® RT8488 High Voltage 6-CH LED Driver Controller General Description Features The RT8488 is a 6-CH LED driver controller that delivers well matched LED current to each channel of LED string. With external current sources, the number of LEDs per string is only limited by the current source and the VIN/ Wide Operation Voltage Range : 7V to 28V VOUT conditions. The current mode PWM boost type controller operates at a programmable switching frequency of up to 1MHz, with a wide VIN range covering from 7V to 28V. The switch driver is designed to drive industrial grade high power MOSFETs. Programmable Channel Current 3% Current Matching Accuracy between Channels Programmable Switching Frequency Easy Analog and Digital Dimming Control Programmable Soft-Start Automatic Open Channel Detection Programmable Output Over Voltage Protection Under Voltage Lockout and Thermal Shutdown 32-Lead WQFN Package The PWM loop selects and regulates the LED string with the highest voltage string to 0.7V, thus allowing voltage mismatches between the LED strings. The RT8488 automatically detects and excludes any open and/or broken strings during operation from the PWM loop to prevent VOUT from over voltage. The LED currents on all channels are simply programmed with a resistor on each channel. Three convenient dimming methods are provided : 1. Analog dimming is linearly controlled by an external voltage. 2. True digitally controlled PWM dimming can regulate the duty cycle of the LED current. 3. For noise free PWM dimming, use an on board output clamping amplifier as a low pass filter to convert PWM dimming signals into analog dimming signals. Applications Building and Street Lighting LED TV Backlight LED Monitor Backlight Industrial Display Backlight Marking Information RT8488GQW : Product Number RT8488 GQWYMDNN YMDNN : Date Code Pin Configurations Other protection features include programmable output over voltage protection, PWM switch current limit and thermal shutdown. Ordering Information RT8488 Package Type QW : WQFN-32L 5x5 (W-Type) Lead Plating System G : Green (Halogen Free and Pb Free) Note : Suitable for use in SnPb or Pb-free soldering processes. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 ISENSE GND GATE GBIAS VCC LED1 GATE1 SENSE1 1 24 2 23 3 22 4 October 2014 21 GND 5 20 6 19 7 33 18 17 8 LED2 GATE2 SENSE2 LED3 GATE3 SENSE3 LED4 GATE4 10 11 12 13 14 15 16 NC SENSE6 GATE6 LED6 SENSE5 GATE5 LED5 SENSE4 RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. 32 31 30 29 28 27 26 25 RSET EN OVP SS VC ACTL DCTL NC 9 Richtek products are : (TOP VIEW) WQFN-32L 5x5 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8488 Typical Application Circuit L1 VIN * 12V CIN R3 D1 CB RT8488 VCC VOUT COUT GBIAS CVCC GATE MSW 6 x N LEDs ISENSE RSENSE RSET RSET LED1 GATE1 M1 SENSE1 VOUT RS1 R1 OVP R2 LED2 GATE2 M2 SENSE2 RS2 VC LED3 RVC GATE3 CVC M3 SENSE3 RS3 SS CSS LED4 GATE4 Chip Enable M4 SENSE4 EN RS4 LED5 DCTL GATE5 M5 SENSE5 RS5 ACTL LED6 GND GATE6 M6 SENSE6 RS6 * : If VIN is operated above 12V, it is recommended to keep the V CC at 12V for optimal application Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 Functional Pin Description Pin No. Pin Name Pin Function 1 RSET Switching Frequency Set Pin. Put a resistor from RSET to GND to program the switching frequency. f SW = 310kHz when RSET = 40k. 2 EN Chip Enable (Active High). 3 OVP Over Voltage Protection Pin. OVP pin threshold is around 1.23V. Use a resistor divider from Output to GND to program the OVP level. 4 SS Soft-Start Pin. Use a soft start cap from SS pin to GND to program the soft start time period. Around 5.5A is sourcing out of SS pin. 5 VC Loop Compensation Pin. 6 ACTL Analog/PWM Dimming Control Pin. When used in analog dimming, ACTL control range is from 0.5V to 1.4V. 7 DCTL Digital PWM Dimming Control Pin. By adding a 0.1F filter capacitor on the ACTL pin, the PWM dimming signal on the DCTL pin will be averaged out and converted into analog dimming signal on the ACTL pin. NC No Internal Connection. SENSEx Source Pin of External MOSFETx (x = 1 to 6). The SENSEx pins are regulated around 225mV. Connect a sense resistor from this pin to GND. The LED current is programmed by I LED = 225mV / (sense resistance) when VACTL is greater than 1.4V. GATEx Gate Pin of External MOSFETx (x = 1 to 6). For LED drivers. LEDx Drain Pin of External MOSFETx (x = 1 to 6). For LED drivers. Short the pin to GND if not used. 28 VCC Power Supply Pin. For good bypass, a low ESR capacitor is needed between this pin and GND. 29 GBIAS Internal Gate Driver Bias Voltage (around 10V) Pin. Need a good bypass capacitor between this pin and GND. 30 GATE Gate Pin of External MOSFET. For the Boost PWM control loop. 8, 9 25, 22, 19 16, 13, 10 26, 23, 20 17, 14, 11 27, 24, 21 18, 15, 12 31, GND 33 (Exposed Pad) 32 ISENSE Ground Pin. