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MC10SX1130 Datenblatt(PDF) 7 Page - ON Semiconductor |
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MC10SX1130 Datenblatt(HTML) 7 Page - ON Semiconductor |
7 / 9 page MC10SX1130 http://onsemi.com 7 First, the RSET resistor must be chosen to set the desired nominal modulation current based on the following equation: RSET = VSET/IMOD (Equation 1) The voltage at VSET is a function of the RTCO tracking resistor, so the desired tracking rate (VTR) must also be chosen. To determine this, the equation must be normalized to correspond to how the LED has been specified. Temp Co = VTR/VSET (Equation 2) The data sheet has three temperature tracking rates for different values of the RTCO resistor. By using the VSET values at 25 °C and substituting those numbers into Equation 2, normalized tracking rates can be calculated. Table 5. Normalized Tracking at 25°C RTCO Tracking %/°C Short +0.20 1 KW +0.52 2 KW +0.89 To match the LED chosen, a 1 k W resistor can be used. Now that this is known, the value of the voltage at the VSET can be substituted into Equation 1 to determine the value of RSET resistor which, for this example is 10 W. The Stretch circuit can be used to compensate for the turn-on/turn-off delay of the LED. The circuit has been designed for ease of use so the pin is designed to be strapped to one of the two power plane levels to select the pre-distortion value. If no pre-distortion is desired, the pin can be left open. In this +5 V example, the maximum amount of pre-distortion is desired, so the STRETCH pin is connected to ground. In addition a resistor must be placed between IOUT and VCC. In selecting this resistor, just as in the case of the RSET, the resistor type should be chosen to dissipate the worst case power and derated for the worst case temperature. As a rule of thumb, the voltage drop across the resistor should match the forward voltage across the diode. The voltage can be larger to minimize the power dissipated on chip when the LED is not ’ON’. Although, the voltage drop across this resistor should not be greater than 2 V. For this example: R @ IOUT = VF/IMOD IMOD(max) + VSET@85 C RSET + 855mV 10W + 86mA ° R @ IOUT = 1.5V/86mA = 17W Because of the positive tracking circuitry in the LED driver, the modulation current will increase over temperature. It is important to now go back and re-calculate the numbers under the worst case environmental conditions to ensure that operating conditions have not been exceeded. Thermal Management LED devices tend to require large amounts of current for most efficient operation. This requirement is then translated into the design of the LED Driver. When large modulation currents are required, power dissipation becomes a critical issue and the user must be concerned about the junction temperature of the device. The following equation can be used to estimate the junction temperature of a device in a given environment: TJ = TA + PD * qJA (Equation 3) TJ Junction Temperature TA Ambient Temperature PD Power Dissipation qJA Average Thermal Resistance (Junction-Ambient) A specially designed thermally enhanced leadframe has been used to house the LED Driver. Below is a graph of the average qJA plotted against air flow. Figure 4. Typical qJA versus Airflow 70 80 90 100 110 0 100 200 300 400 500 AIRFLOW (LFPM) The power dissipation of the device has two components; the quiescent power drain related to the pre-drive circuitry, and the power dissipated in the current switch when driving the LED. Pd = Pstatic + Pswitching (Equation 4) The power dissipated in the current switch is a function of the IMOD current, the LED forward voltage, and the value of RSET. For example in a +5 V application, the following equations can be used: Pstatic = VCC * ICC (Equation 5) Pswitching = (VCC-VF-VSET)* IMOD (Equation 6) |
Ähnliche Teilenummer - MC10SX1130_06 |
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Ähnliche Beschreibung - MC10SX1130_06 |
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