25 50 75 100 125 VFB vs. Input Voltage 0.505 0.5075 0.503 0.5065 0.501 0.5055 V FB (V) V FB (V) 0 Temperature (°C) Temperature (°C) 0.499 0.497 0.5045 0.5035 VIN = 5V 0.495 TA = 25°C 0.5025 -50 -25 0 25 50 75 Temperature (°C) DS9288A-02 April 2011 100 125 3 5.1 7.2 9.3 11.4 13.5 Input Voltage (V) www.richtek.com 7 RT9288A ILED vs. Duty Power On 25 200Hz 2kHz 20 VIN (5V/Div) I LED (mA) 200kHz 15 20kHz VOUT (10V/Div) 10 VLX (10V/Div) 5 VIN = 5V, IOUT = 100mA 0 0 20 40 60 80 100 Time (5ms/Div) Duty (%) Power Off Enable Operating VIN (5V/Div) VEN (5V/Div) VOUT (10V/Div) VOUT (10V/Div) VLX (10V/Div) VCOMP (2V/Div) VIN = 5V, IOUT = 100mA Time (5ms/Div) Time (5ms/Div) Disable Operating Stability VEN (5V/Div) VOUT_ac (50mV/Div) VOUT (10V/Div) VLX (10V/Div) VCOMP (2V/Div) I LOAD (0.5A/Div) VIN = 5V, IOUT = 100mA Time (5ms/Div) www.richtek.com 8 VIN = 5V, IOUT = 100mA VIN = 5V, VOUT = 12V, L = 4.7uH, IOUT = 100mA Time (500ns/Div) DS9288A-02 April 2011 RT9288A Stability Stability VOUT_ac (100mV/Div) VOUT_ac (100mV/Div) VLX (10V/Div) VLX (10V/Div) I LOAD (0.5A/Div) I LOAD (1A/Div) VIN = 5V, VOUT = 12V, L = 4.7uH, IOUT = 200mA Time (500ns/Div) VIN = 5V, VOUT = 12V, L = 4.7uH, IOUT = 300mA Time (500ns/Div) PWM Dimming by EN VIN = 12V, L = 4.7uH, Duty = 50% VEN (5V/Div) VCOMP (0.5V/Div) VEXT (10V/Div) fPWM = 2kHz, CCOMP = 100nF Time (2.5ms/Div) DS9288A-02 April 2011 www.richtek.com 9 RT9288A Application Information The RT9288A is a boost controller for DC to DC conversion. The main switch of the power stage can stand significant current that is greater than the internal main switch. There is no significant power dissipated in the RT9288A, therefore the thermal performance could be excellent. For the RT9288A, determine the maximum input current is the first step of the design procedure. Inductor Selection For the inductor selection, the inductance value depends on the maximum input current. Generally the inductor ripple current range is 20% to 40% of the maximum input current. Take 40% as an example, the value can be calculated as follows : VOUT × IOUT(MAX) η × VIN = 0.4 × IIN(MAX) IIN(MAX) = (1) IRIPPLE (2) Where η is the efficiency, IIN(MAX) is the maximum input current and IRIPPLE is the inductor ripple current. Beside, the input peak current is the maximum input current plus half of the inductor ripple current. IPEAK = 1.2 × IIN(MAX) (3) Note that the saturated current of inductor must be greater than IPEAK. The inductance value can be eventually determined as follows : L= η × (VIN )2 × (VOUT − VIN ) (4) 0.4 × (VOUT )2 × IOUT(MAX) × fOSC Where fOSC is the switching frequency. Consider the system performance, a shielded inductor is preferred to avoid EMI issue. IPEAK IL IIN(MAX ) IRIPPLE Diode Selection Schottky diode is a good choice for an asynchronous Boost converter due to the small forward voltage. However, power dissipation, reverse voltage rating and pulsating peak current are the important parameters of Schottky diode consideration. It is recommended to choose a suitable diode whose reverse voltage rating is greater than the maximum output voltage. Input Capacitor Selection Low ESR ceramic capacitors are recommended for input capacitor applications. Low ESR will effectively reduce the input ripple voltage caused by switching operation. A 10uF is sufficient for most applications. Nevertheless, this value can be decreased with lower output current requirement. Another consideration is the voltage rating of input capacitor must be greater than the maximum input voltage. Output Capacitor Selection Output ripple voltage is an important index for estimating the performance. This portion consists of two parts, one is the product of (IIN − IOUT) and ESR of the output capacitor, another part is formed by charging and discharging process of output capacitor. Refer to figure 5, evaluate ΔVOUT1 by ideal energy equalization. According to the definition of Q that is calculated as follows : ⎡ ⎤ Q = 1 × ⎢⎛⎜ IIN + 1 ΔIL − IOUT ⎞⎟ + ⎛⎜ IIN − 1 ΔIL − IOUT ⎞⎟ ⎥ 2 ⎣⎝ 2 2 ⎠ ⎝ ⎠⎦ V × IN × 1 = COUT × ΔVOUT1 VOUT fOSC Where TS is the inverse of switching frequency and the ΔIL is the inductor ripple current. Move COUT to left side to estimate the value of ΔVOUT1 as : ΔVOUT1 = IOUT(MAX) D × IOUT tON (6) η × COUT × fOSC Finally, the output ripple voltage can be determined as : ΔVOUT = (IIN − IOUT ) × ESR + 0A (5) D × IOUT