tm TE CH T6309A Micropower Step-up DC/DC Converter FEATURES GRNERAL DESCRIPTION • Matched LED Current with Overvoltage Protection • Drives up to 6 LEDs in series • Up to 85% Efficiency • Low 400mV Feedback Reference, Optional 1.19V Feedback Reference • Internal Current Limit 300mA The T6309A is a step-up DC/DC converter that drives LED with a constant current. The device can driver up to 6 series LEDs from a single-cell Li-Ion battery. Series connection of the LEDs provides constant LED current that results in uniform brightness. The T6309A features a minimum off-time current-limited PFM control scheme. The LED current is set with an external sense resistor (Rs) and is regulated by the feedback pin (FB) that regulates the voltage across the sense resistor to 400mV (typ). The low 400mV feedback reference voltage reduces power loss and improves efficiency for LED driver applications. Optional 1.19V feedback reference is available for normal output voltage applications like LCD bias. Output overvoltage protection (OVP) is integrated to prevent damage in case of output open circuit condition (e.g. faulty LED). The OVP pin can monitor the output voltage and turn off the converter in the event of overvoltage condition. The T6309A is available in SOT23-6 packages. • Under Voltage Lock Out Circuits • 0.1uA Low Shutdown Supply Current • 6-pin SOT-23 Package • ESD Human Body Mode Over 5KV APPLICATIONS • Cellular Phones • Portable Communication Devices • Handheld Electronics • PDAs • LED/Display Back Light Driver PART NUMBER EXAMPLES T6309A-AX Feedback Referemce Voltage 400mV T6309A-BX 1.19V Part no. TM Technology Inc. reserves the right to change products or specifications without notice. P. 1 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A PIN ARRANGEMENT SOT23-6 (Top view) SW 1 6 VIN GND 2 5 OVP FB 3 4 CE PIN DESCRIPTION SYMBOL Pin No. SW 1 GND 2 FB 3 CE 4 OVP 5 VIN 6 DESCRIPTION Switch pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. Ground pin. Connect to local ground plane. Feedback pin. Reference voltage is 400mV (T6309A-A). Optional reference voltage is 1.19V (T6309A-B). Connect cathode of lowest LED and current sense resistor here. Chip enable pin. Tie CE pin to 1.5V or higher to enable device; 0.4V or less to shutdown the device. Do not float this pin. Overvoltage protection. Connect his pin to the output capacitor of the converter. Input supply pin. Must be locally bypassed. BLOCK DIAGRAM V IN m in im u m o ff-tim e o n e -sh o t u n d e r v o lta g e lo c k o u t o v e rv o lta g e p ro te c tio n + - e rro r a m p lifie r F B + O V P 1 .1 9 V S W c o n tro l lo g ic b u ffe r d riv e r v o lta g e re fe re n c e G N D c u rre n t lim it C E Fig. 1 TM Technology Inc. reserves the right to change products or specifications without notice. P. 2 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A Absolute Maximum Ratings Supply Voltage …………………………………………………………. -0.3V to 5V SW Pin Switch Voltage ………………………………………………… -0.3V to 31V OVP Pin Overvoltage Protection ……………………………………….. -0.3V to 31V CE Pin Voltage …………………………………………………………. -0.3V to (VDD+0.3V) SW Pin Switch Current ………………………………………………… 300mA Continuous power dissipation, PD @ TA = 25°C SOT-23-6 …………. 0.35W Package Thermal Resistance SOT-23-6, θJA ………………………….. 150°C /W Operating Juction Temperature ………………………………………… 150°C Operating temperature range …………………………………………… -40°C to +85°C Storage temperature range, TSTG ……………………………………… -55°C to +150°C Lead temperature (soldering, 10sec) …………………………………… 260°C ESD Human Body Mode Over ……………………………………………5KV Electrical Characteristics ( VIN=+3V, TA = 25°C, unless otherwise noted ) Symbol VIN Description Conditions Input Voltage VREF Reference Voltage Min. Typ. Max 2.5 5 V 460 mV T6309A-A 340 T6309A-B 1.15 1.19 1.23 V 300 ns TOFF Switch Minimum Off Time VOVP Over voltage Threshold 400 Unit 400 22 V uA IDD2 Switch Off Current (VOUT) FB=0.5V 60 IOFF Shutdown Current (VIN) CE=0V 0.1 VIHCE CE Input Voltage Hight 550 1 1.5 uA V VILCE CE Input Voltage Low 0.4 IICE CE Input Bias Current 0.1 uA IIFB FB Input Bias Current 0.1 uA UVLO Under Voltage Lockout 1.55 1.75 1.95 V RDS(on) Switch ON Resistance 0.6 ohm 300 mA ILIM Current Limit TM Technology Inc. reserves the right to change products or specifications without notice. P. 3 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A Typical Operating Characteristics (VIN=+3.3V,Load=4 LEDs, L=10uH, CIN=4.7uF, COUT=4.7uF, T A=+25 ºC, unless otherwise noted.) Io=20mA VFB Temperature curve 90 0.43 85 0.42 80 VFB(V) Efficiency(%) Efficiency (4WLED) 75 70 Vin=3.1V Load 4LED 0.41 0.4 0.39 