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LM3421 Datasheet(PDF) 21 Page - Texas Instruments |
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LM3421 Datasheet(HTML) 21 Page - Texas Instruments |
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21 / 72 page  x PA 23 = HYS V ¨¨ © § + 2 UV R ¸¸ ¹ · 1 UV R ( + x 1 UV R ) 2 UV R UVH R 2 UV R A 23 x 2 HYS V = 1 ¸ ¸ ¹ · ¨ ¨ © § x = - ON TURN 24V . 1 V 1 UV R + 2 UV UV R R 1.24V 23 2A LM3421/23 RUV2 RUV1 VIN UVLO nDIM RUVH (optional) LM3421, LM3421-Q1, LM3423, LM3423-Q1 www.ti.com SNVS574F – JULY 2008 – REVISED JULY 2015 Feature Description (continued) Figure 23. UVLO Circuit When using the nDIM pin for UVLO and PWM dimming concurrently, the UVLO circuit can have an extra series resistor to set the hysteresis. This allows the standard resistor divider to have smaller resistor values minimizing PWM delays due to a pulldown MOSFET at the nDIM pin (see PWM Dimming section). In general, at least 3 V of hysteresis is preferable when PWM dimming, if operating near the UVLO threshold. The turnon threshold (VTURN-ON) is defined using Equation 26. (26) The hysteresis (VHYS) is defined as follows: 7.3.11.1 UVLO Only (27) 7.3.11.2 PWM Dimming and UVLO (28) When zero current shutdown and UVLO are implemented together, the EN pin can be used to escape UVLO. The nDIM pin will pull up to VIN when EN is pulled low; therefore, if VIN is within the UVLO hysteretic window when EN is pulled high again, the controller will start-up even though VTURN-ON is not exceeded. 7.3.12 PWM Dimming The active low nDIM pin can be driven with a PWM signal which controls the main NFET and the dimming FET (dimFET). The brightness of the LEDs can be varied by modulating the duty cycle of this signal. LED brightness is approximately proportional to the PWM signal duty cycle, (that is, 30% duty cycle equals approximately 30% LED brightness). This function can be ignored if PWM dimming is not required by using nDIM solely as a VIN UVLO input as described in Input Undervoltage Lockout (UVLO) or by tying it directly to VCC or VIN. Copyright © 2008–2015, Texas Instruments Incorporated Submit Documentation Feedback 21 Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1 |
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