η × COUT × fOSC (7) Figure 4. The Waveform of the Inductor Current www.richtek.com 10 DS9288A-02 April 2011 RT9288A L1 LX VIN Input Current VOUT IL CIN ΔIL D1 Inductor Current COUT VDD EXT Output Current RF1 RT9288A FB Time (1-D)T S M1 RF2 Output Ripple Voltage (ac) Time ΔV OUT1 V IN Figure 5. The Output Ripple Voltage without the Contribution of ESR Main Switch Selection The RT9288A uses an N-MOSFET as the main switch to achieve power conversion. The main switch stays in two states in the operation, one is the on state and the other is the off state. The potential of switching point, LX, is 0V in the on state. Nevertheless, the potential of LX rises to output voltage plus the forward voltage of D1 in the off state, this potential is the highest voltage in the Boost converter. Thus, the absolute VDS rating of the main switch must be greater than this voltage to prevent main switch damage in the off state or reliability problem. Another key parameter of main switch is the maximum continuous drain current. For a safety design, it is important to choose a maximum continuous drain current at two times the maximum input current. Energy saving is the trend in recent years. Therefore, design a high efficiency system is the important course. Conduction loss and switching loss play important roles for the efficiency in heavy load and light load respectively. Main switch with a small on resistance leads to lower conduction loss, however, it also means a greater gate capacitance. Great gate capacitance prolongs rising and falling transition in LX, t1 and t2. IL and VLX produce the main switching loss during t1 and t2. Thus, choose a main switch with proper gate capacitance could reduce switching loss. V TH V EXT Time t1 t2 V OUT + V D1 V LX Time ΔIL IL IOUT Time Figure 6. The Waveforms of EXT, LX and Inductor Current Related to the Switching Loss Loop Compensation It is easy to compensate the loop stability for the RT9288A's application in LED driving. Compensation network only contains a capacitor between COMP pin and GND as shown in figure 1. The best criterion to optimize the loop compensation is by inspecting the transient response and adjusting the compensation network. Layout Consideration The PCB layout is a very important issue for switching converter circuits design. There are some recommended layout guidelines that are shown as follows : ` The power components M1, L1, D1, CIN and COUT should be placed as close to the IC as possible to reduce the ac current loop. The connections between power components must be short and wide as possible due to large current stream flowing through these traces during operation. ` The function of C1 is to regulate VDD. Place C1 close to pin 1 is necessary. DS9288A-02 April 2011 www.richtek.com 11 RT9288A ` RF1 and RF2 formed a voltage divider to set correct output voltage. Pin 3 is connected to the branch of voltage divider and is a very sensitive point, placed this trace short and wide as possibly and far away from the switching point to avoid perturbation. ` Pin 4 is the compensation point for system stability. Place the compensation components as close to pin 4 as possibly, no matter the compensation is RC or capacitance. Note that, the GND of the compensation components should be connected with pin 5. Then, short it to system ground by via or trace. This will provide a clean reference for the IC. GND C IN C OUT M1 L1 V IN V OUT D1 GND R1 VDD 1 6 EXT EN 2 5 GND FB 3 4 COMP C1 R F1 RC CC R F2 GND Figure 7. Sketch Map of PCB Layout. www.richtek.com 12 DS9288A-02 April 2011 RT9288A Outline Dimension H D L C B b A A1 e Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.889 1.295 0.031 0.051 A1 0.000 0.152 0.000 0.006 B 1.397 1.803 0.055 0.071 b 0.250 0.560 0.010 0.022 C 2.591 2.997 0.102 0.118 D 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 H 0.080 0.254 0.003 0.010 L 0.300 0.610 0.012 0.024 SOT-23-6 Surface Mount Package Richtek Technology Corporation Richtek Technology Corporation Headquarter Taipei Office (Marketing) 5F, No. 20, Taiyuen Street, Chupei City 5F, No. 95, Minchiuan Road, Hsintien City Hsinchu, Taiwan, R.O.C. Taipei County, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611 Tel: (8862)86672399 Fax: (8862)86672377 Email: [email protected] Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek. DS9288A-02 April 2011 www.richtek.com 13