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T6309B
Micropower Step-up DC/DC Converter
FEATURES
* Matched LED Current with Overvoltage Protection * Drives LEDs in series * Up to 85% Efficiency * Low 400mV Feedback Reference, Optional 1.19V Feedback Reference * Internal Current Limit 250mA * Under Voltage Lock Out Circuits * 0.1uA Low Shutdown Supply Current * 6-pin SOT-23 Package * ESD Human Body Mode Over 5KV
GRNERAL DESCRIPTION
The T6309B is a step-up DC/DC converter that drives LED with a constant current. The device can driver up to 3 series LEDs from a single-cell Li-Ion battery. Series connection of the LEDs provides constant LED current that results in uniform brightness. The T6309B 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 T6309B is available in SOT23-6 packages.
APPLICATIONS
* Cellular Phones * Portable Communication Devices * Handheld Electronics * PDAs * LED/Display Back Light Driver
PART NUMBER EXAMPLES
Part no. T6309B-AX T6309B-BX Feedback Referemce Voltage 400mV 1.19V
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SW GND FB
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T6309B
PIN ARRANGEMENT SOT23-6 (Top view)
1 2 3
6 5 4
VIN OVP CE
PIN DESCRIPTION
SYMBOL SW GND FB CE OVP VIN Pin No. 1 2 3 4 5 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 (T6309B-A). Optional reference voltage is 1.19V (T6309B-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 this pin to the output capacitor of the converter. Power supply Input pin.
BLOCK DIAGRAM
V
IN
m in im u m o ff-tim e o n e -sh o t e rro r a m p lifie r
under 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
+ 1 .1 9 V
OVP
FB
+
c o n tro l lo g ic b u ffe r d riv e r
SW
v o lta g e re fe re n c e
GND
c u rre n t lim it
CE
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T6309B
Absolute Maximum Ratings
Supply Voltage ................................................................... -0.3V to 5V SW Pin Switch Voltage ......................................................... -0.3V to 20V OVP Pin Overvoltage Protection ............................................... -0.3V to 20V CE Pin Voltage ................................................................... -0.3V to (VDD+0.3V) SW Pin Switch Current ......................................................... 250mA Continuous power dissipation, PD @ TA = 25C SOT-23-6 ............. 0.35W Package Thermal Resistance SOT-23-6, JA ................................ 150C /W Operating Juction Temperature ................................................ 150C Operating temperature range ................................................... -40C to +85C Storage temperature range, TSTG ............................................. -55C to +150C Lead temperature (soldering, 10sec) .......................................... 260C ESD Human Body Mode ......................................................... 5KV
Electrical Characteristics
( VIN=+3V, TA = 25C, unless otherwise noted )
Symbol VIN
Description Input Voltage T6309B-A T6309B-B
Conditions
Min. Typ. Max 2.5 340 300 400 400 14 60 0.1 1.5 0.4 0.1 0.1 1.55 1.75 1.95 0.6 250 1 5 460 550
Unit V mV V ns V uA uA V V uA uA V ohm mA
VREF Reference Voltage TOFF Switch Minimum Off Time VOVP Voervoltage Threshold IDD2 IOFF Switch Off Current (VOUT) Shutdown Current (VIN)
1.15 1.19 1.23
FB=0.5V CE=0V
VIHCE CE Input Voltage Hight VILCE CE Input Voltage Low IICE IIFB CE Input Bias Current FB Input Bias Current
UVLO Under Voltage Lockout RDS(on) Switch ON Resistance ILIM Current Limit
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T6309B
Typical Operating Characteristics
(VIN=+3.3V,Load=3 LEDs, L=10uH, CIN=4.7uF, COUT=4.7uF, T A=+25 C, unless otherwise noted.)
