HT7945 White LED Backlight Driver Features Applications • Max input voltage: 26V • LED backlights for notebook and tablet PC • Integrated Power Mosfet • Output voltage up to 40V driving 11 series LEDs – LED Vf(max)=3.5V per string – absolute max rating up to 44V General Description The HT7945 is a high efficiency DC-DC controller which can drive multiple WLEDs connected in a series/parallel configuration. The device has a wide input voltage, ranging from 4.5V to 26V, and an adjustable 15mA to 30mA WLED current, setup using an external resistor. In total, the device can support up to 88 WLED. • Channel Phase Shift PWM Dimming • Drives up to 12 LED strings under Vf(max.)=3.3V condition • Low string feedback voltage: 0.8V at 20mA LED current • Switching frequency: 500kHz/1MHz In addition, eight current sink regulators provide ±1.5% high precision current matching between strings. The brightness can be adjusted by an external PWM signal with frequency up to 20kHz. Once an open/short string is detected, that string is disabled while the other strings continue to operate normally. • 8-string constant current output • LED current adjustable from 15mA to 30mA • ±1.5% current matching between strings • PWM dimming control • 1% minimum dimming duty-cycle at 2kHz Other protection includes soft-start, under voltage lockout, programmable over voltage protection, switch current limit and thermal shutdown. • Integrated soft start function • LED failure detection: open and short circuit • Capacitor type: ceramic The HT7945 is supplied in a QFN 24 pin 4x4 tiny footprint package type • Protection: OVP, OTP, UVLO, SW current limit • Small 24-pin outline package: 4mm×4mm, thin QFN type Rev. 1.00 1 April 13, 2012 HT7945 Block Diagram IN Over voltage Comparator Over temperature Protection 5.2V LDO 1.28V OVP VREF AVDD UVLO PWM Comparator LX1,2 Control Logic Driver 1MHz/500kHz Oscillator OSC M1 PGND1.2.3 Current Limit Slope Compensation Current Sense Fault1 LED1 LED open/short detector EN LED open/short detector Fault8 LED8 LED1 Error Amp. Min voltage selector COMP LED8 Soft-start AGND1.2 VREF = 0.8V LED1 VREF Fault1 Current Source 0.6V Phase Shift & PWM Controller Fault2 Current Source LED2 Current Source LED7 Current Source LED8 Fault7 Fault8 ISET Rev. 1.00 DIM 2 April 13, 2012 HT7945 Pin Assignment PGND1 IN VCC5 AVDD OVP EN OSC TOP VIEW 24 1 19 18 AGND1 PGND3 COMP LX1 HT7945 24 QFN-A 4mm x 4mm DIM LX2 ISET LED1 PGND2 LED8 6 7 LED7 LED6 LED5 AGND2 LED4 LED3 LED2 Exposed Pad 13 12 Pin Description PIN 1 2, 10 Name EN Description Enable Input. When low, the device is powered down. If tied high or left open, the device is active. AGND1, AGND2 Analog ground Error Amplifier Output. A simple RC series is connected between this pin and the analog ground for boost regulator loop compensation. 3 COMP 4 DIM Dimming Input. LED backlight strings PWM control pin. 5 ISET Full Scale LED Current Adjustment Pin. Selection implemented by connection a resistor beteen this pin and the analog ground. 6~9, 11~14 LED1 ~ LED8 LED current sink. Internal regulator open-drain outputs Can sink up to 30mA. If unused, the pins should be left open. 15, 16 LX2, LX1 Switching Node. Internal Power MOSFET drain output. Inductor and Schottky diode are connected to these pins. 17~19 PGND1 ~ PGND3 Power Ground. Power MOSFET return path. 20 IN Input Voltage. Input voltage range from 4.5V to 26V. Bypass IN to analog ground directly at the pin with 0.1uF or greater ceramic capacitor. 21 VCC5 Internal 5.2V LDO Output. Bypass to analog ground with a 10uF or greater ceramic capacitor. If VIN is less than or equal to 6.0V, connect VCC5 to IN to disable the internal LDO. 22 AVDD Power MOSFET Gate Drive Supply. Bypass AVDD to analog ground with a ceramic capacitor of 10uF or greater. 23 OVP Over Voltage Protection. Used to set the desired OVP threshold using an external resistor divider. The detector threshold is 1.28V (typ.) VOVP=VOUT+3V. 24 OSC Oscillator Frequency Selection. Connect OSC to VCC5 to set the oscillator frequency to 1MHz. Connect OSC to analog ground to set the frequency to 500kHz. — E.P. Exposed Pad. Connect to GND plane of the PCB. Rev. 1.00 3 April 13, 2012 HT7945 Absolute Maximum Ratings IN, EN.....................................................................28V DIM.........................................................................30V LED1~LED8, LX1, LX2........................................44V Operating Temperature Range................ -40°C~+85°C AVDD, OVP..............................................................6V Maximum Junction Temperature......................+165°C Note: These are stress ratings only. Stresses exceeding the range specified under “Absolute Maximum Ratings” may cause substantial damage to the device. Functional operation of this device at other conditions beyond those listed in the specification is not implied and prolonged exposure to extreme conditions may affect device reliability. Electrical Characteristics Refer to circuit of Figure 1, EN =VIN = 12V, AVDD = VCC5, Ta=25°C, unless otherwise specified. (Note) Parameter Test Conditions Min. Typ. Max. Unit VIN=VCC5 4.5 — 6.0 VCC5=Open 6.0 — 26.0 EN=VIN — 3.3 — mA EN=GND — — 10 μA VCC5 Output Voltage 6.0V < VIN < 29V, (Only for internal circuit used) 4.9 5.2 5.6 V VCC5 UVLO Threshold Rising edge, typical hysteresis=85mV 3.6 3.8 4.0 V OSC=AGND 335 500 665 kHz Supply Selection IN Input Voltage IN Quiescent Current V Boost Selection Switching Frequency 0.67 1.00 1.33 MHz LX_Internal MOSFET Current Limit OSC=VCC5 — — 2.2 — A LX_Internal MOSFET RDS(ON) — — 0.35 1.50 Ω Maximum Duty Cycle — — 94 — % Enable High Level Threshold Voltage — 2 — — Enable Low Level Threshold Voltage — — — 0.8 Dimming PWM Frequency — — 2 — Dimming PWM High Level Threshold — 2 — — Dimming PWM Low Level Threshold — — — 0.8 Control Selection V kHz V LED_Selection LED_Current RISET=24K 19.2 20.0 20.8 mA LED_Current Regulation Between Strings ILED=20mA — ±1.5 ±2.5 % LED_Open Detector Threshold LED_=Open 300 400 500 mV LED_Short Detector Threshold LED_=VOUT 5.2 5.6 6.0 V — 1.21 1.28 1.35 V Thermal Shutdown Temperature — — 140 — Thermal Shutdown Hysteresis — — 50 — OVP Threshold Voltage Thermal Selection °C Note: Specifications over the -40°C to 85°C operating temperature range are assured by design. Rev. 1.00 4 April 13, 2012 HT7945 Typical Application Circuit VIN C1 4.7uF L1 D1 10uH 1N5819 C2 0.1uF VOUT C3 4.7uF/50V IN LX1 LX2 OSC C4 10uF VCC5 AVCC PGND1 PGND2 PGND3 EN AGND1 AGND2 R3 C5 1.5nF 24kW PWM HT7945 R1 1MW OVP R2 33kW COMP LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 ISET DIM RSET EP Figure 1 Functional Description current reaches the current limit threshold of 2.2A, the MOSFET will be turned off. It is import to note that this current limit will not protect the output from excessive current if the output is short circuited. If an output short circuit has occurred, excessive current can damage both the inductor and diode. VIN Under-Voltage Lockout – UVLO The device contains an Input Under Voltage Lockout (UVLO) circuit. The purpose of the UVLO circuit is to ensure that the input voltage is high enough for reliable operation. For low input voltage operation of 4.5V to 6V, the VREF pin is connected to the VIN pin (input voltage) to bypass the voltage regulator in which the inherent voltage drop can degrade low voltage operation. When input voltage level is below this range (4.5V to 6V) to 4.0V, then proper use is not possible. When the input voltage falls below the under voltage threshold, the internal MOSFET switch will be turned off. If the input voltage rises beyond the under voltage lockout hysteresis, it can return to the original operating situation and does not required to be powered on again. The UVLO threshold is set below the minimum input voltage of 3.8V to avoid any transient VIN drops under the UVLO threshold causing the converter to turn off. Output Voltage Protection Over-Voltage Protection The device includes an over-voltage protection function. If the one of ISEN pins is shorted to ground or an LED is disconnected from the circuit, the voltage on the ISEN pin will fall to zero and the internal power MOSFET will switch with its full duty cycle. This may cause the output voltage to exceed its maximum voltage rating, possibly damaging the device and external components. The internal over-voltage protection circuitry turns off the power MOSFET and shuts down the device as soon as the output voltage exceeds the VOVP threshold. As a result, the output voltage falls to the level of the input supply voltage. The device remains in this shutdown mode until the VOVP is less than its setup threshold. Current Limit Protection The device has a cycle-by-cycle current limit to protect the internal power MOSFET. If the inductor Rev. 1.00 5 April 13, 2012 HT7945 Application Information LED Open Detector Protection The device includes an LED open protection function. If any one of ISEN pins is disconnected from the LED