® RT8537 Current Mode Boost-Type LED Driver with Programmable Dimming Control General Description Features The RT8537 is a current mode Boost-type LED driver with programmable brightness dimming control for portable devices. With the 40V integrated MOSFET, the RT8537 can support up to 10 LEDs in series and wide input voltage range from 2.9V to 18V. The Boost converter runs at 1.2MHz switching frequency which allows for the use of small external components. z The LED current is adjustable by an external resister at FB pin and the feedback voltage is regulated to 200mV typically. The RT8537 provides PWM dimming mode and 1-wire digital dimming mode for accurate LED current control from EN pin. In PWM dimming mode, the feedback reference voltage is changed with the PWM duty cycle proportionally and the available PWM frequency range is from 5kHz to 50kHz. In 1-wire digital dimming mode, it provides a programmable 32-step brightness dimming function with the EN pin setting. The RT8537 provides protection functions including LED open protection, input under voltage lockout, current limit and over temperature protection. z z z z z z z z z z z 32-Step Programmable Digital Dimming PWM Brightness Dimming 2.9V to 18V Input Voltage Range 38V Open LED Protection for 10 LEDs 200mV Reference with 4% Accuracy 1.2MHz Switching Frequency Built-In 1.2A Power Switch Built-in Internal Soft-Start Over Temperature Protection Current Limit Protection Tiny Package with WDFN Package RoHS Compliant and Halogen Free Applications z z z Mobile Phone and Smart Phone Digital Camera and GPS Portable DVD Player Marking Information 0J : Product Code W : Date Code 0JW The RT8537 is available in the WDFN-6SL 2x2 package. Simplified Application Circuit L VIN COUT CIN …… LX 3 x 9 LEDs RT8537 VIN FB EN Digital Dimming PWM Dimming RSET COMP GND CCOMP Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8537 Ordering Information Pin Configurations RT8537 (TOP VIEW) Lead Plating System G : Green (Halogen Free and Pb Free) FB 1 COMP GND 2 GND Package Type QW : WDFN-6SL 2x2 (W-Type) 3 7 Note : 6 VIN 5 EN LX 4 WDFN-6SL 2x2 Richtek products are : ` RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. ` Suitable for use in SnPb or Pb-free soldering processes. Function Pin Description Pin No. Pin Name Pin Function 1 FB Feedback Voltage Input. Connect a resistor between this pin and GND to set the current. 2 COMP Compensation Node. Connect a suitable capacitor to this pin for stability. GND Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 4 LX Switch Node. Connect the LX pin to the external Inductor. This pin is also used to sense the output voltage for open LED protection. 5 EN Enable Control Input. This pin can be Used for PWM dimming and 1-wire digital dimming. 6 VIN Supply Voltage Input. 3, 7 (Exposed Pad) Function Block Diagram VIN Pre-Regulator UVLO Shutdown OTP OSC Soft-Start PWM Control OVP LX LGATE Shutdown Delay + + - OCP EN EN Dimming GND VREF Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 + EA - Soft-Start COMP FB is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Operation The RT8537 is a constant frequency, current mode Boosttype LED driver. In normal operation, the N-MOSFET is turned on when the PWM Control circuit is set by the oscillator and is turned off when the current comparator resets the PWM Control circuit. While the N-MOSFET is turned off, the inductor current conducts through the external diode. Shutdown Delay When the EN voltage is logic low for more than 2.5ms, the driver will be shut down. In shutdown mode, the input supply current for the device is less than 1μA. OCP The driver provides cycle-by-cycle current limit function to control the current on power switch. Pre-Regulator The regulator provides low voltage power to supply the internal control circuits. ULVO When the input voltage is lower than the UVLO threshold (2.2V typ.), the