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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
General Description
The FS8807 is step-up DC/DC converter that drives white LED with a constant current. The device can driver up to 4 series white LEDs from a single-cell Li-Ion battery. Series connection of the LEDs provides constant LED current that results in uniform brightness. The FS8807 features a minimum off-time current-limited PFM control scheme, which combines the advantages of PWM (higher output power and efficiency) and those of traditional PFM (low quiescent current). 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 white LED driver applications. Optional 1.23V feedback reference is available for normal 12V to 18V 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 FS8807 is available in SOT-23-6 packages.
Features
Matched LED Current with Overvoltage Protection Drives up to 4 LEDs in series Up to 85% Efficiency Over 80mA Output Current Capacity Low 400mV Feedback Reference, Optional 1.23V Feedback Reference Internal Current Limit 400mA Under Voltage Lock Out Circuits 0.1A Low Shutdown Supply Current 6-pin SOT-23 Package
Applications
Cellular Phone PDA, Packet PC, Smart Phone Handheld Computers and Devices Digital Still and Video Camera Small LCD display
Pin Configurations
SOT-23-6
(TOP VIEW)
Typical Application
VIN 2.5V~5.5V L1 22uH D1 1 ILED = 15mA
SW 1 GND 2 FB 3
6
VIN
5 OVP 4 CE
SW
6
Ordering Information
OVP
5
VIN
FS8807-x x V Package Type V : SOT-23-6,
CIN
1F
FS8807-ACV
4 ON
CE GND
2
FB
3 Rs 26.6 Ohm
OFF
COUT
4.7F
Operating Temperature Range C : Commercial Standard P : Commercial Standard, Lead (Pb) Free and Phosphorous (P) Free Package Feedback Reference Voltage A : 400mV B : 1.23V
1/11 TD-0408013 Rev. 1.1
Fig 1. FS8807 Typical Application for driving 4 White LEDS
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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
Package Marking Information
Part Number FS8807-ACV FS8807-BCV FS8807-APV FS8807-BPV Marking 07AA 07BA 07AA * 07BA * SOT-23-6 EXFS8807-APV .....
.
07AA
...
W
Pin Description
Pin No. SOT-23-6 1 2 3 4 5 6 Symbol SW GND FB CE OVP Vin
Top Point: Lot No. Bottom Point: Year w : week, A~Z & A ~ Z A dot after marking : Pb/P Free
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 (FS8807-A). Optional reference voltage is 1.23V (FS8807-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.
Functional Block Diagram
Overvoltage Protection Minimum Off-Time One-Shot
+ 1.23V
OVP
VIN
Under Voltage Lock Out
SW
Error Amplifier Control Logic Buffer Driver
FB
+
Maximum On-Time One-Shot
CE
Voltage Reference
Current Limit
GND
Fig 2. FS8807 Simplified Functional Block Diagram
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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
Typical Application Circuit
VIN 2.5V~5.5V L1 22uH D1 1 Dimming Control 0V~2V ILED = 15mA
SW
6
VIN
OVP
5 R2 100K
CIN 1F 4 ON OFF
FS8807-ACV
CE GND
2 COUT 4.7F
FB
3 R1 25K Rs 33.3 Ohm
Fig 3. White LED application with DC voltage dimming control
L1 22uH
VIN 2.5V~5.5V
D1 1
18V 20mA
SW
6
VIN
OVP
5 R1 450K 3 R2 33K
CIN 1F 4 ON OFF
FS8807-BCV
CE GND
2 COUT 4.7F
FB
Fig 4. Typical 18V application for line drivers or LCD bias
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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
Absolute Maximum Ratings
Supply Voltage -------------------------------------------------------------------------------------------- -0.3V to 10V LX Pin Switch Voltage ----------------------------------------------------------------------------------- -0.3V to (Vout +0.3V) FB Pin Voltage --------------------------------------------------------------------------------------------- -0.3V to (Vout +0.3V) CE Pin Voltage -------------------------------------------------------------------------------------------- -0.3V to (Vout +0.3V) LX Pin Switch Current ----------------------------------------------------------------------------------- 1.2A Continuous power dissipation, PD @ TA = 25C SOT-23-6 ------------------------------------------------------------------------------------------ 0.3W Package Thermal Resistance SOT-23-6, JA ------------------------------------------------------------------------------------ 250C /W Maximum Operating Junction Temperature ------------------------------------------------------- 150C Operating temperature range -------------------------------------------------------------------------- -20C to +70C Storage temperature range, TSTG -------------------------------------------------------------------- -55C to +150C Lead temperature (soldering, 10sec) ---------------------------------------------------------------- 260C ESD ratings Human Body Model, per MIL-STD-883D-3015.7 ------------------------------------------ 1.0KV Machine Model, per MIL-STM5.2-1999 ------------------------------------------------------ 75V
