TB62756FUG TOSHIBA BiCD Digital Integrated Circuit Silicon Monolithic TB62756FUG Step-up Type DC/DC Converter for White LEDs The TB62756FUG is a high efficiency step-up type DC/DC converter that is designed especially for use as a constant current driver of white LEDs. It is possible to drive 2 - 6 white LEDs connected in series using a lithium-ion battery. This IC incorporates an N-ch-MOS transistor requiredfor switching of an external inductor. The forward current of the LEDs can be controlled by an external resistor. A pulse input system (PWM) can be used as a brightness control function. This IC is best suited for use as a driver of white LED back lighting in color LCDs in PDAs, cellular phones and handy terminal devices. This device is Pb-free product. Weight: 0.016 g (typ.) Features * * * * * * Can drive 2-6 white LEDs connected series Variable LED Current IF is set with a external resistor : 20 mA (Typ.) @RSENS = 16 Output Power : Available for 400mW LED loading High Efficiency : 87% @Maximum (Using recommended external parts: Typ. 4LEDs) IC Package : SSOP6-P-0.95 (SOT23-6) Switching Frequency : 1.1 MHz (Typ.) Company Headquarters 3 Northway Lane North Latham, New York 12110 Toll Free: 800.984.5337 Fax: 518.785.4725 Web: www.marktechopto.com | Email: info@marktechopto.com California Sales Office: 950 South Coast Drive, Suite 225 Costa Mesa, California 92626 Toll Free: 800.984.5337 Fax: 714.850.9314 TB62756FUG Block Diagram 4 VIN 3 Monostable multivibrator for reference Monostable multivibrator for off time control CTL AMP. SW NC 2 Circuit on/off SHDN 1 Error AMP. 6 FB 5 GND Pin Assignment (top view) SHDN 1 6 FB Q (NC) 2 5 GND 4 SW VIN 3 Note 1: The IC may break if mounted 180 degrees in reverse. Ensure the device is correctly orientated before assembley. Pin Functions No. Symbol Function Input pin for IC ON/OFF control and variable LED IF. SHDN=H Operation Mode SHDN=L Shutdown Mode (IC shutdown) 1 SHDN PWM signal input for IF control (see p.5) This pin must be set to a certain logic level, as unstable output could result if the pin is left open. 2 3 4 5 6 NC VIN SW GND FB No Connection or Connected to GND. (Note 2) Supply voltage pin. Supply voltage range : 2.8V to 5.5V DC-DC converter switching pin - switch incorporates N-ch MOSFET. Ground pin. Connected to the cathode of LED. Note 2: The NC terminal is not connected to the internal circuit. 2 2005-11-8 TB62756FUG I/O Equivalent Pin Circuits 1. SHDN pin VIN 2. NC pin SHDN 1 NC 2 The NC pin is not connected to any internal circuit. 3. SW pin 4. FB pin SW 4 VIN FB 6 3 2005-11-8 TB62756FUG Application Circuit Example VIN 4.7-10H 4 VIN 3 Monostable multivibrator for reference SW 2 NC Monostable multivibrator for off time control CTL AMP. Vz=24V 2.2F Circuit on/off 500 Error AMP. 1F 6 FB 16 @20mA SHDN PWM 1 5 GND Protection at the time of LED opening The zener diode in the application circuit example is necessary for the provision of over-voltage protection for when the LED becomes open. As the IC does not incorporate a voltage protection circuit, it is strongly advised that a zener diode be connected. The zener diode should satisfy the following conditions: i) Less than maximum output voltage of 24V ii) Greater than the total series LED VF iii) Less than the maximum output capacitance C2. Moreover, by connecting a protection circuit such as R_ZD in the figure below, it is possible to control the output current when the LED becomes open, and to use a zener diode of lower tolerance. An example of IZD control by R_ZD connection. (RSENS = 16 ) S-Di R_DZ () 500 100 IZD (mA) 0.6 (Typ.) 2.8 (Typ.) R_ZD GND FB RSENS SW IZD IF 1 F C2 In order to avoid adverse effects on driver characteristics, Toshiba recommends a resistance of 500 ohms or less. Protection circuit application 4 2005-11-8 TB62756FUG Output-side Capacitor Setting It is recommended that the value of C2 be equal to, or greater than 1.0 (F). External Inductor Size Setting For each number of LEDs, the selected inductance should be greater than the value indicated in the table below. Number of LEDs 2 3 4 5 6 10 Inductance (Unit: H) 4.7 6.8 IF = 20 mA Note Control of IF The resistance RSENS is connected between the FB pin and the GND pin. The average current is controlled by the RSENS value, and calculated using the following equation: IF (mA) = [ 325mV / RSENS() ] Margin of error is 5%. Dimming using PWM signal input A dimming function can also by applied using a PWM signal. [Notes] When using a PWM signal, the minimum pulse width of the PWM should be greater than 33s. Duty ratio of PWM function should be set at 10% - 90%. The recommended PWM frequency should be 100Hz - 10kHz. < |