driver will turn off. There is a 70mV for the UVLO hysteresis control. Soft-Start When the device is enabled, the internal VREF ramps up to the target voltage in a specific time. This ensures that the output voltage rises slowly to reduce the input inrush current. OVP The over voltage protection function monitors the output voltage via LX pin voltage. The OVP threshold voltage is 38V typically. Once the LED is open, the output voltage reaches the OVP threshold, the driver will be shut down. OTP The over temperature protection function will shut down the switching operation when the junction temperature exceeds 160°C. Once the junction temperature cools down by approximately 15°C, the converter will automatically resume switching. EN Dimming The EN pin is used for the control input for both PWM dimming mode and digital dimming mode. The dimming mode is decided when the device is enabled. The default dimming mode is PWM dimming mode. To enter digital mode, a certain digital pattern on the EN pin must be recognized when the IC starts from shutdown mode. Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8537 Absolute Maximum Ratings z z z z z z z z z (Note 1) VIN, EN to GND -----------------------------------------------------------------------------------------------------------FB, COMP to GND -------------------------------------------------------------------------------------------------------LX to GND ------------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C WDFN-6SL 2x2 -----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WDFN-6SL 2x2, θJA ------------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------- Recommended Operating Conditions z z z −0.3V to 20V −0.3V to 3V −0.3V to 40V 2.99W 33.5°C/W 260°C 150°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VIN ----------------------------------------------------------------------------------------------- 2.9V to 18V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VIN = 3.6V, TA = 25°C unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Operating Quiescent Current into VIN IQ Device PW M Switching No Load -- -- 2.3 mA Shutdown Current ISHDN EN = GND, VIN = 4.2V -- -- 1 μA VUVLO VIN Falling -- 2.2 2.5 V -- 70 -- mV Under Voltage Lockout Threshold Under Voltage Lockout Hysteresis EN Input Voltage VUVLO_Hys Logic-High VIH VIN = 2.9V to 18V 1.2 -- -- Logic-Low VIL VIN = 2.9V to 18V -- -- 0.4 -- 3 10 μA EN High to Low 2.5 -- -- ms EN Pin Low 260 -- -- μs 100 -- -- μs 1 -- -- ms 192 200 208 mV -- -- 2 μA EN Pull Down Current EN Pulse width to Shutdown tOFF Digital Dimming Detection Time tES_Det Digital Dimming Detection Delay tES_Delay Digital Dimming Detection tES_Win Window Time Measured from EN High V Feedback Reference Voltage VREF Feedback Input bias Current IFB Oscillator Frequency fOSC 1 1.2 1.5 MHz Maximum Duty Cycle D MAX 90 93 -- % VFB = 200mV Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Parameter Symbol Test Conditions Min Typ Max Unit Minimum On Pulse Width tMIM_ON -- 40 -- ns COMP Pin Sink Current ISink -- 60 -- μA COMP Pin Source Current ISource -- 60 -- μA Error Amplifier transconductance G EA 240 320 400 μA/V N-MOSFET On-Resistance RDS(ON) VIN = 3V -- 0.35 0.7 Ω N-MOSFET Leakage Current ILN_NFET VLX = 35V, EN = Low -- -- 1 μA N-MOSFET Current Limit ILIM 0.96 1.2 1.44 A Open LED Protection Threshold VOVP 37 -- 42 V Start Time of Program Stream tStart 3 -- -- μs End Time of Program Stream tEOS 3 -- 360 μs High Time Low Bit tH_LB Logic 0 3 -- 180 μs Low Time Low Bit tL_LB Logic 0 2 x tH_LB -- 360 μs High Time High Bit tH_HB Logic 1 2 x tL_HB -- 360 μs Low Time High Bit tL_HB Logic 1 3 -- 180 μs Thermal Shutdown Threshold TSD -- 160 -- °C -- 15 -- °C Measured on the LX pin, L = 22μH Thermal Shutdown Hysteresis ΔTSD 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. 