Electrical Characteristics
(VIN=+3.0V, TA=25C, unless otherwise noted.) Symbol PARAMETER VIN VREF TON TOFF VOVP IDD2 IOFF Input Voltage Range Reference Voltage Switch Maximum On Time Switch Minimum Off Time Overvoltage Threshold Switch Off Current (VIN) Shutdown Current (VIN) CE Input Voltage Threshold CE Input Bias Current FB Input Bias Current UVLO RDS(on) ILIM Under Voltage Lockout Switch ON Resistance Current Limit 300 FB = 0.2V 2.0 40 2.2 0.6 400 FB = 0.5V CE = 0V 0.4 FS8870-ACV FS8870-BCV CONDITIONS MIN 2.5 360 1.19 3.60 300 18 -TYP -400 1.23 4.80 400 19 30 0.1 0.7 MAX 5.5 440 1.27 7.20 600 20 60 1 1.5 1 80 2.4 1.0 450 UNITS V mV V S nS V A A V A nA V mA
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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
Typical Operating Characteristics
(VIN=+3.0V,Load=4 LEDs, L=22H, CIN=1F, COUT=4.7F, TA=+25, unless otherwise noted.)
FS8807-B Output Voltage vs Input Voltage, Vout=18V Iout=1mA 20 15 10 5 0 0 1 2 3 4 5 Input voltage(V) FS8807-A Input Current vs Input Voltage, Load=4 LED ILED=15mA 300 Input Current(mA) 250 200 150 100 50 0 2.4 2.8 3.2 3.6 4 Input voltage(V) 4.4 4.8
Quiescent Current (uA) 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 2.4 3.0 (-40) 25 5.4 85 6.0
FS8807-B No Load Input Current vs Input Voltage, Vout=18V 800 No Load Current(uA) 700 600 500 400 300 200 100 0 2.4 2.8 3.2 3.6 4 4.4 4.8 Input voltage(V)
Switch Off Quiescent Current vs Input Voltage and Temperature
Output voltage(V)
3.6 4.2 4.8 Input Voltage (V)
Switching Frequency vs Temperature
Feedback Voltage vs Temperature
300
Feedback Voltage (V)
250
Frequency (KHz)
200 150 100 50 0 -40 -20 0 20 40 60 Temperature (oC) 80 100
0.4 0.398 0.396 0.394 0.392 0.39 0.388 0.386 0.384 0.382 0.38 -50 -25 0 25 50 Temperature(oC) 75 100
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TD-0408013
Rev. 1.1
Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
FS8807-A Efficiency, Load = 4 LED 90 85 80 Efficiency(%) 75 70 65 60 55 50 Vin=2.5V Vin=3.6V Vin=4.2V 0.1 1 10 Output Current(mA) 100
FS8807-B Efficiency, Vout=18V 90 80 Efficiency(%) 70 60 50 40 30 0.1 1 Iload(mA) 10 100 Vin=2.5V Vin=4V Vin=3V Vin=5V
FS8807-A Efficiency vs Input Voltage, ILED=15mA 89 88 87 Efficiency(%) 86 85 84 83 82 81 80 2 3 4 Input Voltage(V) 5 6
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Detail Description
Semiconductor Corporation
FS8807
White LED Step-up Converter
The FS8807 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 FS8807 can be used as a current source with overvoltage protection to drive LEDs. The device can generate output voltage up to 18V at 20mA load with the 20V internal switch and the internal 400mA MOSFET switch. This allows up to 4 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 4. 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 for a period of on-time decided by maximum on-time 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 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 FS8807 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
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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 400mA. Overvoltage Protection Since FS8807 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 20V maximum voltage rating of the internal main switch. An overvoltage protection circuit is integrated to prevent the 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 FS8807 enters shutdown to reduce quiescent current to typically 0.5A when CE pin is low . For normal operation, drive CE high or connect CE to VOUT. 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 LX remains high impedance. The capacitance and load at VOUT determine the rate at which VOUT decays. Shutdown can be pulled as high as 6V, regardless of the voltage at VOUT. 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 FS8807 when input voltage is under 2.2V (typ). The device remains off as long as the input voltage is under the UVLO threshold.
