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| HT7937 Built-in OVP White LED Step-up Converter Features * Integrated open-circuit protection * Over voltage protection * Up to 84% efficiency at VIN=3V, 3LEDs, * Low standby current: 0.1mA (typ.) (VSHDN=0V) * Matches LED current * Small value inductor and capacitors * Small outline SOT23-6 package ILED= 20mA * 1.2MHz fixed switching frequency Applications * Cellular phones * PDAs * DSCs * Handheld devices * White LED display backlighting General Description The HT7937 is high efficiency boost converter with a constant current output to provide back light functions in handheld devices. Applications with series connected LEDs ensure constant and identical LED currents resulting in uniform brightness. A continuous LED output current is setup using the FB pin regulated voltage across an external sense resistor, RFB connected between the FB pin and ground. The integrated open load protection circuitry prevents damage resulting from an open circuit condition. The low 95mV feedback voltage minimises power losses in the current setting resistor which improves efficiency. The HT7937 has a high switching frequency of up to 1.2MHz which permits the use of lower value extremely small outline inductor and filter capacitor. The HT7937 is supplied in a space-saving, 6-lead SOT23-6 package type. Block Diagram OVP 28V SW SHDN FB C o n tro l L o g ic 95m V a GND 1 .2 M H z O s c illa to r V IN Rev 1.10 1 January 18, 2010 HT7937 Pin Assignment S O T 2 3 -6 V IN 6 OVP 5 SHDN 4 T o p V ie w 1 SW 2 GND 3 FB Pin Description Pin No. 1 2 3 4 5 6 Pin Name SW GND FB SHDN OVP VIN Switching pin Ground pin Output voltage sense node. Connect the cathode of the LED to this pin. A resistor from this pin to ground sets the LED current. Internally compares to 95mV (Typ.). Shutdown device pin. Connect to 1.5V or higher to enable device (ON), 0.3V or lower to disable device (OFF). For over voltage protection connect to the output. Input voltage pin. 2.5V to 5.5V for internal circuitry. Description Absolute Maximum Ratings Input Voltage..............................................................6V FB Voltage .................................................................6V OVP Voltage ............................................................36V Maximum Junction Temperature..........................125C 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. SW Voltage..............................................................36V SHDN ........................................................................6V Operating Temperature Range ...............-40C to 85C Rev 1.10 2 January 18, 2010 HT7937 Electrical Characteristics Symbol VIN Parameter Input Voltage Switching IIN Supply Current Non-switching VSHDN= 0V VFB fOSC DC RDS(ON) ISW(OFF) VIH VIL VOVP Note: Feedback Voltage Switching Frequency Measurement at SW pin Maximum Duty Cycle SW On Resistance Switch Leakage Current SHDN Voltage High SHDN Voltage Low OVP Threshold No load 3/4 3/4 3/4 3/4 85 3/4 3/4 1.5 3/4 23 90 1.4 0.1 3/4 3/4 28 3 LEDs VSHDN=3V; VIN=3V; Ta=25C (Unless otherwise specified)(Note) Test Conditions 3/4 Min. 2.5 3/4 3/4 3/4 85 0.8 Typ. 3 1.0 50 0.1 95 1.2 Max. 5.5 1.25 100 1 105 1.6 3/4 5 1 3/4 0.3 33 Unit V mA mA mA mV MHz % W mA V V V Specifications are production tested at Ta=25 degree. Specifications over -40 to 85 degree operating temperature range are assured by design, characterization. Rev 1.10 3 January 18, 2010 HT7937 Functional Description * Shutdown The shutdown pin, SHDN, must not be allowed to float. When the SHDN pin voltage is taken below 0.3V, the internal MOSFET, voltage reference, error amplifier, comparators and biasing circuitry will all be switched off reducing the quiescent supply current to less than 1mA. If the SHDN pin has a value greater than 1.5V, then the device will be fully enabled and operational. This pin also can be used as a PWM signal from 100Hz to 1kHz to allow brightness control. * Over voltage protection - OVP A DC signal on the FB pin This method of dimming control uses a DC voltage circuit as shown in Figure 2. The LED brightness is directly proportional to the LED current which is given by the following equation: Where VFB = Feedback voltage is 95mV VDC = DC voltage R1 and R2 >> RFB L V IN With an open circuit output, such as when no LEDs are connected, the FB pin will be pulled down to ground via the sense resistor RFB. As the device will now react by trying to increase the output voltage by generating a maximum duty cycle signal, this may cause the SW pin to