® RT8561A High Voltage 8-CH LED Driver General Description Features The RT8561A is a 40V 8-CH LED driver capable of delivering 30mA to each channel with 10 LEDs (3.6V per diode), total of 80 LEDs with one driver. The RT8561A is a current mode boost converter operated at 1MHz, wide VIN range covers from 6V to 24V and the on-chip current switch is rated at 2.5A. z z z z z z z z z z z Applications z z z UMPC and Notebook Computer Backlight GPS, Portable DVD Backlight Desk Lights and Room Lighting Pin Configurations Note : Lead Plating System G : Green (Halogen Free and Pb Free) Richtek products are : ` RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. ` GND SW SW GND 21 20 19 18 VCC LED2 2 17 CREG LED3 3 16 LED8 LED4 4 15 LED7 LED5 5 14 LED6 OVP 6 13 EN GND 25 7 8 9 10 11 12 DCTL Package Type QW : WQFN-24L 4x4 (W-Type) 22 ACTL RT8561A 23 1 RISET Ordering Information 24 LED1 SS The RT8561A is packaged with a tiny footprint package of WQFN-24L 4x4 packages. GND (TOP VIEW) GND Other protecting features include programmable output over voltage protection, LED current limit, PWM switch current limit and thermal shutdown. z VC The 1.5% matched LED currents on all channels are simply programmed with a resistor or a current sink. Both analog dimming and digitally controlled PWM dimming are supported by the RT8561A. Analog dimming is linearly controlled by an external voltage. With an on-chip output clamping amplifier and a 300kΩ resistor, PWM dimming signal is easily low-pass filtered to an analog dimming signal with one external capacitor for noise-free PWM dimming. A very high contrast ratio true digital PWM dimming can be achieved by driving ACTL pin with a PWM signal. z NC The PWM output voltage loop regulates the LED pins to 0.6V with an auto-adjustment circuit allowing voltage mismatches between LED strings. The RT8561A automatically detects and disconnects any unconnected and/or broken strings during operation from PWM loop to prevent VOUT from over voltage. z High Voltage : VIN up to 24V, VOUT up to 40V, Driving up to 80 x 3.6V LEDs (10 each channel) Channel Current Programmable 10mA to 30mA and Matched to 1.5% Current Mode PWM 1MHz Boost Converter Easy Analog and Digital Dimming Control Programmable Soft Start Automatic Detecting Unconnected and/or Broken Channel Programmable Over Voltage Protection Disconnects LED in Shutdown Providing High Accuracy Digital Dimming by PWM at ACTL Pin VIN Under Voltage Lockout Over Temperature Protection Current Limiting Protection Small 24-Lead WQFN Package RoHS Compliant and Halogen Free WQFN-24L 4x4 Marking Information CZ= : Product Code CZ=YM DNN YMDNN : Date Code Suitable for use in SnPb or Pb-free soldering processes. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8561A Typical Application Circuit VIN 6V to 24V VOUT 40V MAX L 10µH D COUT 10µF CVIN1 10µF RVIN1 10 : : : : RT8561A 19, 20 SW 18 CVIN1 2.2µF 1k 5V R1 VOUT LED1 1 2 LED2 VCC OVP LED7 ... :: : : : : 10 LED String : : . . . 13 EN 6 : : : : 15 LED8 16 R2 17 4.7µF 0.1µF RVC 1.8k CREG RISET 9 SS 7 VC 12 DCTL GND ACTL RISET 4.75k 10 ILED (mA) = 20 x 4.75 RISET (kΩ) 21, 22, 23, 24, 25 (Exposed Pad) 1k 11 PWM Signal CVC 3.9nF Figure 1. 1MHz, 20mA Full Scale Current PWM Dimming Control VIN 6V to 24V VOUT 40V MAX L 10µH D COUT 10µF CVIN1 10µF RVIN1 10 : : : : RT8561A 19, 20 SW 18 CVIN1 2.2uF 1k 5V R1 VOUT LED1 1 2 LED2 VCC 13 EN 6 OVP LED7 17 CREG RISET 9 SS 0.1µF RVC 1.8k CVC 3.9nF ... :: : : : : 10 LED String : : . . . LED8 16 R2 4.7µF 15 : : : : 7 VC 12 DCTL GND ACTL 10 RISET 4.75k ILED (mA) = 20 x 4.75 RISET (kΩ) 21, 22, 23, 24, 25 (Exposed Pad) 11 1k Analog Dimming DCTL can be floating or connected to ACTL Figure 2. 