65 60 0.38 2.0 2.5 3.0 Stability for driving 4LED / Vin=3.0V 3.5 4.0 Vin(V) 4.5 5.0 (time=1.0us/Div) -30 -20 -10 0 10 20 30 40 50 60 70 Temperature (°C) Stabilityfor driving 4LED/ Vin=3.6V Vout 1.0V/Div Vout 1.0V/Div Vsw 5.0V/Div Vsw 5.0V/Div Stability for driving 4LED/ Vin=5.2V (time=1.0us/Div) (time=1.0us/Div) Vout 1.0V/Div Vsw 5.0V/Div TM Technology Inc. reserves the right to change products or specifications without notice. P. 4 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A Typical Operating Characteristics (VIN=+3.3V,Load=6 LEDs, L=10uH, CIN=4.7uF, COUT=4.7uF, T A=+25 ºC, unless otherwise noted.) Efficiency (6WLED) - Io=15mA Vin=3.1V Load 6LED VFB Temperature curve 90 0.43 0.42 80 VFB(V) Efficiency(%) 85 75 70 0.41 0.4 65 0.39 60 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.38 -30 Vin(V) Stability for driving 6LED/Vin=3.0V Vout Vsw -20 -10 0 10 20 30 40 50 60 70 Temperature (ºC) Stability for driving 6LED/Vin=3.6V Vout Vsw Stability for driving 6LED/Vin=5.2V Vout Vsw TM Technology Inc. reserves the right to change products or specifications without notice. P. 5 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A transferred to output filter capacitor and the load. As the energy stored in the inductor is depleted, the current ramps down and the output diode turns off. The output filter capacitor stores the charge while the inductor current is higher than the output current, then sustains the output voltage until the next switching cycle. Functional Description The T6309A is like a standard current-limit PFM step-up DC-DC converter, but regulates the voltage across the sense resistor Rs instead of the output voltage. This results in a constant LED current regardless the input voltage and number of LEDs connected. With integrated overvoltage protection (OVP), the T6309A can be used as a current source with overvoltage protection to drive LEDs. This allows up to 6 LEDS to be connected in series to the output. The low 400mV feedback reference voltage reduces power loss and improves efficiency. Current Limit The T6309A utilizes cycle-by-cycle current limiting by means of protecting the output MOSFET switch from overstress and preventing the small value inductor from saturation. Current limiting is implemented by monitoring the output MOSFET current build-up during conduction, and upon sensing an over-current conduction immediately turning off the switch for the duration of the oscillator cycle. The current through the output MOSFET is monitored and compared against a reference signal. When the threshold is reached, a signal is sent to the PFM controller block to terminate the power switch conduction. The current limit threshold is typically set at 300mA. Step-Up Converter The step-up DC-DC converter operation can be understood by referring to the block diagram in Figure 1. PFM comparator monitors the output voltage via the sense resistor. When the feedback voltage is higher than the reference voltage, the MOSFET switch is turned off. As the feedback voltage is lower than reference voltage and the MOSFET switch has been off for at least a period of minimum off-time decided by the minimum off-time one-shot, the MOSFET switch is then turned on one-shot, or until the current limit signal is asserted. During the internal MOSFET switch is turned on, current ramps up in the inductor and store energy in a magnetic field. When the MOSFET is turned off, the voltage across the inductor reverses and forces current through the diode to the output filter capacitor and load, so the energy in the inductor is TM Technology Inc. reserves the right to change products or specifications without notice. Overvoltage Protection Since T6309A is configured as current source, the output voltage rises as the output impedance increases or output is open-circuit (e.g. fault LED). The output voltage may exceed the 22 V maximum voltage rating of the internal main switch. An overvoltage protection circuit is integrated to prevent the main switch from burning. When the output P. 6 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A voltage exceeds the OVP threshold voltage, the main switch is turned off. It remains off until the output voltage falls below the OVP threshold voltage. The step-up converter continues normal operation as long as the output voltage is under the OVP threshold. Shutdown The T6309A enters shutdown to reduce quiescent current under 1uA when CE pin is low . For normal operation, drive CE high. During shutdown, the reference, all feedback and control circuitry are off. The boost converter’s output drops to one