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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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SP7616EB датащи(PDF) 7 Page - Exar Corporation |
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SP7616EB датащи(HTML) 7 Page - Exar Corporation |
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7 / 11 page  S SP P7 76 61 16 6 H Hiig gh h A Ac cc cu urra ac cy y 4 4--C Ch ha an nn ne ell L Lo ow w S Siid de e L LE ED D D Drriiv ve err ffo orr C CC CF FL L R Re ep plla ac ce em me en ntt © 2009 Exar Corporation 7/11 Rev. 2.0.4 THEORY OF OPERATION INTRODUCTION The SP7616 is a four-channel constant current source LED driver with programmable output current level. The design consists of a regulator reference voltage source, current amplifier, and output driver. The precision reference voltage ensures good performance over voltage and temperature. The four outputs are tightly coupled allowing for excellent LED current matching. SETTING LED CURRENT The current in the LED strings is set by adjusting the RSET resistor connected between the ISET pin and ground. The LED current is set using the following the following equation. mA R K I SET LED = Where: K = Output Current Multiplication Ratio ILED is the desired LED current PWM DIMMING The LED dimming control is done through the PWM pin. The acceptable frequency range of this signal is 100Hz to 5kHz. The acceptable duty cycle range of the signal is 10% to 90% at 1KHZ. When the PWM pin is driven low, only the LED current sources are disabled while the rest of the chip is still enabled. ANALOG DIMMING CONTROL Besides digital PWM control, the LED current can be controlled continuously (from high to low LED current) by raising the voltage at the bottom of RSET, VMOD, from 0.0V to 1V maximum. This configuration is shown in figure 1 below. Figure 1 The RSET value may be determined from the previous equation for VMOD = 0V. The equation below shows the effect of using a VMOD in the circuit on the LED current. () mA R K V V I SET MOD LED − ≈ 1 Where: 1V = Typical ISET Voltage K = Output Current Multiplication Ratio ILED is the desired LED current VMOD is the adjustment voltage Note that this method of current control is not as precise as adjusting RSET. The voltage at the ISET pin is adjusted slightly during manufacturing to ensure that K, the output current multiplication ratio, is as accurate as possible. When the SP7616 was originally released at the end of 2006, the multiplication ratio and ISET voltage were specified separately. However, market feedback for better accuracy when using RSET alone prompted the change to the way the part is specified today. SHUTDOWN USING ISET PIN In normal operation, the voltage at ISET pin is around 1V. To ensure fast turn on at low duty cycle and high PWM frequency, only the output drivers are switching in PWM mode. However, the whole chip can be shutdown by pulling the voltage at ISET above 3V minimum. |
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