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LM3431-Q1 Datenblatt(PDF) 11 Page - Texas Instruments |
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LM3431-Q1 Datenblatt(HTML) 11 Page - Texas Instruments |
11 / 36 page VSS/SH 1V/Div ILED 100 mA/Div Vout 10V/Div VEN 5V/Div 4 ms/DIV C6 = tss x 19 PA 1.85V LM3431 www.ti.com SNVS547F – NOVEMBER 2007 – REVISED MARCH 2009 As input voltage approaches the nominal output voltage, duty cycle and switch on time are reduced. When the on time reaches minimum, pulse skipping will occur. This increases the output ripple voltage and can cause regulator saturation and poor LED current regulation. If input voltage equals or exceeds the set output voltage, switching will stop and the output voltage will become unregulated. This will force an increase in the LED cathode voltage and NPN regulator power dissipation. Although this condition can be tolerated, it is not recommended. Therefore, input voltage should be restricted to keep on time above the minimum (see Switching Frequency section) and at least 1V below the set output voltage. ENABLE and UVLO The EN pin is a dual function pin combining both enable and programmable undervoltage lockout (UVLO). The shutdown threshold is 0.75V. When EN is pulled below this threshold, the LM3431 will shutdown and IQ will be reduced to 15 µA typically. The typical EN pin UVLO threshold is 1.23V. When the EN voltage is above this threshold, the LM3431 will begin softstart. Below the UVLO threshold, the LM3431 will remain in standby mode. A resistor divider, shown as R1 and R2 in Figure 14, can be used to program the UVLO threshold at the EN pin. This feature is used to shutdown the IC at an input voltage higher than the internal VCC UVLO threshold of 4.4V. The EN UVLO should be set just below the minimum input voltage for the application. The internal UVLO is monitored at the VCC pin. When VCC is below the threshold of 4.4V, the LM3431 is in standby mode (See VCC section). Soft-Start The SS/SH pin is a dual function softstart and sample/hold pin. The SH function is described in sections below. When the EN pin is pulled above the programmable UVLO threshold and VCC rises above the internal UVLO threshold, the SS/SH pin begins sourcing current. This charges the SS cap (C6) and the SS pin voltage in turn controls the output voltage ramp-up, sensed via the AFB pin. The softstart capacitor is calculated as shown below, where 20 µA is the typical softstart source current: (3) The LED current regulators are held off until softstart is completed. During softstart, current limit is active and the CFB pin is monitored for a cathode short fault (See LED Protection section). When the SS/SH voltage reaches 1.85V, the current regulators are activated, LED current begins flowing, and output voltage control is transferred to the CFB pin. Typical startup is shown below in Figure 15. Figure 15. Typical Startup Waveforms (From power-on, DIM = high) Copyright © 2007–2009, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3431 |
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