7/11
TD-0408013
Rev. 1.1
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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
Application Information
LED Current Control The LED current is set by the sense resistor Rs. The voltage across the sense resistor is regulated to the reference voltage 400mV. In order to have accurate LED current, high precision resistors are preferred (1% is recommended). The LED current can be calculated as: ILED = 400mV / Rs ILED (mA) 5 10 15 20 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 FS8807 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 1KHz to 10KHz. 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 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. Rs (ohm) 80 40 26.6 20 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. Select the maximum LED current IMAX (e.g. 15mA) and 0mA as the minimum LED current. Calculate R2 to achieve a feedback current in the range of I1 = 3A to 10A as the LEDs are fully turned on: R2 = VREF / I1 (e.g. 0.4V/4A =100K) Calculate R1 to meet the equation: R1 / (R1+R2) = VREF / VMAX (e.g. VREF=0.4V, VMAX=2.0V, calculated R1 is 25K) R2=100K, the
FS8807-ACV
FB VDC
R2 100K R1 25K Rs 33.3 Ohm
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/15mA = 33.3) 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:
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FS8807
White LED Step-up Converter
Output Voltage Selection Referring to Fig.3, select an output voltage for FS8807-BCV (1.23V reference) by connecting FB to a resistive divider between the output and GND. The VOUT can be set as: VOUT = (1+R1/R2) x 1.23V 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 1M Inductor Selection The FS8807 is designed to work well with a 22H 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 DC-resistance inductor, usually less than 1 to minimize loss. It is necessary to choose an inductor with saturation current greater than the peak current that the inductor will 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 500mA rating or greater would be suitable for the FS8807. 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 1F to 4.7F 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 4.7F to 10F ceramic output capacitor with about 50m to 150m ESR to provide stable switching and good line regulation. Schottky Diode Selection
Rs
FS8807-ACV
FB PWM Signal
10K R2 100K R1 25K Rs 33.3 Ohm
0.1nF
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.
FS8807-ACV
FB
Logic Signal
R2
R1
Start-Up and Inrush Current The FS8807 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 FS8807 is about 200mA. If soft-start is required, a recommended external circuit is shown in the below figure.
D1 2.2nF
D2
FS8807-ACV
FB GND
4.7K 1K
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,
9/11 TD-0408013 Rev. 1.1
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Fo r tun e
Semiconductor Corporation
FS8807
White LED Step-up Converter
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. Precaution When testing the FS8807 on socket with external components, keep the wiring from inductor to SW pin and the wiring from SW pin to Schottky diode as short and thick as possible to prevent the device from damage. Long and thin wires would have equivalent small inductance between SW pin and Schottky diode, which could result in unexpected high voltage and damage the SW pin. Always connect Vout to Vovp pin to protect the FS8807. Unexpected open load or incorrect circuit connections could result in high voltage at Vout and damage the SW pin.
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 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
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Package Outline
SOT-23-6
D b
Semiconductor Corporation
FS8807
White LED Step-up Converter
Unit : m m
S YMBO L A A1 MIN. 1.00 0.00 0.70 0.35 0.10 2.70 2.60 1.40 TYP . 1.10 -0.80 0.40 0.15 2.90 2.80 1.60 0.95 BS C . 1.90 BS C . 0.37 1 -5 -9 MAX. 1.30 0.10 0.90 0.50 0.25 3.10 3.00 1.80
L
A2 b C
H
E
D H E e e1
e e1
C
L
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A1
A2
A
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TD-0408013
Rev. 1.1

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