® RT8532 6-String 43V White LED Driver with Boost Regulator General Description Features The RT8532 is a high efficiency white LED driver. It is designed for LCD panel that employs a LED array as the lighting source. An integrated switch current mode Boost controller drives six strings in parallel and supports up to 10 pieces of LEDs per string. The internal current sinks support maximum ±2% current matching for excellent brightness uniformity in each LED string. To provide enough headroom for the operating of current sink, the Boost controller monitors the minimum voltage of feedback pins and regulates an optimized output voltage for power efficiency. Wide Input Voltage : VIN 2.5V to 24V High Output Voltage : VOUT up to 43V Programmable Full Channel Current from 5mA to 50mA and Matched to 2% Channel Current Regulation with ±3% Accuracy Dimming Controls Direct PWM Dimming up to 20kHz and Minimum On-Time to 500ns PWM to Mixed Analog and PWM Dimming up to 20kHz with Maximum 9 bit Resolution Built-In Soft Start to Prevent Inrush Current without External Capacitor Disconnects LED in Shutdown Protection Strings Open Detection Current Limit Protection Programmable Over Voltage Protection Over Temperature Protection 20-Lead WQFN Package RoHS Compliant and Halogen Free The RT8532 has wide input voltage range from 2.5V to 24V and provides adjustable 5mA to 50mA LED current. The internal 200mΩ, 43V power switch with current-mode control provides cycle-by-cycle over current protection. The RT8532 also integrates PWM and analog dimming functions for accurate LED current control. The input PWM dimming frequency can be operated operate from 100Hz to 20kHz without any inrush current in LED. The RT8532 is available in WQFN-20L 3x3 package. Applications UMPC and Notebook Computer Backlight Simplified Application Circuit D1 L1 VIN VOUT COUT CIN Chip Enable VIN EN LX RT8532 ROVP2 OVP ROVP1 PWM MIX MIX PWM Signal COMP CC RCOMP CCOMP RISET RFSW : : : : : : : : : : : : : : : LED1 LED2 LED3 LED4 LED5 LED6 ISET FREQ AGND PGND Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 COVP : : : is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8532 Ordering Information Pin Configurations RT8532 (TOP VIEW) Lead Plating System G : Green (Halogen Free and Pb Free) Note : Richtek products are : RoHS compliant and compatible with the current require- COMP VIN VDC PWM LX Package Type QW : WQFN-20L 3x3 (W-Type) 20 19 18 17 16 EN FREQ ISET MIX AGND 15 2 14 GND 3 4 13 21 5 12 11 6 7 8 LX PGND PGND OVP LED1 9 10 LED6 LED5 LED4 LED3 LED2 ments of IPC/JEDEC J-STD-020. 1 Suitable for use in SnPb or Pb-free soldering processes. WQFN-20L 3x3 Marking Information 89= : Product Code 89=YM DNN YMDNN : Date Code Functional Pin Description Pin No. Pin Name 1 EN 2 FREQ Pin Function Chip Enable (Active High). There is an internal pull low resistor 400k for the EN pin. Switching Frequency of Boost Converter Setting. Connect a resistor between this pin and AGND to set the switching frequency. 25 RFSW 1.1 6 k F (MHz) SW LED Current Setting. LED current is set by the resistor RISET connected from the ISET pin to ground. 3 ISET 4 MIX Dimming Mode Selection. There is an internal pull high 400k resistor connected to VDC. Floating or pulling high for 25% Mixed mode dimming, pulling low for direct PWM mode dimming. Recommended to mixed mode for high LED efficiency. 5 AGND Analog Ground of LED Driver. LED6 to LED1 Current Sink for LED String. (Leave the pin unconnected or short to GND, if not used.) OVP Over Voltage Protection Sense Input. The detecting threshold is 1.2V (typ.). 13, 14 PGND Power Ground of Boost Converter. 15, 16 LX Switching Pin of Boost Converter. 17 PWM PWM Dimming Control Input. 18 VDC Internal Regulator Voltage. Connect a capacitor from this pin to ground. 19 VIN Power Supply Input. 