1MHz, 20mA Full Scale Current Analog Dimming Control Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A VIN 6V to 24V VOUT 40V MAX L 10µH D COUT 10µF CVIN1 10µF RVIN1 10 : : : : RT8561A 19, 20 SW 18 CVIN1 2.2µF 1k 5V LED1 1 2 LED2 VCC 6 OVP LED7 R2 ... :: : : 10 LED String : : : : . . . 13 EN R1 VOUT : : : : 15 LED8 16 17 4.7µF CREG RISET RISET 4.75k 10 ILED (mA) = 20 x 4.75 RISET (kΩ) 9 SS 0.1µF 7 RVC 1.8k 1k 12 VC GND DCTL ACTL 21, 22, 23, 24, 25 (Exposed Pad) 11 0.1µF CVC 3.9nF PWM Signal Figure 3. 1MHz, 20mA Full Scale Current PWM to Analog Dimming Control VIN 4V to 36V VOUT 40V MAX L 10µH D COUT 10µF CVIN1 10µF VAVDD 6V to 24V 19, 20 SW RVIN1 10 18 CVIN1 2.2µF VOUT 1k 5V R1 VCC 6 OVP CREG 9 SS RVC 1.8k LED1 1 2 LED2 LED7 15 : : : : ... :: : : : : 10 LED String : : . . . LED8 16 17 0.1µF RT8561A 13 EN R2 4.7µF : : : : 7 12 VC DCTL CVC 3.9nF RISET GND ACTL 10 RISET 4.75k ILED (mA) = 20 x 4.75 RISET (kΩ) 21, 22, 23, 24, 25 (Exposed Pad) 11 PWM Signal Note : 1. Due to the limitation of maximum duty 5V input can support typically to VOUT = 33V. 2. Due to the limitation of maximum duty 4V input can support typically to VOUT = 26V. Figure 4. Wide Range VIN Application by Connecting VCC Pin to LCD Driver Power AVDD Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8561A Function Block Diagram SW OSC VCC - 5.6V LED1 S + + R OVP 1.2V - + - EN LED2 - 1.6V CREG - R + + + Shutdown - 5V LDO VOUT Regulation Unit VC LED7 + 5V - 5uA SS LED8 + + 1.2V DCTL + - - 300k - ACTL Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 RISET is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A Functional Pin Description Pin No. Pin Name Pin Function 1, 2, 3, 4, 5 LED1, LED2, LED3, LED4, LED5 Channel 1 to Channel 5 LED current sink. Leave the pin unconnected if not used. 6 OVP Over Voltage Protection. PWM boost converter turns off when VOVP goes higher than 1.2V. 7 VC PWM boost converter loop compensation node. 8 NC No Internal Connection. 9 SS Soft Start Pin, a capacitor of at least 10nF is required for soft start. 10 RISET A resistor or a current from DAC on this pin programs the full LED current. 11 ACTL Analog/Digital dimming control. When using analog dimming, 20 x 4.75 ILED (mA) = for VACTL ≥ 1.2V. RISET (kΩ) 12 DCTL By adding a 0.1μF filtering 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. 13 EN Chip enable pin, when pulled low, chip is in shutdown mode. LED6, LED7, LED8 Channel 6 to Channel 8 LED current sink. Leave the pin unconnected if not used. 17 CREG 1μF capacitor should be placed on this pin to stabilize the 5V output of the internal regulator. This regulator is for chip internal use only. 18 VCC Power supply of the chip. For good bypass, a low ESR capacitor is required. 19, 20 SW PWM boost converter switch node. GND Ground pin of the chip. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 14, 15, 16 21, 22, 23, 24, 25 (Exposed Pad) Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8561A Absolute Maximum Ratings z z z z z z z z z z (Note 1) Supply Voltage, VCC ----------------------------------------------------------------------------------------------------SW Pin Voltage at Switching Off -------------------------------------------------------------------------------------LED1 to LED8 Pin --------------------------------------------------------------------------------------------------------DCTL, ACTL, EN, OVP Pin Voltage ---------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C WQFN-24L 4x4 -----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WQFN-24L 