HT7937 Built-in OVP White LED Step-up Converter Features · Integrated open-circuit protection · Low standby current: 0.1mA (typ.) (VSHDN=0V) · Over voltage protection · Matches LED current · Up to 84% efficiency at VIN=3V, 3LEDs, · Small value inductor and capacitors ILED= 20mA · 1.2MHz fixed switching frequency · Small outline SOT23-6 package Applications · Cellular phones · Handheld devices · PDAs · White LED display backlighting · DSCs 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. Selection Table Note: Part No. Package Marking HT7937 SOT23-6 7937# 7937+ Both lead free and green compound devices are available. ²#² stands for Lead-free devices. ²+² stands for green compound devices, which are Lead-free and Halogen-free. Rev 1.40 1 November 6, 2013 HT7937 Block Diagram O V P 2 8 V S W S H D N C o n tro l L o g ic F B 9 5 m V å G N D 1 .2 M H z O s c illa to r V IN Pin Assignment S O T 2 3 -6 V IN 6 O V P 5 S H D N 4 7 9 3 7 1 2 S W G N D T o p V ie w 3 F B Pin Description Pin No. Pin Name Description 1 SW 2 GND 3 FB 4 SHDN 5 OVP For over voltage protection connect to the output. 6 VIN Input voltage pin. 2.5V to 5.5V for internal circuitry. Rev 1.40 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). 2 November 6, 2013 HT7937 Absolute Maximum Ratings Input Voltage..............................................................6V SW Voltage..............................................................36V FB Voltage .................................................................6V SHDN ........................................................................6V OVP Voltage ............................................................36V Operating Temperature Range ...............-40°C to 85°C Maximum Junction Temperature..........................125°C Note: These are stress ratings only. Stresses exceeding the range specified under ²Absolute Maximum Ratings² 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 Symbol VIN Parameter VSHDN=3V; VIN=3V; Ta=25°C (Unless otherwise specified)(Note) Test Conditions Min. Typ. Max. Unit ¾ 2.5 3 5.5 V Switching ¾ 1.0 1.25 mA Non-switching ¾ 50 100 mA VSHDN=0V ¾ 0.1 1 mA 3 LEDs 85 95 105 mV 0.8 1.2 1.6 MHz 85 90 ¾ % Input Voltage IIN Supply Current VFB Feedback Voltage fOSC Switching Frequency DC Maximum Duty Cycle RDS(ON) SW On Resistance ¾ ¾ 1.4 5 W ISW(OFF) Switch Leakage Current ¾ ¾ 0.1 1 mA VIH SHDN Voltage High ¾ 1.5 ¾ ¾ V VIL SHDN Voltage Low ¾ ¾ ¾ 0.3 V VOVP OVP Threshold 23 28 33 V Note: Measurement at SW pin No load Specifications are production tested at Ta=25 degree. Specifications over -40 to 85 degree operating temperature range are assured by design, characterization. Rev 1.40 3 November 6, 2013 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. 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 · Over voltage protection - OVP R1 and R2 >> RFB 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. V C 1 1 m F ¨ P W M f= 1 0 0 H z ~ 1 k H z S W S H D N O V P G N D H T 7 9 3 7 F B V D C F B U p to 6 W L E D s R 1 1 k W R R 2 5 1 k W F B 4 .7 W Where VFB = Feedback voltage is 95mV VPWM = PWM high level voltage D = PWM duty cycle R1 and R2 >> RFB PWM frequency >> U p to 6 W L E D s R V F B 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: C 2 1 m F 1 N 5 8 1 9 O V P 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. D 1 1 