exceed its maximum rated voltage, which may damage the internal N-MOS switching transistor. The OVP function is designed to prevent damage to the internal NMOS switching transistor. When the output voltage rises above the OVP threshold voltage, typically 28V, the converter will clamp the output voltage to this level. When the output voltage returns to a value below the OVP threshold, it will automatically resume normal switching operation. * Dimming control D1 1N5819 SW OVP FB V DC C1 1mF 10mH V IN SHDN GND H T7937 C2 1mF U p to 6 W LEDs V FB R1 1kW R2 51kW R 4 .7 W FB Figure 2. Dimming Control Using a DC Voltage A filtered PWM signal on the FB pin For frequencies greater than 1kHz, dimming can be implemented by using the circuit shown in Figure 3. The PWM control circuitry is connected to the FB pin. Reducing the duty cycle on the PWM signal results in increased LEDs brightness levels. The LED brightness is directly proportional to the LED current which is given by the following equation: There are three methods to control the LEDs brightness as listed below: A PWM signal on the SHDN pin A PWM signal is applied to the SHDN pin as shown in Figure 1. The magnitude of the PWM signal should be higher than the enable voltage of the SHDN pin, the LEDs operate with either zero or full current. The average LED current is proportional to the duty cycle of the applied PWM signal with a duty cycle increase resulting in higher LEDs brightness. Typical PWM frequencies should be between 100Hz and 1kHz. L V IN Where VFB = Feedback voltage is 95mV VPWM = PWM high level voltage D = PWM duty cycle R1 and R2 >> RFB PWM frequency >> D1 1N5819 SW OVP FB R C2 1mF U p to 6 W LEDs V FB C1 1mF PW M f= 1 0 0 H z ~ 1 k H z 10mH V IN SHDN GND H T7937 L IN D1 1N5819 SW OVP FB V R2 51kW R3 5 .1 k W C3 0 .1 m F FB 4 .7 W C1 1mF 10mH V IN SHDN C2 1mF U p to 6 W LEDs R 4 .7 W FB Figure 1. Dimming Control with PWM Signal GND H T7937 VPW M 0V R1 1kW Figure 3. Dimming Control Using a Filter PWM Signal Rev 1.10 4 January 18, 2010 HT7937 Component Selection * Setting the LED Current The step-up converter regulates the LED current by regulating the voltage across the current sense resistor, RFB. To ensure the generation of accurate LED currents, it is recommended that a precision resistor is used for RFB. The voltage across the sense resistor is regulated to the internal reference voltage of 95mV. The LED current is calculated using the following equation: VFB ILED RFB Where VFB = Feedback voltage is 95mV * Inductor Selection Where IL(PEAK) = Peak Inductor Current CO(ESR) = Output Capacitors Equivalent Series Resistance IO = Output Current VO = Output Voltage Vi = Input Voltage CO = Output Capacitance FS = Switching Frequency is 1.2MHz Layout Considerations 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, please follow the guidelines below: * All tracks should be as wide as possible. * The input and output capacitors, C1 and C2, should The selected inductor must have a saturation current greater than the maximum peak current of the step-up converter. A recommended value of inductor for 3 to 6 white LED applications is 4.7mH to 22mH. For good efficiency the inductor should have low core loss and low DCR (DC Resistance). The peak inductor current is calculated using the following equation: be placed close to the VIN, VOUT and GND pins. * The Schottky diode, D1, and inductor, L1, must all be placed close to the SW pin. * Feedback resistor, R1, must be placed close to the FB and GND pins. * A full ground plane is always helpful for better EMI performance. Where VO = Output Voltage Vi = Input Voltage IO = Output Current h = Efficiency L = Inductance FS = Switching Frequency is 1.2MHz * Diode Selection A recommended PCB layout with component locations is shown below. The diode must have a rating greater than the output voltage and output current. In switching applications both forward voltage drop and diode capacitance are important considerations. A Schottky diode is used due to its low forward voltage drop and fast switching speeds which improves efficiency. The Schottky diode has average current of IO, and a peak current which is the same as the inductors peak current and a voltage rating at least 1.5 times the output voltage. A recommended Schottky diode type is the 1N5819. * Capacitor Selection Top Layer A low ESR (Equivalent Series Resistance) capacitor should be used for the input/output capacitors to minimize ripples. Both X5R and X7R types are suitable due to their wider voltage and temperature ranges. Input and output ceramic