Efficiency (2WLED) Io=20mA
90 85
Efficiency (3WLED)
Io=20mA
Efficiency (%)
Efficiency(%)
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
80 75 70 65 60
80 75 70 65 60
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Vin(V)
Vin(V)
Stability for driving 3 LEDs
Vin=3.6V
(2.0V/Div)
(Time=2.0us/Div)
Stability for driving 3 LEDs
Vin=5.2V
(2.0V/Div)
(Time=2.0us/Div)
Vout
(200mV/Div)
Vout
(200mV/Div)
VSW
(5.0V/Div)
VSW
(5.0V/Div)
Vin
3
Vin vs. temperature
0. 43
VFB Temperature curve
2.8
0. 42
2.6
VFB(V)
0. 41 0. 4
2.4
2.2
0. 39
2
Vi n=3. 0V Load 3LED
-30 -20 -10 0 10 20 30 40 50 60 70
0. 38
1.8 20 30 40 50 60 70
Temperature
Temperature (C)
note: Efficiency=(VoutIout) / (VinIin)
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T6309B
Functional Description
The T6309B 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 T6309B can be used as a current source with overvoltage protection to drive LEDs. This allows up to 3 LEDS to be connected in series to the output. The low 400mV feedback reference voltage reduces power loss and improves efficiency. 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
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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. Current Limit The T6309B 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 250mA. Overvoltage Protection Since T6309B 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 14V maximum voltage rating of the internal main switch. An overvoltage protection circuit is integrated to prevent the
Publication Date: SEP. 2005 Revision: A
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T6309B
main switch from burning. When the output 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 T6309B 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 T6309B when input voltage is under 1.75V (typ). The device remains off as long as the input voltage is under the UVLO threshold.
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T6309B
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 T6309B 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. 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 2. 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 20mA. 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. * Select the maximum LED current IMAX
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(e.g. 20mA) 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 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)
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:
Publication Date: SEP. 2005 Revision: A
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T6309B
circuit is shown in the below figure.
T6309B
D1 2.2nF D2 1K RS
PW M Signal
10K 0.1nF R2 100K
FB R1 25K
T6309B FB
RS 25 Ohm
GND 4.7K
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.
T6309B
FB Logic Signal
Output Voltage Selection Referring to Fig. 3, select an output voltage for T6309B-A (0.4V reference) by connecting FB to a resistive divider between the output and GND. The VOUT can be set as: VOUT = (1+R1/R2) x 0.4V 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. Inductor Selection The T6309B 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
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R2
R1
Start-Up and Inrush Current The T6309B 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 T6309B is about 200mA. If softstart is required, a recommended external
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T6309B
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 voltage larger than output voltage. 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. 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.
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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 T6309B. 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 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. 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.
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V IN 2 .5 to 5 .5 V
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T6309B
TYPICAL APPLICATION CIRCUITS
IL E D = 2 0 m A L1 10uH 1 6 C IN 4 .7 u F 4 ON OFF V IN SW OVP 5 D1
T6309B CE GND 2 FB 3 C OUT 4 .7 u F RS 20 O hm
Fig. 1 Typical application for driving 3 white LEDs
ILED=20m A
V IN 2.5 to 5.5V L1 10uH 1 6 C IN 4.7uF 4 ON O FF V IN SW OVP 5 R2 100K 3 C OUT 4.7uF R1 25K RS 25 O hm D1 D im m ing C ontrol 0V ~2V
T 6309B
CE FB
GND 2
Fig. 2 White LED application with DC voltage dimming control
V IN 2 .5 to 5 .5 V
L1 1 0u H 1 6 C IN 4 .7 u F 4 ON OFF V IN SW OVP
D1 5
1 1V 2 0m A
R1 2 65 K
T 6 3 09 B
CE FB 3 C OUT 4 .7 u F R2 1 0K
GND 2
Fig. 3 Typical 11V application for line drivers
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T6309B
PACKAGE DIMENSIONS SOT23-6
B B1 B2
A1
A
C1 C2
C F
I
D
H
G
E
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) 5~15
Max. 3.00 1.60 3.00
Min. 0.102 0.055 0.106
1.45 1.30 0.150 0.60 0.22
0.037 0.035 0 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 0~8 5~15
Max. 0.118 0.063 0.118
0.057 0.051 0.06 0.023 0.009
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