0 m H V IN C 2 1 m F 1 N 5 8 1 9 S W Figure 2. Dimming Control Using a DC Voltage 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. IN V IN H T 7 9 3 7 A PWM signal on the SHDN pin A PWM signal is applied to the SHDN pin as shown in Figure 1. V D 1 1 0 m H G N D There are three methods to control the LEDs brightness as listed below: L C 1 1 m F S H D N · Dimming control ¨ L IN V F B 4 .7 W IN L C 1 1 m F D 1 1 0 m H V IN S W S H D N O V P G N D Figure 1. Dimming Control with PWM Signal H T 7 9 3 7 V P W M 0 V C 2 1 m F 1 N 5 8 1 9 V F B F B R 1 1 k W R 2 5 1 k W R 3 5 .1 k W R U p to 6 W L E D s F B 4 .7 W C 3 0 .1 m F Figure 3. Dimming Control Using a Filter PWM Signal Rev 1.40 4 November 6, 2013 HT7937 Component Selection Where IL(PEAK) = Peak Inductor Current CO(ESR) = Output Capacitor¢s Equivalent Series Resistance IO = Output Current VO = Output Voltage Vi = Input Voltage CO = Output Capacitance FS = Switching Frequency is 1.2MHz · 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 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: · Inductor Selection 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: · All tracks should be as wide as possible. · The input and output capacitors, C1 and C2, should 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. A recommended PCB layout with component locations is shown below. Where VO = Output Voltage Vi = Input Voltage IO = Output Current h = Efficiency L = Inductance FS = Switching Frequency is 1.2MHz · Diode Selection 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 inductor¢s peak current and a voltage rating at least 1.5 times the output voltage. A recommended Schottky diode type is the 1N5819. Top Layer · Capacitor Selection 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 1.40 5 November 6, 2013 HT7937 Typical Performance Characteristics 3 WLEDs Efficiency vs. LED Current 105 90 103 VIN=5V Reference Voltage (mV) VIN=4V 85 Efficiency(%) 80 VIN=3.5V VIN=3V 75 101 99 97 95 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 93 91 89 87 70 85 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) Input Voltage VS Reference Voltage 60 0 5 10 15 20 LED Current (mA) 3 WLEDs Efficiency vs. LED Current 5 WLEDs Efficiency vs. LED Current 90 2.0 Switching Frequency (MHz) VIN=5V 85 Efficiency(%) 80 VIN=4V VIN=3.5V VIN=3V 75 70 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 1.6 1.4 1.2 1.0 0.8 L: 10uH, SR0602 10ML (ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 65 1.8 -45 5 10 15 -15 0 15 30 Temperature ( 60 0 -30 ¢J) 45 60 75 90 Temperature VS Switching Frequency 20 LED Current (mA) 105 95 103 94 Switching Duty Cycle (%) Reference Voltage (mV) 5 WLEDs Efficiency vs. LED Current 101 99 97 95 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 93 91 89 87 93 92 91 90 3 WLEDs, VIN=3V L: 10uH, SR0602 10ML(ABC Electronics Corp.) D1: 1N5819 C1, C2: 1uF 89 88 87 86 85 85 -45 -30 -15 0 15 30 Temperature ( ¢J) 45 60 75 90 -45 -15 0 15 30 Temperature ( ¢J) 45 60 75 90 Temperature VS Switching Duty Cycle Temperature VS Reference Voltage Rev 1.40 -30 6 November 6, 2013 HT7937 Start-Up from Shutdown Waveform Note: Operation Waveform Note: VIN=3.6V, 3 WLEDs, L=10mH, D= 1N5819, Ci=Co=1mF, ILED=20mA VIN=3.6V, 3 WLEDs, L=10mH, D=1N5819, Ci=Co=1mF, ILED=20mA Start-Up from Shutdown Waveform Note: Operation Waveform Note: VIN=3.6V, 