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. The reference voltage accuracy is ±2.5% at recommended ambient temperature range, guaranteed by design. Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8537 Typical Application Circuit L 10µH to 22µH VIN 3V to 6V CIN 4.7µF COUT 1µF 4 LX …… 3 x 9 LEDs RT8537 6 VIN FB 1 5 EN Digital Dimming PWM Dimming RSET 1.1 2 COMP GND 3, 7 (Exposed Pad) 220nF Figure 1. Drive 27 LEDs for Media form Factor Display L 10µH to 22µH VIN 5V CIN 4.7µF COUT 1µF 4 LX High Brightness LED RT8537 6 VIN FB 1 5 EN Digital Dimming PWM Dimming 350mA 2 COMP GND RSET 0.57 3, 7 (Exposed Pad) 220nF Figure 2. Application Circuit for 3 High Brightness LEDs L 10µH to 22µH VIN 12V CIN 4.7µF COUT 1µF 4 LX High Brightness LED RT8537 6 VIN 5 EN Digital Dimming PWM Dimming FB 1 350mA 2 COMP 220nF GND RSET 0.57 3, 7 (Exposed Pad) Figure 3. Application Circuit for 6 High Brightness LEDs Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Timing Diagram PWM Dimming Mode Power on time > 2µs Period1 EN Period2 5kHz < fPWM < 50kHz TSHDN > 2.5ms Duty2 Duty1 Duty1 x 200mV Duty2 x 200mV FB 1-Wire Digital Dimming Mode Enter 1-wire digital dimming mode detection window > 1ms Program Code 1 Shutdown Delay Program Code 2 Digital dimming mode detection time > 260µs ……… EN Digital dimming detection delay time > 100µs ……… Program Value 1 Program Value 2 FB Start-up Delay Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8537 Typical Operating Characteristics Efficiency vs. Input Voltage Efficiency vs. Output Current 100 100 90 95 VIN = 3.3V VIN = 3.6V VIN = 4.2V 70 90 Efficiency (%) Efficiency (%) 80 60 50 40 30 IOUT = 0.1A IOUT = 0.2A IOUT = 0.05A 85 80 75 70 20 65 10 VOUT = 9.5V VOUT = 9.5V 60 0 0 0.04 0.08 0.12 0.16 2.8 3.2 3.6 0.2 4 Operating Quiescent Current vs. Temperature 6.4 6.8 7.2 Operating Quiescent Current vs. Input Voltage 1.70 Operating Quiescent Current (mA)1 Operating Quiescent Current (mA)1 6 Input Voltage (V) Output Current (A) 1.60 1.50 1.40 1.30 VIN = 3.6V 1.20 1.7 1.6 1.5 1.4 1.3 1.2 1.1 -50 -25 0 25 50 75 100 125 2 4 6 Temperature (°C) 8 10 12 14 16 18 Input Voltage (V) Oscillator Frequency vs. Input Voltage Reference Voltage vs. Input Voltage 1.30 0.210 0.208 1.25 Reference Voltage (V) Oscillator Frequency (MHz)1 4.4 4.8 5.2 5.6 1.20 1.15 1.10 1.05 3 Power LEDs, VIN = 3.6V, ILED = 200mA 1.00 0.206 0.204 0.202 0.200 IOUT = 0.2A IOUT = 0.1A IOUT = 0.01A 0.198 0.196 0.194 0.192 0.190 2.5 3.8 5.1 6.4 7.7 Input Voltage (V) Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 9 2.8 3.68 4.56 5.44 6.32 7.2 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Reference Voltage vs. 1-Wire Dimming Step Reference Voltage vs. Output Current 220 0.204 Reference Voltage (mV) Reference Voltage (V) 0.203 0.202 0.201 VIN = 3.3V VIN = 3.6V VIN = 4.2V 0.200 0.199 0.198 176 132 88 44 0.197 VIN = 3.6V 0 0.196 0 0.05 0.1 0.15 0 0.2 4 8 12 16 20 24 28 32 1-Wire Dimming (Step) Output Current (mA) Reference Voltage vs. PWM Dimming Duty Cycle Enable Pull Down Current vs. Input Voltage 16 200 Pull Down Current (µA)1 Reference Voltage (mV) 14 160 120 80 5kHz 10kHz 20kHz 40kHz 50kHz 40 12 10 8 6 4 2 VIN = 3.6V 0 0 0 10 20 30 40 50 60 70 80 90 100 2 4 6 8 10 12 14 PWM Dimming Duty Cycle (%) Input Voltage (V) VIN Shutdown Current vs. Input Voltage Power On from EN 16 18 Shutdown Current (µA)1 0.5 VEN (5V/Div) 0.4 VIN (2V/Div) 0.3 VFB (200mV/Div) 0.2 0.1 I IN (200mA/Div) 10 white LEDs, VIN = 3.6V, ILED = 20mA 0.0 2 4 6 8 10 12 14 16 18 Time (5ms/Div) Input Voltage (V) Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8537 PWM Dimming from EN Power Off from EN VEN (5V/Div) VEN (2V/Div) VIN (2V/Div) VFB (200mV/Div) I IN (200mA/Div) I LED (5mA/Div) 10 white LEDs, VIN = 3.6V, ILED = 20mA Time (5ms/Div) Time (5μs/Div) PWM Dimming from EN 1-Wire Dimming from EN On VEN (2V/Div) I LED (5mA/Div) 10 white LEDs, VIN = 3.6V, RSET = 10Ω, f = 50kHz, Duty cycle = 50% VEN (2V/Div) VIN (5V/Div) 10 white LEDs, VIN = 