20 COMP Compensation Note for Boost Converter. Connect a compensation network to this pin for stability. 6, 7, 8, 9, 10, 11 12 21 GND (Exposed Pad) ILED mA 240 RISET k Ground. The Exposed Pad must be Soldered to a Large PCB and Connected to GND for Maximum Power Dissipation. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8532-04 January 2014 RT8532 Function Block Diagram VIN VDC FREQ OVP + 1.2V - OSC EN LDO S Q R Q LX OCP OTP + + PWM Controller PGND + COMP PWMO Generator PWM-to-DC 6 Mini LED Selection LED Open Detection MUX PWM - GND 0.6V MIX DAC …… LED1 LED2 LED6 + + + - - - …… + - ISET AGND Operation Enable Control When VIN is higher than the UVLO voltage and EN pin input voltage is higher than rising threshold, the VDC will be regulated around 3.8V if VIN is higher than 3.8V. OSC The switching frequency is adjustable by the external resistor connected between the FREQ pin and GND. MOSFET will be turned off until the temperature is lower than the 120°C (typically). OVP When OVP pin voltage is higher than 1.2V, the LX N-MOSFET is turned off immediately to protect the LX N-MOSFET. Minimum LED Selection PWM Controller This controller includes some logic circuit to control LX N-MOSFET on/off. This block controls the minimum on time and max duty of LX. This block detects all LEDx voltage and select a minimum voltage to EA (Error Amplifier). This function can guarantee the lowest of LED pin voltage is around 600mV and VOUT can be boost to the highest forward voltage of LED strings. OCP & OTP LED Open Detection When LX N-MOSFET peak current is higher than 2.5A (typically), the LX N-MOSFET is turned off immediately and resumed again at next clock pulse. When the junction temperature is higher than 150°C (typically), the LX N- If the voltage at LEDx pin is lower than 100mV, this channel is defined as open channel and the Minimum LED Selection function will discard it to regulate other used channels in proper voltage. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8532 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VIN to GND -----------------------------------------------------------------------------------EN, PWM, ISET, COMP, MIX, FREQ to GND ---------------------------------------------------------------------LX, OVP, LED1 to LED6 to GND --------------------------------------------------------------------------------------LX to GND ------------------------------------------------------------------------------------------------------------------< 500ns ---------------------------------------------------------------------------------------------------------------------VDC to GND ---------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C −0.3V to 26.5V −0.3V to 26.5V −0.3V to 48V −0.3V to 48V −1V to 48V −0.3V to 7V WQFN−20L 3x3 -----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WQFN−20L 3x3, θJA -----------------------------------------------------------------------------------------------------WQFN−20L 3x3, θJC -----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------- 1.471W Recommended Operating Conditions 68°C/W 7.5°C/W 260°C 150°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VIN ----------------------------------------------------------------------------------------------- 2.5V to 24V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VIN = 5V, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max IVIN PWM = COMP = 0V, Not Switching -- 3 -- IVIN_SW PWM = COMP = 2V, Switching -- 4 -- VIN Shutdown Current ISHDN VIN = 4.5V, EN = 0V -- -- 10 VIN Under Voltage Lockout Threshold VUVLO VIN Rising -- 2.3 -- VIN Falling -- 2.1 -- PWM Dimming Frequency fPWM 0.1 -- 20 VIN Quiescent Current Unit mA A V kHz Control Input EN, PWM Input High Voltage Low VIH VIN = 2.5V to 24V 1.3 -- 24 VIL VIN = 2.5V to 24V -- -- 0.5 RFSW = 22k 0.8 1 1.2 RFSW = 51k 0.4 0.5 0.6 