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. Switch Current Sense Pin. Connect a sense resistor from this pin to GND. The switch current sense signal is used for boost current mode PWM loop control and power switch over current protection. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 October 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8488 Function Block Diagram VCC 10V OSC RSET + - GBIAS 5.5V GATE + - 1.23V OVP PWM Control - ISENSE + 3.2V GBIAS + - + GATE1 - SENSE1 LED1 VC LED2 LED3 8µA SS 3.2V GBIAS + GATE2 SENSE2 3.2V LED4 + LED5 - GBIAS SENSE3 3.2V GBIAS + EN + Shutdown 3.2V GATE4 SENSE4 3.2V DCTL + - LED4 - - 1.4V LED3 GATE3 LED6 1.4V LED2 - VOUT Regulation Unit 3.2V LED1 1.4V GBIAS + GATE5 + SENSE5 - ACTL 3.2V + GND LED5 GBIAS LED6 GATE6 - SENSE6 Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 Absolute Maximum Ratings (Note 1) VCC --------------------------------------------------------------------------------------------------------------------------- 32V ISENSE DC ------------------------------------------------------------------------------------------------------------------------------- 2V < 200ns ------------------------------------------------------------------------------------------------------------------------ 6V GBIAS ------------------------------------------------------------------------------------------------------------------------- 14V SENSE1 to SENSE6 DC ------------------------------------------------------------------------------------------------------------------------------- 1V < 200ns ------------------------------------------------------------------------------------------------------------------------ 6V LED1 - LED6 (Note 5) ---------------------------------------------------------------------------------------------------- 20V DCTL, ACTL, EN, OVP --------------------------------------------------------------------------------------------------- 10V Power Dissipation, PD@TA = 25°C WQFN-32L 5x5 ------------------------------------------------------------------------------------------------------------- 2.778W Package Thermal Resistance (Note 2) WQFN-32L 5x5, θJA -------------------------------------------------------------------------------------------------------- 36°C/W WQFN-32L 5x5, θJC ------------------------------------------------------------------------------------------------------- 6°C/W Junction Temperature ------------------------------------------------------------------------------------------------------ 150°C Storage Temperature Range --------------------------------------------------------------------------------------------- −65°C to 150°C Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------- 260°C ESD Susceptibility (Note 3) HBM (Human Body Model) ----------------------------------------------------------------------------------------------- 2kV MM (Machine Model) ------------------------------------------------------------------------------------------------------ 200V Recommended Operating Conditions (Note 4) Supply Voltage, VCC ------------------------------------------------------------------------------------------------------- 7V to 28V Junction Temperature Range --------------------------------------------------------------------------------------------- −40°C to 125°C Electrical Characteristics (VCC = 12V, No Load, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Overall Supply Current IVCC VVC 0.4V (Switching off) -- 5 8 mA Shutdown Current ISHDN VEN 1.2V -- 5 -- A EN Threshold Voltage Logic-High VIH 2 -- -- Logic-Low VIL -- -- 0.5 -- 2 -- A 214 225 236 mV -- 1.5 3 % -- -- 10 A -- 0.4 -- V EN Input Current IEN VEN 3.3V V LED Current Programming SENSE1-SENSE6 Threshold 6V > VGATEx > 2V V(MAX) V(MIN) SENSE Voltage CH to CH Matching 2 V(avg) Analog Dimming ACTL Input Current IACTL LED Current Off Threshold at ACTL VACTL_OFF Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 October 2014 VACTL ≦ 6V is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8488 Parameter Symbol Test Conditions Min Typ Max Unit -- 1.4 -- V LED Current On Threshold at ACTL VACTL_ON DCTL Input Current IDCTL VDCTL ≦ 6V -- -- 1 A f SW RSET = 40k 220 280 340 kHz