4x4, θJA ------------------------------------------------------------------------------------------------------WQFN-24L 4x4, θJC -----------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Mode) ---------------------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------------------ Recommended Operating Conditions z z z −0.3V to 28V −0.3V to 45V −0.3V to 35V −0.3V to 5.5V 1.923W 52°C/W 7°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------- 6V to 24V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VCC = 17V, TA = 25°C, unless otherwise specified) Parameter Symbol Supply Current I VCC VIN Under Voltage Lockout Threshold VUVLO Shutdown Current I SHDN EN Threshold Voltage Test Conditions Min Typ Max Unit VC ≤ 0.2V (Switching off) -- 3 5 mA VIN Rising -- 5.6 5.95 VIN Falling -- 4.9 -- VEN ≤ 0.7V -- -- 10 Logic-High VEN_H 1.6 -- 5 Logic-Low VEN_L -- -- 1 V μA V PWM Threshold Logic-High Voltage Logic-Low VPWM_H 1.3 -- 5 VPWM_L -- -- 0.15 EN Pin Input Current I EN VEN ≤ 5V -- -- 0.1 μA I LED 2V > VLED > 0.6V, RISET = 4.75kΩ, VACTL > 1.2V 19 20 21 mA 2V > VLED > 0.6V, RISET = 4.75kΩ, VACTL > 1.2V, Calculating (I(MAX) − I(MIN)) / I Average x 100% -- -- 1.5 % 1.17 1.2 1.23 V -- 1 2 μA V LED Current Programming LED Current LEDs Current Matching RISET Pin Voltage VRISET 3.6kΩ ≤ RISET ≤ 9.6kΩ, VACTL > 1.2V Input Current of ACTL I ACTL 0.3V ≤ VACTL ≤ 1.2V Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A Parameter Symbol Test Conditions Min Typ Max Unit Threshold of ACTL VACTL LED Current Off -- 0.2 -- V Input Current of DCTL IDCTL 0.3V ≤ VDCTL ≤ 5.5V -- 1 2 μA Un-connection -- 0.1 -- V 0.8 1 1.2 MHz -- 100 -- ns 0.5 0.6 0.7 V VLED Threshold PWM Boost Converter Switching Frequency Minimum On Time Regulated VLED Highest Voltage LED String Amplifier (gm) Output Current 2.4V > VC > 0.2V -- ±15 -- μA VC Threshold PWM Switch Off 0.1 0.2 -- V -- 0.3 0.5 Ω 2.5 -- -- A 80 88 -- % 1.1 1.2 1.3 V SW RDS(ON) SW Current Limit ILIM SW Maximum Duty OVP & Soft-Start OVP Threshold VOVP OVP Input Current IOVP VOVP ≤ 3V -- -- 50 nA Soft-Start Current ISS VSS ≤ 2.5V 3 5 8 μA -- 150 -- °C -- 20 -- °C Thermal Shutdown Temperature Thermal Shutdown Hysteresis TSD 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. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8561A Typical Operating Characteristics LED Current vs. Input Voltage Efficiency vs. Input Voltage 26 100 90 24 80LEDs LED Current (mA) Efficiency (%) 80 70 60 50 40 30 20 22 20 LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 18 16 14 12 10 10 0 8 10 12 14 16 18 20 22 8 24 10 12 16 18 20 22 24 VRISET vs. Temperature LED Current vs. Temperature 24 1.24 23 1.22 22 1.20 21 VRISET (V) LED Current (mA) 14 Input Voltage (V) Input Voltage (V) 20 19 1.18 1.16 1.14 18 1.12 17 VIN = 12V 16 -50 -25 0 25 50 75 100 VIN = 12V 1.10 -50 125 -25 Temperature (°C) 0 25 50 75 100 125 Temperature (°C) VRISET vs. Input Voltage LED Current vs. ACTL PWM Duty Cycle 25 1.25 1.24 20 LED Current (mA) V RISET (V) 1.23 1.22 1.21 1.20 1.19 15 ACTL = 200Hz ACTL = 1kHz ACTL = 10kHz PWM = 30kHz 10 5 1.18 1.17 ACTL = 0V to 3V, VIN = 12V 0 8 10 12 14 16 18 20 22 Input Voltage (V) Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 24 0 0 0.1 10 0.2 1 20 0.3 30 0.4 40 0.5 50 0.6 60 0.7 70 0.8 80 0.9 90 100 Duty Cycle (%) is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A LED Current vs. ACTL Analog Voltage LED Current vs. DCTL PWM Duty Cycle 28 25 24 LED Current (mA) LED Current (mA) 20 DCTL = 200Hz DCTL = 1kHz DCTL = 10kHz 15 10 20 16 12 8 5 4 DCTL = 0V to 3V, VIN = 12V 0 0 10 20 30 40 50 60 70 