3.6V, RSET = 10Ω, f = 5kHz, Duty cycle = 50% Time (50μs/Div) VFB (100mV/Div) I IN (100mA/Div) 10 white LEDs, VIN = 3.6V, RSET = 10Ω, Dimming step = 0 to 31 Time (5ms/Div) 1-Wire Dimming from EN Off VEN (2V/Div) VIN (5V/Div) VFB (100mV/Div) I IN (100mA/Div) 10 white LEDs, VIN = 3.6V, RSET = 10Ω, Dimming step = 31 to 16 Time (5ms/Div) Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Application Information Soft-Start Where : Soft-start circuitry is integrated into the IC to avoid a high inrush current during start-up. After the device is enabled, the VREF ramps up slowly to the target voltage within a specific time. This ensures that the output voltage rises slowly to reduce the input current. ILED = output current of LEDs VREF = feedback reference voltage (200mV typ.) RSET = current sense resistor The output current tolerance depends on the VREF accuracy and the current sense resistor accuracy. Open LED Protection Open LED protection circuitry prevents IC from damage as the result of LED disconnection. The RT8537 monitors the voltage at the LX pin during each switching cycle. The circuitry turns off the switch and shuts down the IC as soon as the LX voltage exceeds the VOVP threshold (38V typ.). The device remains in shutdown mode until it is enabled by toggling the EN pin logic. LED Brightness Dimming Mode Selection The EN pin is used for the control input for both dimming modes, PWM dimming mode and 1-wire digital dimming mode. The dimming mode for the RT8537 is selected when the device is enabled. The default dimming mode is PWM dimming mode. To enter digital dimming mode, the following digital pattern on the EN pin must be recognized by the IC when the IC starts from the shutdown mode. Shutdown The RT8537 enters shutdown mode when the EN pin is pulled low for 2.5ms. During shutdown, the input supply current for the device is less than 1μA. Although the internal FET does not switch in shutdown, there is still a DC current path between the input and the LEDs through the inductor and Schottky diode. The minimum forward voltage of the LED array must exceed the maximum input voltage to ensure that the LEDs remain off in shutdown. ` Pull the EN pin high to enable the RT8537 and start the detection window (tES_win, 1ms) for digital dimming ` After the digital dimming detection delay time (tES_Delay, 100μs), drive the EN low for more than the detection time (tES_Detect, 260μs). ` Pull the EN pin high after the detection time (260μs) and before the detection window (tES_Win, 1ms), once the above 3 conditions are met, the IC immediately enters the digital 1-wire dimming mode. The digital dimming communication can start before the detection window expires. Once the dimming mode is selected, it can not be changed without another start up. This means the IC needs to be shut down by pulling the EN low for 2.5ms and restarts. See the dimming mode detection and soft-start (see Figure 4) for a graphical explanation. Current Setting The LED current is adjustable by an external current sense resistor in series with the LED string. The LED current can be calculated by the following equation : V ILED = REF RSET Enter 1-wire digital dimming mode detection window > 1ms Program Code 1 Shutdown Delay Program Code 2 Digital dimming mode detection time > 260µs ……… EN Digital dimming detection delay time > 100µs ……… Program Value 1 Program Value 2 FB Start-up Delay Figure 4. Start-Up for Digital Dimming Mode Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8537 PWM Dimming Mode When the EN pin is constantly high, the FB voltage is regulated to 200mV typically. However, the EN pin allows a PWM signal to reduce this regulation voltage; therefore, it achieves LED brightness dimming. The relationship between the duty cycle and FB voltage is given by VFB = Duty x VREF Where : Duty = duty cycle of the PWM signal VREF = internal reference voltage (200mV