RSET = 40k -- 300 -- ns -- 0.1 -- V PWM Boost Converter Switching Frequency Minimum Off-Time VLED Threshold for No Connection VLED Regulated VLED VLED Highest Voltage LED String -- 0.7 -- V Amplifier Output Current IVC 2.4V > VVC > 0.2V -- ±30 -- A -- 0.7 -- V 110 120 140 mV IGBIAS = 20mA -- 10 -- V IGATE = 20mA -- 7.7 -- IGATE = 0.1mA -- 8.2 -- IGATE = 20mA -- 0.7 -- IGATE = 0.1mA -- 0.4 -- 1nF load at Gate -- 20 -- IGATEx = 2mA -- 8.1 -- IGATEx = 0.1mA -- 8.3 -- IGATEx = 2mA -- 0.8 -- IGATEx = 0.1mA -- 0.6 -- -- 1.23 -- V VC Threshold for PWM Switch Off ISENSE Threshold for Current VLIM_ISENSE Limit Switch Gate Driver GBIAS Voltage VGBIAS GATE High Voltage VGATE_H GATE Low Voltage VGATE_L GATE Drive Rise and Fall Time V V ns LED Current Sources Gate Driver GATE1 to 6 High Voltage VGATEx_H GATE1 to 6 Low Voltage VGATEx_L V V OVP and Soft-Start OVP Threshold VOVP OVP Input Current IOVP VOVP 1.23V -- 1 -- A Soft-Start Pin Current ISS VSS 3.2V -- 8 -- A TSD -- 150 -- C TSD -- 20 -- C Thermal Protection Thermal Shutdown temperature Thermal Shutdown Hysteresis Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 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. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Note 5. Adding a series resistor of at least 20kΩ for higher pin voltage. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 October 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8488 Typical Operating Characteristics LED Current vs. ACTL PWM Duty Efficiency vs. Input Voltage 100 250 90 LED1 LED2 LED3 LED4 LED5 LED6 200 LEDx Current (mA) Efficiency (%) 80 70 60 50 40 30 150 100 50 20 10 VIN = 24V, RSx = 0.9Ω 6 x 10LEDs, ILED = 100mA 0 8 12 16 20 24 0 0 28 20 Input Voltage (V) 40 60 80 100 ACTL PWM Duty (%) Frequency vs. RSET Resistance LED Current vs. Input Voltage 120 1000 RSx = 2.2Ω 110 Frequency (kHz)1 LEDx Current (mA) 900 100 LED1 LED2 LED3 LED4 LED5 LED6 90 800 700 600 500 400 300 VIN = 24V 200 100 80 10 13 16 19 22 10 25 20 30 40 50 Input Voltage (V) 70 80 90 100 110 RSET (kΩ) Switching Frequency vs. Input Voltage Supply Current vs. Input Voltage 840 10 9 RSET = 10kΩ 760 Supply Current (mA) 1 Switching Frequency (kHz)1 60 680 600 520 RSET = 20kΩ 440 360 280 RSET = 30kΩ 8 7 6 5 4 3 2 1 200 0 4 8 12 16 20 24 Input Voltage (V) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 28 4 8 12 16 20 24 28 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 Shutdown Current vs. Input Voltage Soft-Start Current vs. Input Voltage 10.0 9.5 6 Soft-Start Current (μA) Shutdown Current (μA)1 7 5 4 3 2 1 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 EN = 0 0 5.0 4 8 12 16 20 24 28 4 8 Input Voltage (V) LED Regulated Voltage vs. Input Voltage 24 28 ISENSE Threshold Voltage vs. Input Voltage ISENSE Threshold Voltage (mV) LED Regulation Voltage(mV) 20 150 690 680 670 660 650 140 130 120 110 100 90 80 70 60 50 4 8 12 16 20 24 28 4 8 12 16 20 Input Voltage(V) Input Voltage (V) Load Transient Response Power On from EN VOUT (50V/Div) ACTL (5V/Div) 24 28 EN (5V/Div) GATE (5V/Div) IOUT (100mA/Div) VIN = 24V, PWM = 1kHz, Duty = 50% Time (250μs/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 16 Input Voltage (V) 700 GATE (5V/Div) 12 October 2014 VOUT (50V/Div) LED1 (500mA/Div) VIN = 24V, ILED = 400mA, CSS = 0.1μF Time (100μs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8488 Application Information The RT8488 is a 6-CH programmable current source controller for LED backlight or lighting application. By detecting the minimum voltage required to drive each LED string and hence to set the boost output accordingly, this topology reduces power dissipation and increases overall efficiency of the LED lighting system. The individual current source channel regulates the current flow to give accurate current sinking for each LED string. The external N-MOSFET current source will accommodate the power dissipation difference among channels resulting from the forward voltage difference between the LED strings. Both digital PWM dimming signal and analog voltage signal can be used to control the LED current of each channel. With high speed current source N-MOSFET drivers, the RT8488 features highly accurate current matching of ±3 percent, while also providing very fast turn-on and turn-off times. This allows a very narrow minimum on or off pulse, which increases dimming range and provides higher linearity. Power On sequence The RT8488 integrates adjustable switching frequency and soft-start, and provides the circuitry for over temperature, over