load, the device will stop driving the ISEN pin, automatically ignoring the open pin. The LED current of the other ISEN pins will not be influenced by any open ISEN pin. When the open ISEN pins are re-connected to the LED load, there will be no current. These ISEN pins will remain disabled until the power is recycled. Inductor Selection The inductor choice affects steady state operation as well as transient behavior and loop stability. There are three important electrical parameters which need to be considered when choosing an inductor: • The inductor value • DCR – copper wire resistance LED Short Detector Protection • The saturation current The device includes an LED short circuit protection function. If more than 2~3 LEDs are short circuited on any ISEN pin or the voltage level of the ISEN pin is greater than 5.6V, the device will turn off that ISEN pin and automatically ignore the shorted pin. The LED current of other ISEN pins will not be influenced by any shorted ISEN pins. If even only one ISEN pin remains operational due to shorts on other pins, it will still maintain normal operation. The shorted ISEN pins remain disabled until the power is recycled. Inductor choice is especially important as it is required to ensure the inductor does not saturate under its peak current conditions. The general rule is to keep the inductor current peak-to-peak ripple at approximately 30% of the nominal output current. As a typical example, when using the HT7943 boost converter, operating in both discontinuous and continuous conduction modes, the typical application circuit value of the inductor, L1, would be around 10μH. Over-Temperature protection – OTP Input/Output Capacitor An internal thermal shutdown function is included to prevent device damage due to excessive heat and power dissipation. Typically, the thermal shutdown threshold of is 140°C. When the thermal shutdown function is activated, the device stops switching until the temperature falls to below 90°C typically. When this occurs the device resumes switching once again. Output Capacitor The output capacitor determines the steady state output voltage ripple. In the compensation parameters, the output capacitor is one of the parameters, and if the capacitance is too big or too small, it can cause system instability. Its value must be based on the application circuit recommended output capacitor value. A low ESR ceramic capacitor is required to keep noise to a minimum. A 4.7μF ceramic capacitor is suitable for typical applications. Soft Start Function Converter operation starts immediately after power on. In order to avoid the possibility of large in-rush currents to the load during this power on period, a soft-start function is implemented to prevent this problem from occurring. Rev. 1.00 Input Capacitor An input capacitor is required to supply the ripple current to the inductor and is also used to limit the input noise, allowing the device to obtain a stable DC power supply. As the input capacitance is not a compensation parameter there are no stability problems, however a capacitor must always be connected along with an input power supply. For typical applications, a 4.7μF ceramic capacitor is sufficient. This capacitor must be connected very close to the VIN pin and inductor, with short traces for good noise performance. 6 April 13, 2012 HT7945 Schottky Diode Layout Considerations It is recommended to use a Schottky diode with a low forward voltage to minimise power dissipation and therefore maximise the converter efficiency. The average and peak current ratings must be greater than the maximum output current and peak inductor current. There are three important electrical parameters to consider when choosing the diode: Circuit board layout is a very important consideration for switching regulators if they are to function properly. Poor circuit layout may result in related noise problems. In order to minimise EMI and switching noise, the guidelines should be noted: • All tracks should be as wide as possible. • The input and output capacitors, CIN and COUT, should be located close to the VIN, VOUT and GND pins. • The diode maximum reverse voltage value must be greater than the maximum output voltage. • Short recovery time and low forward voltage – use a Schottky diode type. • The Schottky diode, D1, and inductor, L1, must be located close to the LX pin. • Diode current rating should be greater than the maximum load current. • The AGND analog ground pins, and PGND power ground