80 90 VIN = 12V 0 0 100 0.25 Frequency vs. Input Voltage 1 1.25 1.5 Shutdown Current (µA) 10 1060 1020 980 940 8 6 4 2 VEN = 0V 0 900 8 10 12 14 16 18 20 22 8 24 10 12 14 16 18 20 22 24 Input Voltage (V) Input Voltage (V) SS Current vs. Temperature Supply Current vs. Input Voltage 4.0 8.0 3.9 7.6 3.8 7.2 3.7 6.8 SS Current (µA) Switch Off Current (mA) 0.75 Shutdown Current vs. Input Voltage 1100 Frequency (kHz) 0.5 ACTL Analog Voltage (V) Duty Cycle (%) 3.6 3.5 3.4 3.3 6.4 6.0 5.6 5.2 4.8 3.2 3.1 VC = 0V 3.0 8 10 12 14 16 18 20 22 Input Voltage (V) Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 24 4.4 VIN = 12V, CSS = 0.1μF 4.0 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8561A SS Current vs. Input Voltage OVP Voltage vs. Temperature 1.30 9 1.28 1.26 7 OVP Voltage (V) SS Current (µA) 8 6 5 4 1.24 1.22 1.20 1.18 1.16 1.14 3 1.12 CSS = 0.1μF 2 8 10 12 14 16 18 20 22 VIN = 12V 1.10 -50 24 -25 0 50 75 100 125 Temperature (°C) Input Voltage (V) ACTL Off Threshold Voltage vs. Temperature OVP Voltage vs. Input Voltage 0.22 ACTL Off Threshold Voltage (V) 1.3 1.2 OVP Voltage (V) 25 1.1 1.0 0.9 0.8 0.21 0.20 0.19 0.18 0.17 0.16 0.15 VIN = 12V 0.14 8 10 12 14 16 18 20 22 24 -50 -25 0 25 50 75 100 Input Voltage (V) Temperature (°C) Power On from EN Line Transient Response 125 VIN = 10.8V to 13.2V VEN (2V/Div) VIN (5V/Div) VOUT (20V/Div) I IN (500mADiv) VIN = 12V, CSS = 0.1μF Time (10ms/Div) Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 IOUT (100mA/Div) Time (50ms/Div) is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A OVP R1 = 3.3MΩ, R2 = 100kΩ VIN (10V/Div) VOUT (20V/Div) SW (20V/Div) VIN = 12V, All LED Pin Open Time (2.5ms/Div) Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8561A Applications Information The RT8561A is a current mode boost converter operated at 1MHz to power up to 80 white LEDs with a programmable current for uniform intensity. The part integrates current sources, soft-start, and easy analog and digital dimming control. The protection block provides the circuitry for over temperature, over-voltage and current limit protection features. Abnormal Power on sequence Power off sequence Power on sequence VIN UVLO PWM EN VIN must be turned off early than EN and PWM signal VIN must be turned on late than EN and PWM signal Input UVLO The input operating voltage range of the RT8561A is 6V to 24V. An input capacitor at the VIN 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 startup. The UVLO threshold of input voltage rising is set at 5.6V and falling is set at 4.9V typically. No Soft-Start If PWM turns on late Soft-Start VOUT Figure 6. Power On Sequence Control by VIN EN and/or VIN should be pulled low once PWM pull low for over 10 ms EN/VIN Power Sequence Please refer to the below Figure 5 and Figure 6. The recommended power on sequence is that the PWM ready before EN and/or VIN ready. If not, the soft-start function will be disabled. As to power off sequence, the EN/VIN must be pulled low within 10ms to prevent “Hard-Start” shown as Figure 7. Power on sequence Power off sequence Abnormal Power on sequence VIN UVLO 10ms Figure 7. To Prevent “Hard-Start” Sequence Soft-Start The RT8561A employs a soft-start feature to limit the inrush current. The soft-start circuit prevents the excessive inrush current and input voltage droop. The soft-start time is determined by capacitor CSS connected to SS with 5μA constant current to charge CSS. The value of capacitor CSS is user-defined to satisfy the designer' requirement. PWM EN PWM EN must be turned on late than VIN and PWM signal Soft-Start VOUT EN must be turned