voltage and current limit protection features. Input UVLO The input operating voltage range of the RT8488 is 7V to 28V. An input capacitor at the VCC pin can reduce ripple voltage. It is recommended to use a ceramic 10μF or larger capacitance as the input capacitor. This IC provides an Under Voltage Lockout (UVLO) function to enhance the stability when start-up. The UVLO rising input voltage threshold is set at 5.5V typically with a 0.7V hysteresis. Power Sequence Refer to below Figure 1 and 2. The recommended power on sequence states that the PWM signal should be ready before EN and/or VIN is ready. Otherwise, the soft-start function will be disabled. As for power off sequence, EN/ VIN must be pulled low within 10ms to prevent “hardstart”as shown as Figure 3. Power Off sequence Abnormal Power-on sequence UVLO VIN PWM EN must be turned on later than VIN and PWM signal EN must be turned off earlier than VIN and PWM signal EN No Soft-Start Soft-Start If PWM turns on late VOUT Figure 1. Power On Sequence Control by EN Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 Power On sequence Power Off sequence Abnormal Power-on sequence UVLO VIN PWM VIN must be turned off earlier than EN and PWM signal EN VIN must be turned on later than EN and PWM signal No Soft-Start If PWM turns on late Soft-Start VOUT Figure 2. Power On Sequence Control by VIN EN/VIN EN and/or VIN should be pulled low once PWM pull low for over 10 ms PWM 10ms Figure 3. To Prevent “Hard-Start”Sequence Soft-Start The soft-start of the RT8488 can be achieved by connecting a capacitor from the SS pin to GND. The built in soft-start circuit reduces the start up current spike and output voltage overshoot. The soft-start time is determined by the external capacitor charged by an internal 8μA constant charging current. The SS pin directly limits the rate of voltage rise on the VC pin, which in turn limits the peak switch current. The soft-start interval is set by the soft-start capacitor selection according to the equation : tSS = CSS 3.2V 8 A (s) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 October 2014 A typical value for the soft-start capacitor is 0.1μF. The soft-start pin reduces the oscillator frequency and the maximum current in the switch. The soft-start capacitor is discharged when EN/UVLO falls below its threshold, during an over-temperature event, or during a GBIAS under voltage event. GBIAS Regulator Operation The GBIAS pin requires a capacitor for stable operation and also to store the charge for the large GATE switching currents. Choose a 10V rated low ESR, X7R or X5R ceramic capacitor for best performance. The value of the capacitor is determined primarily by the stability of the regulator rather than the gate charge of the switching is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8488 N-MOSFET. A 1μF capacitor will be adequate for most applications. Place the capacitor close to the IC to minimize the trace length to the GBIAS pin and also to the IC ground. An internal current limit on the GBIAS protects the RT8488 from excessive on chip power dissipation. The ACTL pin can also be used in conjunction with a thermistor to provide over temperature protection for the LED load, or with a resistive voltage divider to VIN to reduce output power and switching current when VIN is low. Brightness Control where, RSx is the resistor between external regulating N-MOSFET and GND. For LED applications where a wide dimming range is required, two methods are available: analog dimming and PWM dimming. The easiest method is to simply vary the DC current through the LED by analog dimming at the ACTL pin voltage. The PWM dimming offers wider dimming range over the analog dimming. There are two PWM dimming control methods in RT8488 application : the ACTL PWM dimming and the DCTL PWM dimming. The PWM dimming at the ACTL pin achieves dimming by turning the current source MOSFETs under the LED string fully on when PWM is high and fully off when PWM is low via different duty cycle to control the average LED current. The ACTL PWM dimming is more preferred by LED manufacturers than the ACTL analog dimming. The advantage is the chromaticity of the LEDs which remains unchanged since the LED current is either zero or at the full programmed current. But, this advantage comes with a price. The dimming non-linearity and dimming flicker at certain duty spot depending on the PWM dimming frequency can happen due to the load transient response variation in each PWM dimming cycle. To