FS8807 White Step-up Converter Features Matched Current with Over
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FS8807 White Step-up Converter Features
Matched Current with Overvoltage Protection Drives LEDs series Efficiency Over 80mA Output Current Capacity 400mV Feedback Reference, Optional 1.23V Feedback Reference Internal Current Limit 400mA Under Voltage Lock Circuits 0.1µA Shutdown Supply Current 6-pin SOT-23 Package
FS8807 step-up DC/DC converter that drives white with constant current. device driver series white LEDs from single-cell Li-Ion battery. Series connection LEDs provides constant current that results uniform brightness. FS8807 features minimum off-time current-limited control scheme, which combines advantages (higher output power efficiency) those traditional (low quiescent current). current with external sense resistor (RS) regulated feedback (FB) that regulates voltage across sense resistor 400mV (typ). 400mV feedback reference voltage reduces power loss improves efficiency white driver applications. Optional 1.23V feedback reference available normal output voltage applications like bias. Output overvoltage protection (OVP) integrated prevent damage case output open circuit condition (e.g. faulty LED). monitor output voltage turn converter event overvoltage condition. FS8807 available SOT-23-6 packages.
Applications
Cellular Phone PDA, Packet Smart Phone Handheld Computers Devices Digital Still Video Camera Small display
Configuration
SOT-23-6
(TOP VIEW)
Typical Application
2.5V~5.5V 22uH ILED 15mA
Ordering Information
FS8807-XCV Package Type SOT-23, Operating Temperature Range Commercial Standard
FS8807-ACV
COUT 4.7µF
26.6
Feedback Reference Voltage 400mV 1.23V
FS8807 Typical Application driving White LEDs
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TD0310-0305
VER.
Marking Information
Part Number FS8807-ACV FS8807-BCV
FS8807 White Step-up Converter
Marking 807A 807B
Description
Symbol Description Switch pin. Connect inductor/diode here. Minimize trace area this reduce EMI. Ground pin. Connect local ground plane. Feedback pin. Reference voltage 400mV (FS8807-A). Optional reference voltage 1.23V (FS8807-B). Connect cathode lowest current sense resistor here. Chip enable pin. 1.5V higher enable device; 0.4V less shutdown device. float this pin. Overvoltage protection. Connect output capacitor converter. Input supply pin. Must locally bypassed.
Typical Application Schematics
2.5V~5.5V 22uH ILED 15mA Dimming Control 0V~2V
100K
FS8807-ACV
COUT 4.7µF
33.3
White application with voltage dimming control
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FS8807 White Step-up Converter
2.5V~5.5V
22uH
20mA
450K
FS8807-BCV
COUT 4.7µF
Typical application line drivers bias
Function Block Diagram
Overvoltage Protection
Minimum Off-Time One-Shot
1.23V
Under Voltage Lock
Error Amplifier Control Logic Buffer Driver
Maximum On-Time One-Shot
Voltage Reference
Current Limit
FS8807 Simplified Function Block Diagram
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FS8807 White Step-up Converter
Absolute Maximum Ratings
Supply Voltage -0.3V Switch Voltage -0.3V Overvoltage Protection -0.3V Voltage -0.3V (VDD+0.3V) Switch Current -400mA Continuous power dissipation, 25°C SOT-23-6 0.25W Package Thermal Resistance SOT-23-6, 150°C Operating Juction Temperature 150°C Operating temperature range -40°C +85°C Storage temperature range, TSTG -55°C +150°C Lead temperature (soldering, 10sec) 260°C
Electrical Characteristics
(VIN=+3.0V, TA=25°C, unless otherwise noted.) Symbol PARAMETER VREF TOFF VOVP IDD2 IOFF Input Voltage Range Reference Voltage Switch Maximum Time Switch Minimum Time Overvoltage Threshold Switch Current (VIN) Shutdown Current (VIN) Input Voltage Threshold Input Bias Current Input Bias Current UVLO RDS(on) ILIM Under Voltage Lockout Switch Resistance Current Limit 0.2V 0.5V FS8870-ACV FS8870-BCV CONDITIONS 1.19 3.50 -TYP -400 1.23 4.80 1.27 6.50 UNITS
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FS8807 White Step-up Converter
Typical Operating Characteristics
(VIN=+3.0V,Load=4 LEDs, L=22µH, CIN=1µF, COUT=4.7µF, TA=+25, unless otherwise noted.)