VIN > 4.5V 0.18 0.2 0.22 V Boost Converter Switching Frequency LX On Resistance (N-MOSFET) fSW RLX Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 MHz is a registered trademark of Richtek Technology Corporation. DS8532-04 January 2014 RT8532 Parameter Symbol Test Conditions Min Typ Max Unit Minimum On-Time tON(MIN) F SW = 1MHz 40 60 80 ns Maximum Duty DMAX F SW = 1MHz 90 93 96 % LX Current Limit ILIM 2.2 2.5 2.8 A Regulated VLEDx VLEDx 0.5 0.6 0.7 V 3.6 3.8 4 V 19.4 20 20.6 mA -- -- 2 % Highest LED String, I LED = 20mA Low Dropout Linear Regulator LDO Output Voltage Range VDC LED Current Programming LED Current Accuracy ILEDA 2V > VLEDx > 0.5V, RISET = 12k LED Current Matching ILEDM 2V > VLEDx > 0.5V, RISET = 12k, Formulated by (ILEDx IAVG ) / IAVG x 100% ISET Pin Voltage VISET 0.76 1 1.24 V OVP Threshold VOVP 1.16 1.2 1.24 V OVP UVLO Threshold VOVPF -- 50 -- mV Thermal Shutdown Temperature TSD -- 150 -- C OTP Hysteresis TOTP_Hys -- 30 -- C LED Pin Under Voltage Threshold VLSD -- 0.1 -- V Fault Protection Un-Connection 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 © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8532 Typical Application Circuit VIN 5V to 24V L1 10µH CIN 4.7µF D1 SS16 R1 10 19 VIN C1 100nF Chip Enable 1 17 PWM Signal 15, 16 LX ROVP2 500k ROVP1 15k EN MIX PWM VDC LED1 LED2 LED3 LED4 LED5 LED6 RCOMP 5.1k CCOMP 22nF 3 ISET 2 FREQ RFSW 51k OVP 12 RT8532 20 COMP RISET 12k VOUT 43V MAX COUT 4.7µF 4 18 COVP 47pF : : : : : : : : : : : : : : : : : : CDC 0.1µF 11 10 9 8 7 6 AGND PGND GND 5 13, 14 21 (Exposed Pad) Figure 1. For General Application Circuit L1 10µH VIN 2.5V to 24V R1 10 5V 19 VIN C1 100nF 1 17 15, 16 LX RT8532 PWM RCOMP 5.1k CCOMP 22nF RISET 12k 3 ISET 2 FREQ RFSW 51k OVP 12 MIX VDC LED1 LED2 LED3 LED4 LED5 LED6 4 18 on DMAX) ROVP2 500k ROVP1 24k EN 20 COMP PWM Signal VOUT 25V MAX (VOUT depends COUT 4.7µF CIN 4.7µF Chip Enable D1 SS16 COVP 47pF : : : : : : : : : : : : : : : : : : CDC 0.1µF 11 10 9 8 7 6 AGND PGND GND 5 13, 14 21 (Exposed Pad) Figure 2. For Low Input Voltage Application Circuit Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8532-04 January 2014 RT8532 Typical Operating Characteristics LED Current vs. Input Voltage Efficiency vs. Input Voltage 26 100 LED1 LED2 LED3 LED4 LED5 LED6 90 24 Output Current (mA) 80 Efficiency (%) 70 60 50 40 30 20 22 20 18 16 10 9 LEDs per channel, fSW = 500kHz, PWM = 3.3V 54 LEDs, fSW = 500kHz, PWM = 3.3V 14 0 4 8 12 16 20 4 24 8 12 Input Voltage (V) 20 24 VDC vs. Temperature LED Current vs. Temperature 26 5.0 24 4.5 22 4.0 VDC (V) LED Current (mA) 16 Input Voltage (V) 20 3.5 18 3.0 16 2.5 9 LEDs per channel, fSW = 500kHz, PWM = 3.3V 9 LEDs per channel, fSW = 500kHz 2.0 14 -50 -25 0 25 50 75 100 -50 125 -25 0 Temperature (°C) 50 75 100 125 LED Current vs. PWM Duty Cycle fSW vs. RFSW 2000 25 Temperature (°C) 20 9 LEDs per channel 1800 LED Current (mA) 1600 f SW (kHz) 1400 1200 1000 800 600 16 PWM 100Hz PWM 1kHz PWM 10kHz PWM 20kHz 12 8 4 400 9 LEDs per channel, fSW = 500kHz 200 0 0 20 40 60 80 100 120 RFSW (kohm) (kΩ) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 140 0 20 40 60 80 100 PWM Duty Cycle (%) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8532 Quiescent Current vs. Input Voltage OVP Threshold Voltage vs. Input Voltage 2.5 Quiescent Current (mA) OVP Threshold Voltage (V) 1.4 1.3 1.2 1.1 2.3 2.0 1.8 9 LEDs per channel, fSW = 500kHz Not Switching 1.5 1.0 4 8 12 16 20 3 24 5 8 10 13 15 18 20 Input Voltage (V) Input Voltage (V) Line Transient Response Power On in PWM Mode 23 25 VIN (10V/Div) VEN (5V/Div) VIN (10V/Div) VPWM (5V/Div) I LED (20mA/Div) VIN = 12V to 18V, fSW = 500kHz, PWM = 3.3V I LED (20mA/Div) Time (2.5ms/Div) Time (10ms/Div) Power On from EN Power On in Mixed Mode VOUT (20V/Div) VIN (20V/Div) VEN (5V/Div) VPWM (5V/Div) VLX (20V/Div) VEN (5V/Div) I LED (20mA/Div) I LED (20mA/Div) VIN = 12V, fSW = 500kHz, PWM = 3.3V Time (10ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 VIN = 12V, fSW = 500kHz VIN = 12V, fSW = 500kHz Time (10ms/Div) is a registered trademark of Richtek Technology Corporation. DS8532-04 January 2014 RT8532 Application Information The RT8532 is a general purpose 6-CH LED driver capable of delivering an adjustable 5mA to 50mA LED current. The IC is a current mode boost converter integrated with a 43V/2.5A power switch and can cover a wide VIN range from 2.5V to 24V. The part integrates both built-in softstart and with PWM dimming control; moreover, it provides over voltage, over temperature and current limiting protection features. It also integrates PWM and mixed mode dimming function for accurate LED current control. The PWM dimming frequency can operate from 100Hz to 20kHz without inducing any inrush current in LED or inductor. where fSW is the switching frequency and ΔIL is the inductor ripple current. Move COUT to the left side to estimate the value of ΔVOUT1 according to the following equation : VOUT1 D IOUT COUT fSW Finally, taking ESR into account, the overall output ripple voltage can be determined by the following equation : D IOUT VOUT IIN ESR COUT fSW ΔIL Input Capacitor Selection Input Current Low ESR ceramic capacitors are recommended for input capacitor applications. Low ESR will effectively reduce the input ripple voltage caused by the switching operation. Two 2.2μF low ESR ceramic capacitors are sufficient for most applications. Nevertheless, this value can be decreased for applications with lower output current requirement. Another consideration is the voltage rating of the input capacitor, which must be greater than the maximum input voltage. Output Capacitor Selection Output ripple voltage is an important index for estimating chip performance. This portion consists of two parts. One is the product of the inductor current ripple with the ESR of the output capacitor, while the other part is formed by the charging and discharging process of the output capacitor. As shown in Figure 3, ΔVOUT1 can be evaluated based on the ideal energy equalization. According to the definition of Q, the Q value can be calculated as the following equation : 1 1 1 Q IIN IL IOUT IIN IL IOUT 2 2 2 VIN 1 COUT VOUT1 VOUT fSW Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 Inductor Current Output Current Time (1-D)TS Output Ripple Voltage (ac) Time ΔVOUT1 Figure 3. The Output Ripple Voltage without the Contribution of ESR Inductor Selection The inductor value depends on the maximum input current. As a general rule the inductor ripple current is 20% to 40% of maximum input current. If 40% is selected as an example, the inductor ripple current can be calculated according to the following equation : IIN(MAX) VOUT IOUT(MAX) VIN IRIPPLE 0.4 IIN(MAX) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8532 where η is the efficiency of the boost converter, IIN(MAX) is the maximum input current and IRIPPLE is the inductor ripple current. The input peak current can be obtained by adding the maximum input current with half of the inductor ripple current as shown in the following equation : Setting and Regulation of LED Current IPEAK = 1.2 x IIN(MAX) where RISET is the resistor between the ISET pin and GND. This setting is the reference for the LED current at LED1 to LED6 and represents the sensed LED current for each string. The DC/DC converter regulates the LED current according to the setting. Note that the saturated current of inductor must be greater than IPEAK. The inductance can eventually be determined according to the following equation : VIN D VOUT VIN 2 L1 0.4 VOUT IOUT fSW 2 where VOUT is the maximum output voltage, VIN is the minimum input voltage, fSW is the switching frequency, and IOUT is the sum