off early than VIN and PWM signal LED Connection No Soft-Start If PWM turns on late Figure 5. Power On Sequence Control by EN The RT8561A equips 8-CH LED drivers and each channel supports up to 10 LEDs. The 8 LED strings are connected from VOUT to pin 1, 2, 3, 4, 5, 14, 15, and 16 respectively. If one of the LED channel is not used, the LED pin should be opened directly. Setting and Regulation of LED Current The LED current can be calculated by the following equation : ILED (mA) = 20 x 4.75 RISET (kΩ) Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A Where, the RISET is the resister between RISET pin and GND. This setting is the reference for the LED current at LED1-8 and represents the sensed LED current for each string. The DC/DC converter regulates the LED current according to the setting. Brightness Control The RT8561A features both analog and digital dimming control. Analog dimming is linearly controlled by an external voltage (0.3V < VACTL < 1.2V). With an on-chip output clamping amplifier and a 300kΩ resistor, PWM dimming signal is easily low-pass filtered to an analog dimming signal with one external capacitor for noise-free PWM dimming. A very high contrast ratio true digital PWM dimming can be achieved by driving ACTL pin with a PWM signal and the suggested PWM frequency is from 100Hz to 10kHz. Dimming frequency can be sufficiently adjusted from 100Hz to 30kHz.However, LED current cannot be 100% proportional to duty cycle especially for high frequency and low duty ratio because of physic limitation caused by inductor rising time. Refer to Figure 8 and Table 1. LED Current vs. ACTL PWM Duty Cycle 25 Table 1. Dimming Frequency (Hz) 100 < fPWM ≤ 200 200 < fPWM ≤ 500 500 < fPWM ≤ 1k 1k < fPWM ≤ 2k 2k < fPWM ≤ 5k 5k < fPWM ≤ 10k 10k < f PWM ≤ 20k Duty (Min.) 0.16% 0.40% 0.80% 1.60% 4.00% 8.00% 16.00% Duty (Max.) 100% 100% 100% 100% 100% 100% 100% Note : The minimum duty in Table 1 is based on the application circuit and does not consider the deviation of current linearity. Over Voltage Protection The RT8561A equips over voltage protection (OVP) function. When the voltage at the OVP pin reaches a threshold of approximately1.2V, the MOSFET drive output (SW) will turn “OFF”. The MOSFET drive output (SW) will turn “ON” again once the voltage at OVP drops below the threshold of approximately 1.2V. So, the output voltage can be clamped at a certain voltage level and it can be calculated by the following equation : VOUT, OVP = VOVP × ⎛⎜ 1+ R1 ⎞⎟ ⎝ R2 ⎠ Where R1 and R2 are the voltage divider connected to OVP pin. VOVP is typically 1.2V. LED Current (mA) 20 If at least 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. 15 ACTL = 200Hz ACTL = 1kHz ACTL = 10kHz ACTL = 30kHz 10 5 ACTL = 0V to 3V, VIN = 12V 0 0 0 0.1 10 0.2 1 20 0.3 30 0.4 40 0.5 50 0.6 60 0.7 70 0.8 80 0.9 90 100 Duty Cycle (%) Figure 8. LED Current vs. ACTL PWM Dimming Duty Cycle Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 Current Limit Protection The RT8561A can limit the peak current to achieve over current protection. The RT8561A senses the inductor current through SW pin in the switch-on period. The duty cycle depends on the current sense signal summing with the internal slope compensation compared to the VC signal. The internal 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 internal MOSFET is turned on by the oscillator in the next beginning cycle. is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8561A Over Temperature Protection Capacitor Selection The RT8561A has over temperature protection (OTP) function to prevent the excessive power