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SP7121 数据表(PDF) 4 Page - Sipex Corporation |
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SP7121 数据表(HTML) 4 Page - Sipex Corporation |
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4 / 11 page  Sept 11 - 07 Rev E SP7121 Low Dropout LED Driver © 2007 Sipex Corporation Page 4 APPLICATION INFORMATION The SP7121 allows the LED current level to be adjusted by an external resistor connected to the ISET pin. This option lets costumer select LED current amplitude by selecting appropriate external RSET resistor value. The SP7121 circuit consists of enable, bandgap reference, and current amplifier circuit blocks. The Enable circuit block provides the enable and PWM function for the IC. The Bandgap reference provides a stable voltage source from which the output current is derived. Setting the Current in the SP7121 The internal current amplifier provides a current gain function where the RSET resistor is connected from the ISET pin (pin 4) to GND. The current, produced by the RSET resistor is then amplified and delivered to the LED output. The external RSET resistor value for SP7121 is determined by following equation: RSET = 200 x VSET/ILED Where ILED = required LED current value in mA per channel. VSET = 1.22V 200 is the typical gain The LED current level can be set anywhere from 10mA to 25mA per each output. Specifications are not guaranteed for current levels that are set outside this range. The SP7121 doesn’t have protection from ISET pin being shorted to ground or the RSET value being too low. With an operational maximum current of 30mA per channel device may be damaged if the ISET pin is shorted to the ground or the RSET value is below 8k . ENABLE The SP712X drivers have a low current shutdown function. In shutdown mode, the part draws less than 1µA current maximum. The part can be shut down using the EN pin (EN = LOW). The enable pin cannot be tri- stated or left floating. There are no predefined internal states, so leaving this pin open will cause the part to operate incorrectly. PWM Dimming The SP7121 driver allows LED-dimming control by applying a PWM signal to the EN pin. The acceptable frequency range of this signal is 100Hz to 20 kHz. The minimum ON time that is required for the ISET pin to set is 15µs from the time the Enable pin goes high. Thus, for a 2kHz PWM signal, the acceptable duty cycle range is 3% to 100%. To find the minimum PWM duty cycle the following steps need to be followed. Step 1: Determine the time period of the PWM frequency. Where PWMf is the PWM frequency Step 2: Obtain the minimum Enable ON time; this is 15µS for the SP7121 part. |
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