of current from all LED strings. LED Soft-Start Function The soft-start time of the LED boost converter, defined as the period from EN to set I OUT, is several tens of milliseconds according to the difference of PWM or Mixed mode. The LED starts up after VIN, PWM and EN signals are all ready. The soft-start inrush peak current must be less than 2.5A. LED Driver Compensation The control loop can be compensated by adjusting the external components connected to the COMP pin. The COMP pin is the output of the internal error amplifier. The compensation capacitors, CC and CCOMP, will adjust the integrator zero and pole respectively to maintain stability. Moreover, the resistor, RCOMP, will adjust the mid-band gain for fast transient response. Diode Selection The LED current can be calculated by the following equation : 240 ILED mA RISET k PWM Mode and Mixed Mode Brightness Dimming The RT8532 allows two ways of controlling the LED brightness. PWM Mode Dimming : When the MIX pin is connected to GND, the dimming mode operates in PWM Mode. During the PWM dimming, the current source turn-on/off is synchronized with the PWM signal. The LED current frequency is equivalent to PWM input frequency. Mixed Mode Dimming : If the MIX pin is floating or tied to VDC, the dimming mode operates in Mixed Mode. In this mode the PWM and ILED dimming cycle will delay by 2 periods. First cycle delay is required for the period, while the second cycle delay is for the duty rate calculation. (a) When 25% ≤ PWM duty ≤ 100%, the current source outputs are DC dimming, and the PWM duty cycle modulates the amplitude of the currents. (b) When PWM Duty < 25%, the DC dimming will translate to DC-PWM dimming to control the LED current. In this state, the LED current is fixed at 0.25 x ISET, and the dimming duty is 4 x PWM duties. The minimum D/A Converter is 512 steps resolution for ILED regulation. Schottky diodes D1 are recommended for most applications because of their fast recovery time and low forward voltage. Power dissipation, reverse voltage rating, and pulsating peak current are important parameters for consideration when making a Schottky diode selection. Make sure that the diode's peak current rating exceeds IPEAK and reverse voltage rating exceeds the maximum output voltage. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8532-04 January 2014 RT8532 Brightness Control Over Temperature Protection The RT8532 features the digital dimming control scheme. A very high contrast ratio true digital PWM dimming can be achieved by driving PWM pin with a PWM signal and the recommended PWM frequency is 100Hz to 10kHz. Dimming frequency can be sufficiently adjusted from 100Hz to 20kHz. However, LED current cannot be 100% proportional to duty cycle especially for high frequency and low duty ratio because of physical limitation caused by inductor rising time. Please refer to Table 1 and Table2. The RT8532 includes an Over Temperature Protection (OTP) feature to prevent overheating due to excessive power dissipation from damaging the device. The OTP function will shut down LED driver when the junction temperature exceeds 150°C. It will reactivate the device when powered on again. To maintain continuous operation, the junction temperature should be kept below 125°C. Table 1. Mixed Dimming Mode Dimming Frequency (Hz) Duty (Min) Duty (Max) 100 < fPWM 200 0.18% 100% 200 < fPWM 500 0.18% 100% 500 < fPWM 1k 0.2% 100% 1k < fPWM 2k 0.2% 100% 2k < fPWM 5k 0.3% 100% 5k < fPWM 10k 0.3% 100% 10k < fPWM 20k 0.6% 100% LED Driver Over Voltage Protection The LED driver equips an Over Voltage Protection (OVP) function. When the voltage at the OVP pin reaches a threshold of approximately 1.2V, the driver will turn off. The drivers turn on again once the voltage at OVP drops below the threshold voltage. Thus, the output voltage