FS8807-B Output Voltage Input Voltage, Vout=18V Iout=1mA FS8807-B Load Input Current Input Voltage, Vout=18V Load Current(uA)
Output voltage(V) Input voltage(V)
Input voltage(V)
FS8807-A Input Current Input Voltage, Load=4 ILED=15mA Input Current(mA) Input voltage(V) Quiescent Current (uA) 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00
Switch Quiescent Current Input Voltage Temperature
(-40)
Input Voltage
Switching Frequency Temperature
Feedback Voltage Temperature
Feedback Voltage
Frequency (KHz)
Temperature (oC)
0.398 0.396 0.394 0.392 0.39 0.388 0.386 0.384 0.382 0.38 Temperature(oC)
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FS8807 White Step-up Converter
FS8807-A Efficiency, Load Efficiency(%) Vin=2.5V Vin=3.6V Vin=4.2V Output Current(mA)
FS8807-B Efficiency, Vout=18V
Efficiency(%) Iload(mA)
Vin=2.5V Vin=4V Vin=3V Vin=5V
FS8807-A Efficiency Input Voltage, ILED=15mA Efficiency(%) Input Voltage(V)
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Detail Description
FS8807 White Step-up Converter
FS8807 like standard current-limit step-up DC-DC converter, regulates voltage across sense resistor instead output voltage. This results constant current regardless input voltage number LEDs connected. With integrated overvoltage protection (OVP), FS8807 used current source with overvoltage protection drive LEDs. device generate output voltage 20mA load with internal switch internal 400mA MOSFET switch. This allows LEDS connected series output. 400mV feedback reference voltage reduces power loss improves efficiency. Step-Up Converter step-up DC-DC converter operation understood referring block diagram Figure comparator monitors output voltage sense resistor. When feedback voltage higher than reference voltage, MOSFET switch turned off. feedback voltage lower than reference voltage MOSFET switch been least period minimum off-time decided minimum off-time one-shot, MOSFET switch then turned period on-time decided maximum on-time one-shot, until current limit signal asserted. During internal MOSFET switch turned current ramps inductor store energy magnetic field. When MOSFET turned off, voltage across inductor reverses forces current through diode output filter capacitor load, energy inductor transferred output filter capacitor load. energy stored inductor depleted, current ramps down output diode turns off. output filter capacitor stores charge while inductor current higher than output current, then sustains output voltage until next switching cycle. Current Limit FS8807 utilizes cycle-by-cycle current limiting means protecting output
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MOSFET switch from overstress preventing small value inductor from saturation. Current limiting implemented monitoring output MOSFET current build-up during conduction, upon sensing over-current conduction immediately turning switch duration oscillator cycle. current through output MOSFET monitored compared against reference signal. When threshold reached, signal sent controller block terminate power switch conduction. current limit threshold typically 400mA. Overvoltage Protection Since FS8807 configured current source, output voltage rises output impedance increases output open-circuit (e.g. fault LED). output voltage exceed maximum voltage rating internal main switch. overvoltage protection circuit integrated prevent main switch from burning. When output voltage exceeds threshold voltage, main switch turned off. remains until output voltage falls below threshold voltage. step-up converter continues normal operation long output voltage under threshold. Shutdown FS8807 enters shutdown reduce quiescent current typically 0.5A when normal operation, drive high connect VOUT. During shutdown, reference, feedback control circuitry off. boost converter's output drops Schottky diode voltage drop below input voltage remains high impedance. capacitance load VOUT determine rate which VOUT decays. Shutdown pulled high regardless voltage VOUT. also used dimming control. More details dimming control section. Undervoltage Lockout undervoltage lockout (UVLO) circuit prevents
7/11 TD0310-0305 VER.
FS8807 White Step-up Converter
fault operation FS8807 when input voltage under 2.2V (typ). device remains long input voltage under UVLO threshold.
Application Information
Current Control current sense resistor voltage across sense resistor regulated reference voltage 400mV. order have accurate current, high precision resistors preferred recommended). current calculated ILED 400mV ILED (mA) Dimming Control There several types dimming control circuit follows: Using signal When using signal pin, FS8807 turned signal. average current increases proportionally with duty cycle signal. typical frequency range signal 1KHz 10KHz. magnitude signal should greater than threshold voltage voltage high. Using voltage voltage signal used well control brightness. dimming control method shown figure below. When voltage zero, fully turned voltage increases, voltage drop increases voltage drop decreases. Thus current decreases. selection should make current from variable source much smaller than current much larger than bias current.
