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PAM2861 Datasheet(PDF) 8 Page - Diodes Incorporated |
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PAM2861 Datasheet(HTML) 8 Page - Diodes Incorporated |
8 / 12 page PAM2861 Document number: DSxxxxx Rev. 1 - 1 8 of 12 www.diodes.com July 2013 © Diodes Incorporated PAM2861 A Product Line of Diodes Incorporated Application Information Setting Nominal Average Output Current with External Resistor RS The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RS) connected between VIN and ISENSE and is given by: R 1 . 0 I S ) NOM ( OUT The table below gives values of nominal average output current for several preferred values of current setting resistor (RS) in the typical application circuit shown on page 1. RS (Ω) Nominal Average Output Current (mA) 0.1 1000 0.13 760 0.15 667 0.3 333 The above values assume that the VSET pin is floating and at a nominal voltage of VREF (1.25V). Note that RS = 0.1Ω is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. It is possible to use different values of RS if the VSET pin is driven from an external voltage. Capacitor Selection A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. A minimum value of 4.7µF is acceptable if the input source is close to the device, but higher values will improve performance at lower input voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC. For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended. Capacitors with Y5Vdielectric are not suitable for decoupling in this application and should NOT be used. Inductor Selection Recommended inductor values for the PAM2861 are in the range 33µH to 100µH. Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range. The inductor should be mounted as close to the device as possilbe with low resistance connections to the LX and VIN pins. The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. The inductor value should be chosen to maintain operating duty cycle and switch ‘on’/’off’ times within the specified limits over the supply voltage and load current range. The following equations can be used as a guide. LX Switch ‘On’ time ) R R R ( I V V I L T LX L S LED LED IN ON Lx Switch ‘Off’ time ) R R ( I V V I L T L S LED D LED OFF Where: L is the coil inductance; RL is the coil resistance; RS is the current sense resistance ILED is the required LED current; ΔI is the coil peak- peak ripple current (Internally set to 0.25 x ILED); VIN is the supply voltage; VLED is the total LED forward voltage; RLX is the switch resistance (0.3 Ω nominal); VD is the diode forward voltage at the required load current |
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