can be clamped at a certain voltage level. This voltage level can be calculated by the following equation : R VOUT, OVP VOVP 1 OVP2 R OVP1 where ROVP1 and ROVP2 are the resistors in the voltage divider connected to the OVP pin. It is suggested to use 500kΩ for ROVP2 to reduce loading effect. Table 2. PWM Dimming Mode Dimming Frequency (Hz) Duty (Min) Duty (Max) LED Channel Open Circuit Protection 100 < fPWM 200 0.02% 100% 200 < fPWM 500 0.02% 100% 500 < fPWM 1k 0.04% 100% 1k < fPWM 2k 0.06% 100% If at least one channel is in normal operation, the LED driver will automatically ignore the open channels and continue to regulate current for the channels in normal operation. 2k < fPWM 5k 0.15% 100% 5k < fPWM 10k 0.3% 100% 10k < fPWM 20k 0.6% 100% Note : The minimum duty in Table 1 and Table 2 is based on the application circuit and does not consider the deviation of current linearity when fPWM > 10kHz, ILED may not achieve setting current in duty (min.) due to different VOUT / VIN ratio at VIN = 12V. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 Under Voltage Lockout (UVLO) The UVLO circuit compares the LED driver input voltage at VIN with the UVLO threshold to ensure the input voltage is high enough for reliable operation. The 200mV (typ.) hysteresis prevents supply transients from causing a shutdown. Once VIN exceeds the UVLO rising threshold, the LED soft-start will begin after a several ms delay. When VIN falls below the UVLO falling threshold, the controller turns off all LED driver functions. is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8532 Thermal Considerations Layout 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 : PCB layout is very important to design power switching converter circuits. The following layout guide lines should be strictly followed for best performance of the RT8532. The power components L1, D1, CIN and COUT 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 L1 and D1 as close to LX pins as possible. The trace should be short and wide as possible. Place the input capacitor C1 close to VIN pin. Pin 20 is the compensation point to adjust system stability. Place the compensation components to pin 20 as close as possible. 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-20L 3x3 packages, the thermal resistance, θJA, is 68°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) / (68°C/W) = 1.471W for WQFN-20L 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 4 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Maximum Power Dissipation (W)1 1.6 Four-Layer PCB 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 4. Derating Curve of Maximum Power Dissipation Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8532-04 January 2014 RT8532 Place the power components as close as possible. The traces should be wide and short especially for the high-current loop. VIN Locate the C1 as close to VIN as possible. The compensation circuit should be kept away from the power loops and should be shielded with a ground trace to prevent any noise coupling. GND CIN GND CCOMP D1 R1 RCOMP Locate the RISET as close 1 to ISET as possible. VOUT 20 19 18 17 16 COMP VIN VDC PWM LX C1 EN LX 15 PGND 14 2 FREQ GND COUT L1 GND RISET 3 ISET 4 MIX GND PGND 13 OVP 12 LED1 11 LED6 LED5 LED4 LED3 LED2 5 AGND 6 7 8 9 10 Figure 5. PCB Layout Guide Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8532-04 January 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8532 Outline Dimension 1 1 2 2 DETAIL A Pin #1 ID and Tie Bar Mark Options 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 0.150 0.250 0.006 0.010 D 2.900 3.100 0.114 0.122 D2 1.650 1.750 0.065 0.069 E 2.900 3.100 0.114 0.122 E2 1.650 1.750 0.065 0.069 e L 0.400 0.350 0.016 0.450 0.014 0.018 W-Type 20L 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 14 DS8532-04 January 2014