typ.) The RT8537 chops up the internal 200mV reference voltage at the duty cycle of the PWM signal. Therefore, although a PWM signal is used for brightness dimming, only the LED DC current is modulated, which is often referred as analog dimming. This eliminates the audible noise which often occurs when the LED current is pulsed in replica of the frequency and duty cycle of PWM control. For optimum performance, use the PWM dimming frequency in the range of 5kHz to 50kHz. The requirement of minimum dimming frequency comes from the digital dimming detection delay and detection time specification in the dimming mode selection. Power on time > 2µs Period1 Period2 5kHz < fPWM < 50kHz TSHDN > 2.5ms EN Duty1 Duty2 Duty1 x 200mV Duty2 x 200mV FB Figure 5. PWM Dimming Control programmed reference voltage is stored in an internal register. A power reset clears the register value and resets it to default. It is recommended to finish the first address/ data stream before 2ms at CCOMP = 220nF during start-up for the 1-wire dimming, to avoid the possibilities for LED brightness changes from bright to dark. Especially the digital dimming code is below step 5 (VFB = 17mV). The digital dimming interface is based on a master slave structure, where the master is typically a microcontroller or application processor. Figure 6 and Table 2 give an overview of the protocol. The protocol consists of a device specific address byte and a data byte. The device specific address byte is fixed to 72 hex. The data byte consists of five bits for information, two address bits, and the RFA bit always set to 0. The advantage of 1-wire digital dimming compared with other one pin interfaces is that its bit detection is in a large extent independent from the bit transmission rate. It can automatically detect bit rates between 1.7kBit/sec and up to 160kBit/sec. All bits are transmitted MSB first and LSB last. Figure 7 shows the protocol without acknowledge request (Bit RFA = 0). Prior to both bytes, device address byte and data byte, a start condition must be applied. For this, the EN pin must be pulled high for at least tStart (3μs) before the bit transmission starts with the falling edge. If the EN pin is already at a high level, no start condition is needed prior to the device address byte. The transmission of each byte is closed with an end of stream condition for at least tEOS (3μs). The bit detection is based on a logic detection scheme, where the criterion is the relation between tLOW and tHIGH. It can be simplified to : 1-Wire Digital Dimming Mode The RT8537 adopts an 1-wire digital protocol for the digital dimming mode control, which can program the FB voltage to any of the 32 steps with single command. The step increment increases with the voltage to produce pseudo logarithmic curve for the brightness step. See Table 1 for the FB pin voltage steps. The default step is full scale when the device is first enabled (VFB = 200mV). The Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 High Bit : tHIGH > tLOW, but with tHIGH at least 2 x tLOW, see Figure 7. Low Bit : tHIGH < tLOW, but with tLOW at least 2 x tHIGH, see Figure 7. The bit detection starts with a falling edge on the EN pin and ends with the next falling edge. Depending on the relation between tHIGH and tLOW, the logic 0 or 1 is detected. is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Step 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 FB Voltage (mV) 0 5 8 11 14 17 20 23 26 29 32 35 38 44 50 56 Data IN D4 D3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 Table 1. 