pins, must have independent connections, but must be connected together at some final point on the user circuit board. Compensation Components The COMP pin is the output of the error amplifier and must be properly connected to an external RC network to ensure regulator loop stability. Recommended values are: RComp=24kΩ and CComp=1.5nF A recommended PCB layout with component locations is enclosed. Oscillator Frequency Setup There are two frequency options available. The OSC pin default switching frequency is 1MHz when the pin is unconnected and 500kHz when the pin is connected to ground. LED current Setup The LED current can be setup using an external resistor connected from the ISENT pin to ground. The following equation shows how the current is calculated: ILED(mA)= 480 RISET(kΩ) This shows how the Led reference current can be setup at LED1~8 and represents the sensed LED current for each string. The LED current regulation between the strings has good acuracy at ±1.5%. Top Layer Dimming Control The device includes an external PWM signal dimming control. PWM dimming control is achieved by applying an external PWM signal with a frequency of 100Hz to 20kHz. The high level of this signal must be greater than 2.0V and the low level must be less than 0.8V. A 0% duty cycle corresponds to zero LED current while a 100% duty cycle corresponds to full LED current. Bottom Layer Rev. 1.00 7 April 13, 2012 HT7945 Typical Performance Characteristics Temperature vs Quiescent Current W-ILED Current vs PWM Duty Temperature vs LED Current Efficiency vs Input Voltage Temperature vs VREF Output Voltage Temperature vs Input Voltage Rev. 1.00 8 April 13, 2012 HT7945 (CH1=one of LED channel Feedback Voltage, CH2=LED Current, CH3=Dimming signal) Temperature vs Switching Frequency (1MHz) Dimming=2kHz 90% Basic Waveform Dimming Waveform (CH1=VOUT(AC),CH2=IL, CH3=Switching Pin) (CH1=one of LED channel Feedback Voltage, CH2=LED Current, CH3=Dimming signal) Dimming=200Hz 10% Start up Waveform (CH1=VOUT(AC), CH2=VIN(AC)) Rev. 1.00 9 April 13, 2012 HT7945 Package Information Note that the package information provided here is for consultation purposes only. As this information may be updated at regular intervals users are reminded to consult the Holtek website (http://www.holtek.com.tw/english/ literature/package.pdf) for the latest version of the package information. SAW Type 24-pin (4mm×4mm) QFN Outline Dimensions OSE Symbol Nom. Max. A 0.028 ― 0.031 A1 0.000 0.001 0.002 A3 ― 0.008 ― b 0.008 0.010 0.012 D 0.156 0.157 0.159 E 0.156 0.157 0.159 e ― 0.020 ― D2 0.106 0.108 0.110 E2 0.106 0.108 0.110 L 0.010 0.012 0.014 Symbol Rev. 1.00 Dimensions in inch Min. Dimensions in mm Min. Nom. Max. A 0.70 ― 0.80 A1 0.00 0.02 0.04 A3 ― 0.20 ― b 0.20 0.25 0.30 D 3.95 4.00 4.05 E 3.95 4.00 4.05 e ― 0.50 ― D2 2.70 2.75 2.80 E2 2.70 2.75 2.80 L 0.25 0.30 0.35 10 April 13, 2012 HT7945 Holtek Semiconductor Inc. (Headquarters) No.3, Creation Rd. II, Science Park, Hsinchu, Taiwan Tel: 886-3-563-1999 Fax: 886-3-563-1189 http://www.holtek.com.tw Holtek Semiconductor Inc. (Taipei Sales Office) 4F-2, No. 3-2, YuanQu St., Nankang Software Park, Taipei 115, Taiwan Tel: 886-2-2655-7070 Fax: 886-2-2655-7373 Fax: 886-2-2655-7383 (International sales hotline) Holtek Semiconductor Inc. (Shenzhen Sales Office) 5F, Unit A, Productivity Building, No.5 Gaoxin M 2nd Road, Nanshan District, Shenzhen, China 518057 Tel: 86-755-8616-9908, 86-755-8616-9308 Fax: 86-755-8616-9722 Holtek Semiconductor (USA), Inc. (North America Sales Office) 46729 Fremont Blvd., Fremont, CA 94538, USA Tel: 1-510-252-9880 Fax: 1-510-252-9885 http://www.holtek.com Copyright© 2012 by HOLTEK SEMICONDUCTOR INC. The information appearing in this Data Sheet is believed to be accurate at the time of publication. However, Holtek assumes no responsibility arising from the use of the specifications described. The applications mentioned herein are used solely for the purpose of illustration and Holtek makes no warranty or representation that such applications will be suitable without further modification, nor recommends the use of its products for application that may present a risk to human life due to malfunction or otherwise. Holtek's products are not authorized for use as critical components in life support devices or systems. Holtek reserves the right to alter its products without prior notification. For the most up-to-date information, please visit our web site at http://www.holtek.com.tw. Rev. 1.00 11 April 13, 2012