Schottky diode voltage drop below the input voltage and SW remains high impedance. The capacitance and load at VOUT determine the rate at which VOUT decays. The CE pin can also be used as dimming control. More details are in the dimming control section. Undervoltage Lockout An undervoltage lockout (UVLO) circuit prevents fault operation of the T6309A when input voltage is under 1.75V (typ). The device remains off as long as the input voltage is under the UVLO threshold. TM Technology Inc. reserves the right to change products or specifications without notice. P. 7 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A • Select the maximum LED current IMAX and 0mA as the minimum LED current. • Calculate R2 to achieve a feedback current in the range of I1 = 3uA to 10uA as the LEDs are fully turned on: R2 = VREF / I1 (e.g. 0.4V/4uA =100KOhm ) • Calculate R1 to meet the equation: R1 / (R1+R2) = VREF / VMAX (e.g. VREF=0.4V, VMAX=2.0V, R2=100KOhm, the calculated R1 is 25KOhm) • Calculate the sense voltage Vs at maximum Applications Information Dimming Control There are several types of dimming control circuit as follows: 1. Using a PWM signal to CE pin When using a PWM signal at CE pin, the T6309A is turned on or off by the PWM signal. The average LED current increases proportionally with the duty cycle of the PWM signal. The typical frequency range of the PWM signal is 150Hz to 250Hz. The magnitude of the PWM signal should be greater than the threshold voltage of CE voltage high. LED current: Vs = VREF x (1 + R1/R2) (e.g. Vs = 0.4V x (1 + 25K/100K) = 0.5V) • Calculate the sense resistor Rs: Rs = Vs / IMAX (e.g. 0.5V/20mA = 25 Ohm) 2. Using a DC voltage A DC voltage signal can be used as well to control the LED brightness. The dimming control method is shown in the figure below. When the DC voltage is zero, the LED is fully turned on. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on R1 decreases. Thus the LED current decreases. The selection of R1 and R2 should make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 2V, the selection of resistors in this figure gives dimming control of LED current from 0mA to 15mA. Following steps are used to select the resistor values: • Select the voltage VMAX to turn the LEDs off. (e.g. 2.0V) • Select 0V as default to turn the LEDs fully on. TM Technology Inc. reserves the right to change products or specifications without notice. 3. Using a Filtered PWM signal The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the DC voltage source if adjustable analog signal is not available in the system. The circuit is shown in the figure below: T6309A PWM Signal 0.1nF P. 8 FB 10K R2 100K R1 25K RS 25 Ohm Publication Date: MAY. 2007 Revision: A tm TE CH T6309A 4. Using a Logic Signal The LED current can be controlled in discrete steps with a logic signal as shown in the figure below. When the NMOS is off, R1 set the minimum current. When the NMOS is on, R2 sets the LED current increment value. Output Voltage Selection Referring to Fig. 4, select an output voltage for T6309A-B (1.19V reference) by connecting FB to a resistive divider between the output and GND. The VOUT can be set as: VOUT = (1+R1/R2) x 1.19V Higher R1,R2 values reduce quiescent current, but give bad noise immunity. To keep stable feedback loop operation and better noise immunity, select (R1+R2) value less than 1MOhm. T6309A FB Logic Signal Inductor Selection The T6309A is designed to work well with a 10uH inductor in most applications. Low inductance values supply higher output current, but also increase the ripple and reduce efficiency. Higher inductor values reduce ripple and improve efficiency, but also limit output current. Choose a low DCresistance inductor, usually less than 1 Ohm to minimize loss. It is necessary to choose an inductor with saturation current greater than the peak current that the inductor will R1 R2 Start-Up and Inrush Current The T6309A has no internal soft start circuit included to achieve minimum start-up delay. All inductive step-up converters have higher inrush current during start-up. The inrush current for T6309A is about 200mA. If softstart is required, a recommended external circuit is shown in the below figure. encounter in the application. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support and inductance falls. Inductor with 600mA rating or greater would be suitable for the T6309A. D1 2.2nF T6309A D2 1K FB GND 4.7K RS TM Technology Inc. reserves the right to change products or specifications without notice. Capacitor Selection The input capacitor stabilizes the input voltage and minimizes the peak current ripple from the source. The value of the capacitor depends on the impedance of the input source P. 9 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A used. Small ESR ( Equivalent Series Resistance) ceramic capacitor with value of 1uF to 4.7uF would be suitable. The output capacitor is used to sustain the output voltage when the internal MOSFET is switched on and smoothing the ripple voltage. The larger the output ripple, the larger the line regulation, which means the LED current changes if the input voltage changes. Low ESR capacitor should be used to reduce output ripple voltage. Use a 4.7uF to 10uF ceramic output capacitor with about 50mOhm to 150mOhm ESR to provide stable switching and good line regulation. voltage larger than output voltage. Schottky Diode Selection The diode is the largest source of loss in DC-DC converters. The most important parameters which affect the efficiency are the forward voltage drop, VF, and the reverse recovery time. The forward voltage drop creates a loss just by having a voltage across the device while a current flowing through it. The reverse recovery time generates a loss when the diode is reverse biased, and the current appears to actually flow backwards through the diode due to the minority carriers being swept from the P-N junction. A Schottky diode with the following characteristics is recommended: Small forward voltage, VF = 0.3 V Small reverse leakage current Fast reverse recovery time/switching speed Rated current larger than peak inductor current Reverse The output capacitor should be placed close to the output terminals to obtain better smoothing effect on the output ripple. In addition, the ground connection for the sense resistor Rs should be tied directly to the GND pin and not shared with any other component. This ensures a clean, noise-free connection. TM Technology Inc. reserves the right to change products or specifications without notice. Layout Considerations High switching frequencies make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close to the device as possible, and keep their traces short, direct, and wide to reduce power loss so as to improve efficiency. Connect their ground pins at a single common node in a star ground configuration, or at a full ground plane. P. 10 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A TYPICAL APPLICATION CIRCUITS ILE D = 2 0 m A V IN 2 .0 to 5 .5 V L1 10uH D1 1 6 C IN 4 .7 u F 4 V IN SW OVP 5 T6309A CE ON FB GND 2 O FF 3 RS 20 O hm C OUT 4 .7 u F Fig. 2 Typical application for driving 4 white LEDs ILE D = 2 0 m A V IN 2 .0 to 5.5 V L1 1 0u H D1 1 6 C IN 4 .7 u F 4 V IN SW OVP 5 R2 1 00 K T6309A CE ON FB GND 2 O FF D im m in g C ontrol 0V ~ 2 V 3 C OUT 4 .7u F R1 25 K RS 25 O hm Fig. 3 White LED application with DC voltage dimming control V IN 2 .0 to 5 .5 V L1 10uH D1 20m A 1 6 C IN 4 .7 u F 4 ON O FF V IN SW OVP 5 R1 450K T6309A CE FB GND 2 3 C OUT 4 .7 u F R2 33K Fig. 4 Typical application for line drivers or LCD bias TM Technology Inc. reserves the right to change products or specifications without notice. P. 11 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A TYPICAL APPLICATION CIRCUITS ILE D = 1 5 m A V IN 2 .0 to 5 .5 V L1 10uH D1 1 6 C IN 4 .7 u F 4 V IN SW OVP 5 T6309A CE ON FB GND 2 O FF 3 RS 20 O hm C OUT 4 .7 u F Fig. 5 Typical application for driving 6 white LEDs ILE D = 1 5 m A V IN 2 .0 to 5.5 V L1 1 0u H D1 1 6 C IN 4 .7 u F 4 V IN SW OVP 5 R2 1 00 K T6309A CE ON FB GND 2 O FF D im m in g C ontrol 0V ~ 2 V 3 C OUT 4 .7u F R1 25 K RS 25 O hm Fig. 6 White LED application with DC voltage dimming control V IN 2 .0 to 5 .5 V L1 10uH D1 15m A 1 6 C IN 4 .7 u F 4 ON O FF V IN SW OVP 5 R1 450K T6309A CE FB GND 2 3 C OUT 4 .7 u F R2 33K Fig. 7 Typical application for line drivers or LCD bias TM Technology Inc. reserves the right to change products or specifications without notice. P. 12 Publication Date: MAY. 2007 Revision: A tm TE CH T6309A PACKAGE DIMENSIONS SOT23-6 B B 1 B 2 A 1 C 1 A C C 2 F D I Symbol A A1 B B1 B2 C C1 C2 D E F G H I Min. 2.60 1.40 2.70 0.95 0.90 0 0.30 0.08 Dimension in mm Typ. 2.80 1.575 2.85 1.90(BSC) 0.95(BSC) 1.20 1.10 0.075 0.40 0.45 0.15 0.60(REF) G H Max. 3.00 1.60 3.00 Min. 0.102 0.055 0.106 1.45 1.30 0.150 0.037 0.035 0 0.60 0.22 0.012 0.003 Dimension in inch Typ. 0.110 0.062 0.112 0.075(BSC) 0.037(BSC) 0.047 0.043 0.003 0.015 0.018 0.006 to change products or specifications without notice. Max. 0.118 0.063 0.118 0.057 0.051 0.06 0.023 0.009 0~8° 5~15° 5~15° TM Technology Inc. reserves the right E P. 13 Publication Date: MAY. 2007 Revision: A