32-Step Digital Dimming Setting FB Voltage Step D2 D1 D0 (mV) 16 62 0 0 0 17 68 0 0 1 18 74 0 1 0 19 80 0 1 1 20 86 1 0 0 21 92 1 0 1 22 98 1 1 0 23 104 1 1 1 24 116 0 0 0 25 128 0 0 1 26 140 0 1 0 27 152 0 1 1 28 164 1 0 0 29 176 1 0 1 30 188 1 1 0 31 200 1 1 1 D4 D3 D2 D1 D0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Data Byte Device Address Start DA7 DA6 DA5 DA4 DA3 DA2 DA1 DA0 EOS Start RFA A1 0 1 1 1 0 0 1 0 0 0 A0 0 D4 D3 D2 D1 D0 EOS Data OUT Figure 6 Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8537 Table 2. Byte Transmission Description Bit Number Name 7 DA7 0 MSB device address 6 DA6 1 5 DA5 1 4 DA4 3 DA3 2 DA2 0 1 DA1 1 0 DA0 0 LSB device address 7 (MSB) RFA RFA = 0 6 A1 Address bit 1 = 0 5 A0 Address bit 0 = 0 4 D4 Device Address Byte 72 hex Data Byte Description 1 IN 0 Data bit 4 IN 3 D3 Data bit 3 2 D2 Data bit 2 1 D1 Data bit 1 0 (LSB) D0 Data bit 0 Digital 1-Wire Dimming w/o Acknowledge RFA = 0 tStart Data IN Address Byte Data Byte tStart Static High Static High DA7 0 DA0 0 tLOW tHIGH tEOS tLOW Low Bit Logic 0 RFA 0 D0 1 tEOS tHIGH High Bit Logic 1 Figure 7 Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 The recommended value of inductor for 10 LEDs or high brightness LED applications is from 10μH to 22μH. Smaller size and better efficiency are the major concerns for portable devices. The inductor should have low core loss at 1MHz (Min.) and low DCR for better efficiency. The inductor saturation current rating should be considered to cover the inductor peak current. The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curve in Figure 8 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 3.6 Maximum Power Dissipation (W)1 Inductor Selection Capacitor Selection For low ripple voltage, ceramic capacitors with low ESR are recommended. X5R and X7R types are suitable because of their wide voltage range and good operating temperature characteristics. For the application of the RT8537 to drive 10 LEDs in series, a 4.7μF for input capacitor, an 1μF for output capacitor and a 220nF for Four-Layer PCB 3.0 2.4 1.8 1.2 0.6 0.0 0 compensation capacitor are recommended. 25 50 75 100 125 Ambient Temperature (°C) Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For WDFN-6SL 2x2 package, the thermal resistance, θJA, is 33.5°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 : Figure 8. Derating Curve of Maximum Power Dissipation Layout Consideration For best performance of the RT8537, the following layout guidelines must be strictly followed. ` Input and output capacitors should be connected to a strong ground plane for heat sinking and noise protection. ` Keep the main current traces as possible as short and wide. ` LX node of DC/DC converter is with high frequency voltage swing. It should be kept at a small area. ` Place the feedback components as close as possible to the IC and keep away from the noisy devices. PD(MAX) = (125°C − 25°C) / (33.5°C/W) = 2.99W for WDFN-6SL 2x2 package Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8537-00 May 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8537 FB node copper area should be minimized and keep far away from noise sources (LX pin) and RSET should be as close as possible to FB pin. GND ... CCOMP FB 1 COMP GND 2 GND RSET 3 7 6 5 4 CIN VIN EN LX COUT VOUT Output capacitor must be placed between GND and VOUT to reduce noise. Input capacitor must close to VIN to reduce noise. The inductor should be placed as close as possible to the switch pin to minimize the noise coupling into other circuits. LX node copper area should be minimized for reducing EMI. Figure 9. PCB Layout Guide Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS8537-00 May 2013 RT8537 Outline Dimension D2 D L E E2 1 e 2 b A A1 SEE DETAIL A 1 2 1 DETAIL A Pin #1 ID and Tie Bar Mark Options A3 Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.700 0.800 0.028 0.031 A1 0.000 0.050 0.000 0.002 A3 0.175 0.250 0.007 0.010 b 0.200 0.350 0.008 0.014 D 1.900 2.100 0.075 0.083 D2 1.550 1.650 0.061 0.065 E 1.900 2.100 0.075 0.083 E2 0.950 1.050 0.037 0.041 e L 0.650 0.200 0.026 0.300 0.008 0.012 W-Type 6SL DFN 2x2 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. DS8537-00 May 2013 www.richtek.com 17