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SM8121AD Fiches technique(PDF) 9 Page - Nippon Precision Circuits Inc |
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SM8121AD Fiches technique(HTML) 9 Page - Nippon Precision Circuits Inc |
9 / 19 page SM8121A NIPPON PRECISION CIRCUITS INC.—9 LED OPEN-CIRCUIT PROTECTION When there is no load (LED open-circuit), the FB pin is pulled-down and then switching occurs at maximum duty. Consequently, the output voltage continues to be boosted and the SW pin voltage may exceed the maxi- mum rating of 27V. A zener diode can be added so that it acts as the output load when the LED is open-circuit, preventing the SW voltage from rising. The zener diode must be selected so that the zener does not breakdown during normal operation. The zener voltage VZD range is given by the following relationship, where N is the number of LEDs connected in series, VF MAX is the maximum LED forward-bias voltage drop, VOUT MAX is the SW pin maximum output voltage, VFB is the FB pin voltage, and VSBD is the Schottky-barrier diode for- ward-bias voltage drop. (VF MAX × N) ≤ VZD ≤ (VOUT MAX − VFB − VSBD) When the load is applied using a connector (SM8121A and LEDs on separate boards), the zener diode should be mounted on the same board as the SM8121A device so that the SW boost prevention function can operate when the load is disconnected. Zener Diode (ZD) Only Connection When the load is removed (LEDs open circuit), the output voltage is determined by the zener voltage, and the output current is determined by the output current-setting resistance. Consequently, the output current when the load is removed is not limited, and thus the input current cannot be controlled. Zener Diode (ZD) and Current-Limiting Resistance Connection When the load is removed (LEDs open circuit), the output voltage is determined by the zener voltage, and the output current is determined by the sum of the output current-setting resistance and the current-limiting resis- tance. Consequently, the output current is limited when the load is removed, and the input current can be con- trolled. VSS FB L 10 µH CIN 4.7 µF VIN 3.6V SM8121 SBD COUT 1.0 µF LED Open VOUT=VZD +0.5=15.5V IZD=0.5/33=15.15mA 33 Ω ZD (15V) SW VDD CE IIN=(VOUT IZD)/VIN=65mA SW VSS FB L 10 µH CIN 4.7 µF VIN 3.6V SM8121 SBD COUT 1.0 µF LED Open VOUT=VZD +0.5=15.5V IZD=0.5/1033=0.48mA 33 Ω ZD (15V) 1k Ω VDD CE IIN=(VOUT IZD)/VIN=2mA |
Numéro de pièce similaire - SM8121AD |
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Description similaire - SM8121AD |
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