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CAT4004A Datasheet(PDF) 7 Page - ON Semiconductor |
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CAT4004A Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 11 page CAT4002A, CAT4004A http://onsemi.com 7 Basic Operation The CAT400XA uses tightly matched current sinks to accurately regulate LED current in each channel proportional to the current sourced from the RSET pin. There are 32 different settings for LED brightness that can be programmed through the EN/DIM pin. Tight current regulation for all channels is possible over a wide range of input and LED voltages due to independent current sensing circuitry on each channel. Each LED channel needs a minimum of 50 mV headroom to sink a constant regulated current of 20 mA. If the input supply falls below 2.0 V typical, the under−voltage lockout circuit disables all LED channels and resets the circuit to default values. Any unused LED channels should be left open. LED Current Selection After power−up, the LED current is set by the external resistor (RSET) value and the number of pulses (n) on the EN/DIM input as follows: LED current + 125 0.6 V RSET 31 * n 31 The full scale current is calculated from the above formula with n equal to zero. The EN/DIM pin has two primary functions. One function enables and disables the device. The other function is LED current dimming with 32 different levels by pulsing the input signal, as shown on Figure 16. On each consecutive pulse rising edge, the LED current is decreased by about 3.2% (1/31th of the full scale value). After 30 pulses, the LED current is 3.2% of the full scale current. On the 31st pulse, the current drops to zero, and then goes back to full scale on the following pulse. Each pulse width should be between 200 ns and 100 ms. Pulses faster than the minimum TLO may be ignored and filtered by the device. Pulses longer than the maximum TLO may shutdown the device. By pulsing the EN/DIM signal at high frequency, the LED current can quickly be set to zero. The LED driver enters a “zero current” shutdown mode if EN/DIM is held low for 3 ms typical. The dimming level is set by the number of pulses on the EN/DIM after the power−up, as shown in Table 7. Table 7. DIMMING LEVELS Full Scale (Current in %) Dimming Pulses [n] 100 0 97 1 94 2 90 3 87 4 84 5 81 6 77 7 74 8 71 9 68 10 65 11 61 12 58 13 55 14 52 15 48 16 45 17 42 18 39 19 35 20 32 21 29 22 26 23 23 24 19 25 16 26 13 27 10 28 6 29 3 30 0 31 100 32 Figure 16. EN/DIM Digital Dimming Timing Diagram 100% EN/DIM LED Current Shutdown Shutdown 97% 94% 3% 0% 100% 32 Levels TLED TLO THI TPWRDWN |
Número de pieza similar - CAT4004A_11 |
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Descripción similar - CAT4004A_11 |
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