avoid this potential dimming non-linearity and the dimming flicker issues, analog dimming signal should be applied at the ACTL pin. In this case, the second dimming method by PWM dimming at the DCTL pin should be considered. The RT8488 will convert the PWM dimming signal at the DCTL pin into an analog signal at the ACTL pin. To get analog dimming signal at the ACTL pin, the PWM dimming signal at the ACTL pin can also be converted into analog signal through an external RC filter. The ACTL pin should be tied to a voltage higher than 1.4V to get the full scale 225mV (typical) threshold across the sense resistor. The ACTL pin can also be used to dim the LED current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. When the ACTL pin voltage is less than 1.4V, the LED current is : (V 0.4) 225mV ILED = ACTL (mA) RSx The RT8488 features both the analog and the digital dimming controls. The analog dimming is linearly controlled by an external voltage (0.4V to 1.4V) at the ACTL pin. A very high contrast ratio can be achieved by driving the ACTL pin with a PWM signal at the recommended PWM frequency of 100Hz to 10kHz with acceptable dimming linearity. The dimming frequency can be up to 30kHz with observable dimming non-linearity at If the input voltage, VIN, does not exceed 10V, then the GBIAS pin should be connected to the input supply. Be aware that a typical 20mA current will load the GBIAS to shutdown. Loop Compensation The RT8488 uses an internal error amplifier, in which through its compensation pin (VC) the loop response is optimized for specific applications. The external inductor, output capacitor, compensation resistor, and compensation capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop response and stability. The compensation resistor and capacitor are connected in series from the VC pin to GND to provide a pole and a zero for proper loop compensation. The typical compensation values for RT8488 is 1.8kΩ and 3.3nF. LED Current Setting The maximum current of channel 1 to 6 is programmed by placing an appropriate sense resistor at each LED string. When the voltage of ACTL is higher than 1.4V, the LED current can be calculated by the following equation : ILED, MAX = 225mV RSx (mA) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 Frequency vs. RSET Resistance 1000 900 Frequency (kHz)1 the low dimming duty range. The LED current cannot be 100% proportional to the duty cycle, especially at high dimming frequency and in low duty ratio condition, because of the physical limitation on the internal regulation control loop transient response time. Referring to Figure 4, the minimum dimming duty with good dimming linearity can be as low as 1% for the PWM dimming frequency range between 100Hz and 300Hz. For the PWM dimming frequency between 300Hz and 1kHz, the minimum dimming duty with good dimming linearity is around 5%. If the PWM dimming frequency is increased between 1kHz and 30kHz, the minimum dimming duty with good dimming linearity will be around 10%. 800 700 600 500 400 300 VIN = 12V 200 100 10 20 30 40 50 60 70 80 90 100 110 RSET (kΩ) LED Current vs. PWM Duty Cycle Figure 5. Switching Frequence vs RSET 160 LED Current (mA) 140 LED Pin External Resistor Connection 120 The RT8488 equips 6-CH LED drivers and each channel supports numerous LEDs. The 6 LED strings are connected from VOUT to pin LEDx (x = 1 to 6) respectively. 100 200Hz 1kHz 3kHz 80 60 If one of the LED channel is not used, the LEDx (x = 1 to 6) pins should be connected to ground directly. 40 20 0 0 20 40 60 80 100 Duty Cycle (%) Figure 4. LED Current vs. PWM Dimming Duty Cycle Programmable Switching Frequency The RSET frequency adjust pin allows the user to program the switching frequency from 100kHz to 1MHz in order to optimize efficiency and performance or minimize external component size. Higher frequency operation yields smaller component size but increases switching losses and gate driving current, and may not allow sufficiently high or low duty cycle operation. Lower frequency operation gives better performance, but is more costly with larger external component size. An external resistor from the RSET pin to GND is required do not leave this pin open. For an appropriate RSET value, refer to Figure 5. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 October 2014 In typical application, the current source MOSFET drain node is tied to LEDx pin. The LEDx pin voltage is fed back and regulated around 0.7V by the PWM control loop. Hence, the LEDx pin voltage will not exceed the absolute maximum rating at 20V. If the short circuit between the LED string positive node and the negative node could happen during production, to protect the LEDx pins from damage in high Vout applications (with Vout < 50V), a resistor Rx (around 20kΩ) between current source MOSFET drain node and LEDx node as shown in Figure 6 is recommended to limit the breakdown current into the LEDx pins. For applications with Vout greater than 50V, a bigger Rx will be needed to limit the current into the LEDx pins less than 2mA. Since there is leakage current out of the LEDx pins, note that the adding of the resistor Rx introduces voltage offset to the current source MOSFET drain node regulation voltage by the amount of the leakage current times Rx. is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8488 VOUT L= RT8488 LEDx (VOUT VIN ) VIN 2 IOUT f VOUT x 0.3 The inductor must be selected with a saturation current rating greater than the peak current provided by the following equation : Rx GATEx SENSEx RSx IPEAK = GND Figure 6. LED Pin External Resistor Connection V VOUT IOUT VIN VIN OUT 2 L f VOUT VIN where VOUT = maximum output voltage. Output Over Voltage Protection Setting The RT8488 is equipped with Over Voltage Protection (OVP) function. When the voltage at the OVP pin exceeds a threshold of approximately 1.23V, the power switch is turned off. The power switch can be turned on once again after the voltage at the OVP pin drops below 1.23V. The output voltage can be clamped at a certain voltage level set by the following equation : R1 VOUT,OVP = 1.23 1 R2 where R1 and R2 make up the resistive voltage divider from VOUT to GND with the divider center node connected to the OVP pin. As long as one string is in normal operation, the controller will automatically ignore the open strings and continue to regulate the current for the string(s) in normal operation. Over Temperature Protection The RT8488 has an over temperature protection (OTP) function to prevent overheating caused by excessive power dissipation. The OTP function will shut down switching operation when the die junction temperature exceeds 150°C. The chip will automatically start to switch again once the die junction temperature starts cooling down by approximately 20°C. Inductor Selection The inductor for the RT8488 should have a saturation current rating appropriate to the maximum switch current. Choose an inductor value based on the operating frequency, input voltage and output voltage to provide a current mode ramp during the MOS switching. Allow the peak to peak inductor ripple to be ±30% of the output current.The following equations are useful to estimate the inductor value : Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 VIN = minimum input voltage. f = operating frequency. IOUT = sum of current from all LED strings. η is the efficiency of the power converter. The boost converter operates in discontinuous conduction mode over the entire input voltage range when the L1 inductor value is less than this value L. With an inductance greater than L, the converter operates in continuous conduction mode at the minimum input voltage and may be discontinuous at higher voltages. Input Over Current Protection The resistor, RSENSE, between the source of the external switching N-MOSFET and GND should be selected to provide adequate switch current. The RT8488 senses the inductor current through ISENSE pin in the switch on period. The duty cycle depends on the current sense signal summed with the internal slope compensation and compared to the VC signal. The external N-MOSFET will be turned off when the current signal is larger than the VC signal. In the off period, the inductor current will descend. The external N-MOSFET is turned on by the oscillator in the next beginning cycle.To drive the application without exceeding the current limit threshold on the ISENSE pin of the RT8488, select a resistor according to : RSENSE = ISENSE threshold spec minimum valve IOCP IOCP = (1.33 to 1.5) IPEAK Where IPEAK formula can be found in the inductor selection section above. is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 The ISENSE pin input to RT8488 should be a kelvin connection to the positive terminal of RSENSE. Need to minimize the PCB trace resistance between the ISENSE pin and IC GND to avoid the parasitic resistance. Power MOSFET Selection For applications operating at high input or output voltages, the power N-MOSFET switch is typically chosen for drain voltage VDS rating