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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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ZXLD1374 датащи(PDF) 15 Page - Diodes Incorporated |
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ZXLD1374 датащи(HTML) 15 Page - Diodes Incorporated |
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15 / 35 page  ZXLD1374 ZXLD1374 Document number: DS35032 Rev. 1 - 2 Page 15 of 35 www.diodes.com October 2010 © Diodes Incorporated A Product Line of Diodes Incorporated Applications Information (Continued) Component Selection External component selection is driven by the characteristics of the load and the input supply, since this will determine the kind of topology being used for the system. Component selection starts with the current setting procedure and the inductor/frequency setting. Finally after selecting the freewheeling diode and the output capacitor (if needed), the application section will cover the PWM dimming and thermal feedback. Setting the output current The first choice when defining the output current is whether the device is operating with the load in series with the sense resistor (Buck mode) or whether the load is not in series with the sense resistor (Boost and Buck-boost modes). The output current setting depends on the choice of the sense resistor RS, the voltage on the ADJ pin and the voltage on the GI pin, according to the device working mode. The sense resistor RS sets the coil current IRS. The ADJ pin may be connected directly to the internal 1.25V reference (VREF) to define the nominal 100% LED current. The ADJ pin can also be overdriven with an external dc voltage between 125mV and 2.5V to adjust the LED current proportionally between 10% and 200% of the nominal value. ADJ and GI are high impedance inputs within their normal operating voltage ranges. An internal 2.6V clamp protects the device against excessive input voltage and limits the maximum output current to approximately 4% above the maximum current set by VADJ if the maximum input voltage is exceeded. Below are provided the details of the LED current calculation both when the load in series with the sense resistor (Buck mode) and when the load is not in series with the sense resistor (Boost and Buck-boost modes). In Buck mode, GI is connected to ADJ which results in the average LED current (ILED) equal to the average sense resistor/coil current (IRS). A loop gain compensation factor, K, compensates for GI being connected to ADJ. This gives the following equation for ILED: REF ADJ S REF ADJ S Rs LED V V R mV 218 V V R mV 225 K I I = = = where K = 0.97 If ADJ (and GI pin) is directly connected to VREF, this becomes: S Rs LED R mV 218 I I = = Therefore: LED s I mV 218 R = RS ISM VIN REF ADJ GI SGND Figure 30: Buck configuration In Boost and Buck-boost mode GI is connected to ADJ through a voltage divider. With VADJ equal to VREF, the ratio defined by the resistor divider at the GI pin determines the ratio of average LED current (ILED) to average sense resistor/coil current. ⇒ − = D 1 I I LED COIL D 1 xR I xR I V S LED S COIL RS − = = Where ILED = = S REF ADJ ADJ GI R 225 . 0 V V V V S REF ADJ 2 GI 1 GI 1 GI R 225 . 0 V V ) R (R R + = Therefore: REF ADJ LED 2 GI 1 GI 1 GI s V V I mV 225 ) R (R R R + = RS ISM VIN REF ADJ GI SGND RGI2 RGI1 Figure 31: Boost and Buck-boost connection |
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