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FS8807-ACV
100K 33.3
(ohm) 26.6
range from selection resistors this figure gives dimming control current from 15mA. Following steps used select resistor values: Select voltage VMAX turn LEDs off. (e.g. 2.0V) Select default turn LEDs fully Select maximum current IMAX (e.g. 15mA) minimum current. Calculate achieve feedback current range 10µA LEDs fully turned VREF (e.g. 0.4V/4µA =100K) Calculate meet equation: (R1+R2) VREF VMAX (e.g. VREF=0.4V, VMAX=2.0V, R2=100K, calculated 25K) Calculate sense voltage maximum current: VREF R1/R2) (e.g. 0.4V 25K/100K) 0.5V) Calculate sense resistor IMAX (e.g. 0.5V/15mA 33.3) Using Filtered signal
TD0310-0305 VER.
FS8807 White Step-up Converter
filtered signal considered adjustable voltage. used replace voltage source adjustable analog signal available system. circuit shown figure below:
2.2nF
FS8807-ACV
FS8807-ACV
Signal
100K 33.3
4.7K
Output Voltage Selection Referring Fig.3, select output voltage FS8807-BCV (1.23V reference) connecting resistive divider between output GND. VOUT VOUT (1+R1/R2) 1.23V Higher R1,R2 values reduce quiescent current, give noise immunity. keep stable feedback loop operation better noise immunity, select (R1+R2) value less than Inductor Selection
0.1nF
Using Logic Signal current controlled discrete steps with logic signal shown figure below. When NMOS off, minimum current. When NMOS sets current increment value.
FS8807-ACV
Logic Signal
Start-Up Inrush Current FS8807 internal soft start circuit included achieve minimum start-up delay. inductive step-up converters have higher inrush current during start-up. inrush current FS8807 about 200mA. soft-start required, recommended external circuit shown below figure.
FS8807 designed work well with 22µH inductor most applications. inductance values supply higher output current, also increase ripple reduce efficiency. Higher inductor values reduce ripple improve efficiency, also limit output current. Choose DC-resistance inductor, usually less than minimize loss. necessary choose inductor with saturation current greater than peak current that inductor will encounter application. Saturation occurs when inductor's magnetic flux density reaches maximum level core support inductance falls. Inductor with 500mA rating greater would suitable FS8807. Capacitor Selection input capacitor stabilizes input voltage minimizes peak current ripple from source. value capacitor depends impedance input source used. Small (Equivalent Series Resistance) ceramic
9/11 TD0310-0305 VER.
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FS8807 White Step-up Converter
design minimizes excessive feedback paths voltage gradients ground plane, both which result instability regulation errors. Connect inductor, input filter capacitor, output filter capacitor close device possible, keep their traces short, direct, wide reduce power loss improve efficiency. Connect their ground pins single common node star ground configuration, full ground plane. output capacitor should placed close output terminals obtain better smoothing effect output ripple. addition, ground connection sense resistor should tied directly shared with other component. This ensures clean, noise-free connection. Precaution When testing FS8807 socket with external components, keep wiring from inductor wiring from Schottky diode short thick possible prevent device from damage. Long thin wires would have equivalent small inductance between Schottky diode, which could result unexpected high voltage damage pin. Always connect Vout Vovp protect FS8807. Unexpected open load incorrect circuit connections could result high voltage Vout damage pin.
capacitor with value 4.7µF would suitable. output capacitor used sustain output voltage when internal MOSFET switched smoothing ripple voltage. larger output ripple, larger line regulation, which means current changes input voltage changes. capacitor should used reduce output ripple voltage. 4.7µF 10µF ceramic output capacitor with about 150m provide stable switching good line regulation. Schottky Diode Selection diode largest source loss converters. most important parameters which affect efficiency forward voltage drop, reverse recovery time. forward voltage drop creates loss just having voltage across device while current flowing through reverse recovery time generates loss when diode reverse biased, current appears actually flow backwards through diode minority carriers being swept from junction. Schottky diode with following characteristics recommended: Small forward voltage, 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 board layout very important part design. Good
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Package Information
SOT-23-6
FS8807 White Step-up Converter
Unit
YMBO MIN. 1.00 0.00 0.70 0.35 0.10 2.70 2.60 1.40 1.10 -0.80 0.40 0.15 2.90 2.80 1.60 0.95 1.90 0.37 MAX. 1.30 0.10 0.90 0.50 0.25 3.10 3.00 1.80
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TD0310-0305
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