and low gate charge. Consideration of switch on resistance, R DS(ON), is usually secondary because switching losses dominate power loss. The GBIAS regulator on the RT8488 has a fixed current limit to protect the IC from excessive power dissipation at high VIN, so the N-MOSFET should be chosen such that the product of QG at 7V and the switching frequency does not exceed the GBIAS current limit. Schottky Diode Selection The Schottky diode, with their low forward voltage drop and fast switching speed, is necessary for the RT8488 applications. In addition, power dissipation, reverse voltage rating and pulsating peak current are important parameters of the Schottky diode that must be considered. Choose a suitable Schottky diode whose reverse voltage rating is greater than the maximum output voltage. The diode’s average current rating must exceed the average output current. The diode conducts current only when the power switch is turned off (typically less than 50% duty cycle). If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with sufficiently low leakage current. capacitor, COUT, is approximately given by the following equation : I (VOUT VIN ) COUT OUT VRIPPLE VOUT f where V RIPPLE is rhe output voltage ripple, for LED applications, the equivalent resistance of the LED is typically low and the output filter capacitor should be sized to attenuate the current ripple. Use of X7R type ceramic capacitors is recommended. Lower operating frequencies will require proportionately higher capacitor values. 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 of the RT8488, the maximum junction temperature is 125°C and TA is the ambient temperature. The junction to ambient thermal resistance, θJA, is layout dependent. For WQFN32L 5x5 packages, the thermal resistance, θJA, is 36°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 : Capacitor Selection PD(MAX) = (125°C − 25°C) / (36°C/W) = 2.778W for WQFN-32L 5x5 package The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most applications, a 10μF ceramic capacitor is sufficient. A value higher or lower may be used depending on the noise level from the input supply and the input current to the converter. The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. For the RT8488 package, the derating curve in Figure 7 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. In boost applications, the output capacitor is typically a ceramic capacitor selected based on the output voltage ripple requirements. The minimum value of the output Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8488-05 October 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8488 Layout Consideration Maximum Power Dissipation (W)1 3.0 PCB layout is very important when designing power switching converter circuits. Some recommended layout guidelines are suggested as follows : Four-Layer PCB 2.7 2.4 2.1 The power components L1, D1, CIN, MSW and COUT must be placed as close to each other as possible to reduce the ac current loop area. The PCB trace between power components must be as short and wide as possible due to large current flow through these traces during operation. 1.8 1.5 1.2 0.9 0.6 0.3 0.0 0 25 50 75 100 Place L1 and D1, which are connected to N-MOSFET, as close as possible. The trace should be as short and wide as possible. 125 Ambient Temperature (°C) Figure 7. Derating Curves for RT8488 Package The input capacitor, CVCC must be placed as close to the VCC pin as possible. Place the compensation components to the VC pin as close as possible to avoid noise pick up. L1 D1 VIN VOUT LEDs x N COUT CIN CVCC MSW Place the capacitors close to the input or output. M1 RSENSE ISENSE GND GATE GBIAS VCC LED1 GATE1 SENSE1 RS1 32 31 30 29 28 27 26 25 VOUT RSET EN OVP SS VC ACTL DCTL NC 1 24 2 23 3 22 4 21 GND 5 20 6 19 7 18 33 8 17 LED2 GATE2 SENSE2 LED3 GATE3 SENSE3 LED4 GATE4 M2 RS2 M3 LEDs x N RS3 NC SENSE6 GATE6 LED6 SENSE5 GATE5 LED5 SENSE4 9 10 11 12 13 14 15 16 RS4 RS6 M6 RS5 M5 M4 LEDs x N VOUT Figure 8. PCB Layout Guide Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS8488-05 October 2014 RT8488 Outline Dimension D2 D SEE DETAIL A L 1 E E2 e b 1 2 DETAIL A Pin #1 ID and Tie Bar Mark Options A A1 1 2 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 4.950 5.050 0.195 0.199 D2 3.400 3.750 0.134 0.148 E 4.950 5.050 0.195 0.199 E2 3.400 3.750 0.134 0.148 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 32L QFN 5x5 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. DS8488-05 October 2014 www.richtek.com 17