capacitors of 1mF are recommended for HT7937 applications. The output ripple voltage is calculated as the following equation: Bottom Layer Rev 1.10 5 January 18, 2010 HT7937 Typical Performance Characteristics 90 VIN=4V 85 VIN=3.5V VIN=3V VIN=5V 105 103 Reference Voltage (mV) 101 99 97 95 93 91 89 87 85 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 80 Efficiency(%) 75 70 L: 10uH, SR0602 10ML (ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 65 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Input Voltage (V) 60 0 5 10 LED Current (mA) 15 20 Input Voltage VS Reference Voltage 3 WLEDs Efficiency vs. LED Current 90 VIN=5V 2.0 Switching Frequency (MHz) 85 1.8 1.6 1.4 1.2 1.0 0.8 -45 -30 -15 0 15 30 ) 45 60 75 90 Temperature ( 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 80 VIN=4V VIN=3V Efficiency(%) VIN=3.5V 75 70 L: 10uH, SR0602 10ML (ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 65 60 0 5 10 LED Current (mA) 15 20 Temperature VS Switching Frequency 5 WLEDs Efficiency vs. LED Current 105 103 95 94 Switching Duty Cycle (%) 93 92 91 90 89 88 87 86 85 -45 -30 -15 0 15 30 ) 45 60 75 90 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF Reference Voltage (mV) 101 99 97 95 93 91 89 87 85 Temperature ( 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF -45 -30 -15 0 15 30 ) 45 60 75 90 Temperature ( Temperature VS Reference Voltage Temperature VS Switching Duty Cycle Rev 1.10 6 January 18, 2010 HT7937 Start-Up from Shutdown Waveform Note: VIN=3.6V, 3 WLEDs, L=10mH, D= 1N5819, Ci=Co=1mF, ILED=20mA Note: Operation Waveform VIN=3.6V, 3 WLEDs, L=10mH, D=1N5819, Ci=Co=1mF, ILED=20mA Start-Up from Shutdown Waveform Note: VIN=3.6V, 5 WLEDs, L=10mH, D=1N5819, Ci=Co=1mF, ILED=20mA Note: Operation Waveform VIN=3.6V, 5 WLEDs, L=10mH, D=1N5819, Ci=Co=1mF, ILED=20mA 100% 80% 90 Efficiency(%) Average ILED / ILED Max 95 VIN=3.6V, L=10uH, Ci=Co=1uF, 3LEDs 60% 100Hz 200Hz 500Hz 40% 1KHz 85 80 75 70 65 60 2.5 3.0 3.5 VIN(V) 4.0 4.5 5.0 6S1P 3S2P 20% 0% Efficiency vs. VIN 0% 20% 40% 60% 80% 100% Shutdown PIN PWM Duty Dimming Control by Shutdown PIN Rev 1.10 7 January 18, 2010 HT7937 3 WLEDs, VIN=3.6V, f=200Hz, Duty=50% 3 WLEDs, VIN=3.6V, f=1kHz, Duty=50% 3 WLEDs, VIN=3.6V, f=500Hz, Duty=50% Rev 1.10 8 January 18, 2010 HT7937 Application Circuits 6 WLEDs Application Circuit V L IN D1 1N5819 SW OVP FB U p to 6 W LEDs C2 1mF C1 1mF 10mH V IN SHDN GND H T7937 20m A R 4 .7 W FB N o te : "L " S R 0 6 0 2 1 0 0 M L B ( A B C T a iw a n E le c tr o n ic s C o r p .) Dimming Control with a PWM Signal L V IN D1 1N5819 SW OVP FB U p to 6 W LEDs C2 1mF C1 1mF PW M 10mH V IN SHDN GND H T7937 R 4 .7 W FB Dimming Control Using a DC Voltage L V IN D1 1N5819 SW OVP FB VDC 0V~5V R2 51kW R1 1kW U p to 6 W LEDs C2 1mF C1 1mF 10mH V IN SHDN GND H T7937 R 4 .7 W FB Dimming Control Using a Filter PWM Signal L V IN D1 1N5819 SW OVP FB R2 51kW R3 5 .1 k W C3 0 .1 m F R1 1kW U p to 6 W LEDs C2 1mF C1 1mF 10mH V IN SHDN GND H T7937 PW M R 4 .7 W FB Rev 1.10 9 January 18, 2010 HT7937 Package Information 6-pin SOT23-6 Outline Dimensions D C L E H e A2 b A1 A Symbol A A1 A2 b C D E e H L q Dimensions in mm Min. 1.0 3/4 0.7 0.35 0.10 2.7 1.4 3/4 2.6 0.37 1 Nom. 3/4 3/4 3/4 3/4 3/4 3/4 3/4 1.9 3/4 3/4 3/4 Max. 1.3 0.1 0.9 0.50 0.25 3.1 1.8 3/4 3.0 3/4 9 Rev 1.10 10 q January 18, 2010 HT7937 Product Tape and Reel Specifications Reel Dimensions T2 D A B C T1 SOT23-6 Symbol A B C D T1 T2 Description Reel Outer Diameter Reel Inner Diameter Spindle Hole Diameter Key Slit Width Space Between Flange Reel Thickness Dimensions in mm 178.01.0 62.01.0 13.00.2 2.500.25 8.4+1.5/-0.0 11.4+1.5/-0.0 Rev 1.10 11 January 18, 2010 HT7937 Carrier Tape Dimensions D E F P0 P1 t W C B0 D1 P A0 K0 R e e l H o le IC M a r k in g SOT23-6 Symbol W P E F D D1 P0 P1 A0 B0 K0 t C Description Carrier Tape Width Cavity Pitch Perforation Position Cavity to Perforation (Width Direction) Perforation Diameter Cavity Hole Diameter Perforation Pitch Cavity to Perforation (Length Direction) Cavity Length Cavity Width Cavity Depth Carrier Tape Thickness Cover Tape Width Dimensions in mm 8.00.3 4.00.1 1.750.1 3.500.05 1.5+0.1/-0.0 1.5+0.1/-0.0 4.00.1 2.000.05 3.150.1 3.20.1 1.40.1 0.200.03 5.30.1 Rev 1.10 12 January 18, 2010 HT7937 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 Tel: 1-510-252-9880 Fax: 1-510-252-9885 http://www.holtek.com Copyright O 2010 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. Holteks 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 1.10 13 January 18, 2010 |
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