5 WLEDs, L=10mH, D=1N5819, Ci=Co=1mF, ILED=20mA VIN=3.6V, 5 WLEDs, L=10mH, D=1N5819, Ci=Co=1mF, ILED=20mA 100% 80% Average ILED / ILED Max 95 Efficiency(%) 90 85 80 6S1P 3S2P 75 70 65 60 VIN=3.6V, L=10uH, Ci=Co=1uF, 3LEDs 60% 100Hz 200Hz 500Hz 1KHz 40% 20% 2.5 3.0 3.5 4.0 4.5 5.0 VIN(V) 0% Efficiency vs. VIN 0% 20% 40% 60% 80% 100% Shutdown PIN PWM Duty Dimming Control by Shutdown PIN Rev 1.40 7 November 6, 2013 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 1.40 8 November 6, 2013 HT7937 Application Circuits 6 WLEDs Application Circuit V IN L C 1 1 m F U p to 6 W L E D s D 1 1 0 m H V IN S H D N C 2 1 m F 1 N 5 8 1 9 S W O V P G N D 2 0 m A F B R H T 7 9 3 7 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 .) F B 4 .7 W Dimming Control with a PWM Signal V IN L C 1 1 m F P W M U p to 6 W L E D s D 1 1 0 m H V IN 1 N 5 8 1 9 S W S H D N C 2 1 m F O V P G N D F B H T 7 9 3 7 R F B 4 .7 W Dimming Control Using a DC Voltage V IN L C 1 1 m F U p to 6 W L E D s D 1 1 0 m H V IN 1 N 5 8 1 9 S W S H D N O V P G N D F B H T 7 9 3 7 R 1 1 k W V D C 0 V ~ 5 V R R 2 5 1 k W C 2 1 m F F B 4 .7 W Dimming Control Using a Filter PWM Signal V IN L C 1 1 m F 1 0 m H V IN 1 N 5 8 1 9 S W S H D N O V P G N D R 1 1 k W F B H T 7 9 3 7 R 2 5 1 k W P W M Rev 1.40 U p to 6 W L E D s D 1 R 3 5 .1 k W 9 R C 2 1 m F F B 4 .7 W C 3 0 .1 m F November 6, 2013 HT7937 Note: As the above application circuits are unable to show the full drive capabilities and series/parallel drive combinations of this device the following supplemental information is provided. For the general full voltage range situation: 1. Vin Variable Voltage of 2.6V~5.5V 2. LED forward voltage of 3.5V max. Maximum LED Drive Capability Series Parallel Total 3 2 6 4 2 8 5 1 5 6 1 6 It is also important to note that for an input voltage supply which can be maintained at 5.0V or higher, the device possesses even higher drive capabilities which can drive a higher number of parallel connected LEDs. The following table depicts the maximum LED numbers possible for this higher input voltage range. 1. Vin Variable Voltage of 5.0V~5.5V 2. LED forward voltage of 3.5V max. Maximum LED Drive Capability Rev 1.40 Series Parallel Total 3 3 9 4 2 8 5 1 5 6 1 6 10 November 6, 2013 HT7937 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 for the latest version of the package information. Additional supplementary information with regard to packaging is listed below. Click on the relevant section to be transferred to the relevant website page. · Further Package Information (include Outline Dimensions, Product Tape and Reel Specifications) · Packing Meterials Information · Carton information · PB FREE Products · Green Packages Products Rev 1.40 11 November 6, 2013 HT7937 6-pin SOT23-6 Outline Dimensions Symbol A ¾ A1 ¾ A2 0.035 Nom. Max. ¾ 0.057 ¾ 0.006 0.045 0.051 b 0.012 ¾ 0.020 C 0.003 ¾ 0.009 D ¾ 0.114 BSC ¾ E ¾ 0.063 BSC ¾ e ¾ 0.037 BSC ¾ e1 ¾ 0.075 BSC ¾ H ¾ 0.110 BSC ¾ L1 ¾ 0.024 BSC ¾ q 0° ¾ 8° Symbol Rev 1.40 Dimensions in inch Min. Dimensions in mm Min. Nom. Max. A ¾ ¾ 1.45 A1 ¾ ¾ 0.15 A2 0.90 1.15 1.30 b 0.30 ¾ 0.50 C 0.08 ¾ 0.22 D ¾ 2.90 BSC ¾ E ¾ 1.60 BSC ¾ e ¾ 0.95 BSC ¾ e1 ¾ 1.90 BSC ¾ H ¾ 2.80 BSC ¾ L1 ¾ 0.60 BSC ¾ q 0° ¾ 8° 12 November 6, 2013 HT7937 Copyright Ó 2013 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 1.40 13 November 6, 2013