dissipation from overheating. The OTP will shut down switching operation if the junction temperature exceeds 150°C. Main converter starts switching after junction temperature cools down approximately 20°C. The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most applications, a 10uF 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. Inductor Selection The value of the output inductor (L), where the transition from discontinuous to continuous mode occurs is approximated by the following equation : L= (VOUT − VIN ) × VIN2 It is recommended to choose a ceramic capacitor bases on the output voltage ripple requirements. The minimum value of the output capacitor COUT is approximately given by the following equation : COUT = 2 × IOUT × f × VOUT 2 (VOUT − VIN ) × IOUT η × VRIPPLE × VOUT × f Where, Layout Guideline VOUT = maximum output voltage. PCB layout is very important for designing power switching converter circuits. The following layout guide lines should be strictly followed for best performance of the RT8561A. VIN = minimum input voltage. f = operating frequency. IOUT = sum of current from all LED strings. ` The power components L, D, CVIN1, COUT1 and COUT2 must be placed as close as possible to reduce the ac current loop. The PCB trace between power components must be short and wide as possible due to large current flow through these trace during operation. ` Place L and D connected to SW pin as close as possible. The trace should be short and wide as possible. ` It is recommend to place CVIN2 close to VIN pin. ` Pin7 is the compensation point to adjust system stability. Place the compensation components to pin7 as close as possible, no matter the compensation is RC or capacitance. η is the efficiency of the power converter. The boost converter operates in discontinuous 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 mode at the minimum input voltage and may be discontinuous at higher voltages. The inductor must be selected with a saturated current rating that is greater than the peak current provided by the following equation : IPEAK = VOUT × IOUT + VIN × T ⎛⎜ VOUT − VIN ⎞⎟ η × VIN 2 × L ⎝ VOUT ⎠ Diode Selection Schottky diode is a good choice for an asynchronous Boost converter due to the small forward voltage. However, for power dissipation, reverse voltage rating and pulsating peak current are the important parameters of Schottky diode selection. Choose a suitable diode whose reverse voltage rating is greater than the maximum output voltage. Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8561A-02 April 2012 RT8561A Place these components C OUT1 as close as possible. D VOUT COUT2 GND CVIN1 VIN GND GND GND GND SW SW L 24 23 22 21 20 19 GND CVIN2 LED1 1 18 VCC LED2 2 17 CREG LED3 3 LED4 4 LED5 5 OVP 6 GND 9 10 11 12 SS RISET ACTL DCTL VC CVC2 8 NC 25 7 RVC CVC1 RVIN 16 Locate the C VIN2 as close to VCC pin as LED8 possible. 15 LED7 14 LED6 13 EN Locate the compensation components to VC pin as close as possible. GND Figure 9. PCB Layout Guide Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8561A-02 April 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8561A Outline Dimension D2 D SEE DETAIL A L 1 E E2 e b 1 2 2 DETAIL A Pin #1 ID and Tie Bar Mark Options A A3 A1 1 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 3.950 4.050 0.156 0.159 D2 2.300 2.750 0.091 0.108 E 3.950 4.050 0.156 0.159 E2 2.300 2.750 0.091 0.108 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 24L QFN 4x4 Package Richtek Technology Corporation 5F, No. 20, Taiyuen 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 DS8561A-02 April 2012