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LM2751SD-B bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM2751SD-B bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 11 page Application Information (Continued) The voltage rating of the output capacitor should be 10V or more. All other capacitors should have a voltage rating at or above the maximum input voltage of the application. DRIVING WHITE LEDS The desired LED current is set by placing a resistor (R) in series with each LED, and is determined by the equation: I LED =(VOUT -VLED)÷R In the equation above, I LED is the current that flows through a particular LED, and V LED is the forward voltage of the LED at the given current. The output voltage (V OUT)ofthe LM2751 is tightly regulated to 4.5V or 5V depending on the output voltage option. However, LED forward voltage varies from LED to LED, and LED current will vary accordingly. Mismatch of LED currents will result in brightness mismatch from one LED to the next. Therefore it is suggested that LED groups with tightly controlled I-V characteristics ("Binned" LEDs) be used. LEDs with looser tolerance can be used in applications where brightness matching is not critical, such as in keypad or general backlighting. The typical and maxi- mum diode forward voltage depends highly on the manufac- turer and their technology. PWM BRIGHTNESS CONTROL Perceived LED brightness can be adjusted using a PWM control signal on the Enable pin of the LM2751, to turn the voltage output ON and OFF at a rate faster than perceptible by the eye. When this is done, the total brightness perceived is proportional to the duty cycle (D) of the PWM signal (D = the percentage of time that the LED is on in every PWM cycle). A simple example: if the LEDs are driven at 15mA each with a PWM signal that has a 50% duty cycle, per- ceived LED brightness will be about half as bright as com- pared to when the LEDs are driven continuously with 15mA. For linear brightness control over the full duty cycle adjust- ment range, the PWM frequency (f) should be limited to accommodate the turn-on time (typ. T ON = 300µs) of the device. D x (1/f) > T ON f MAX =DMIN ÷TON The minimum recommended PWM frequency is 100Hz. Fre- quencies below this may be visibly noticeable as flicker or blinking. The maximum recommended PWM frequency is 1kHz. Frequencies above this may cause noise in the au- dible range. THERMAL PROTECTION When the junction temperature exceeds 150˚C (typ.), inter- nal thermal protection circuitry disables the device. This feature protects the LM2751 from damage due to excessive power dissipation. The device will recover and operate nor- mally when the junction temperature falls below 140˚C (typ.). It is important to have good thermal conduction with a proper layout to reduce thermal resistance. POWER EFFICIENCY Charge-Pump efficiency is derived in the following two ideal equations (supply current and other losses are neglected for simplicity): I IN =GxIOUT E=(V OUT xIOUT)÷(VIN xIIN)=VOUT ÷(GxVIN) In the equations, G represents the charge pump gain. Effi- ciency is at its highest asGxV IN approaches VOUT. Refer to the efficiency graph in the Typical Performance Character- istics section for the detailed efficiency data. POWER DISSIPATION The power dissipation (P DISSIPATION) and junction tempera- ture (T J) can be approximated with the equations below. PIN is the product of the input current and input voltage, P OUT is the power consumed by the load connected to the output, T Ais the ambient temperature, and θ JA is the junction-to- ambient thermal resistance for the LLP-10 package. V IN is the input voltage to the LM2751, V VOUT is the voltage at the output of the device, and I OUT is the total current supplied to the load connected to V OUT. P DISSIPATION =PIN -POUT =(V IN xIIN)−(VVOUT xIOUT) T J =TA +(PDISSIPATION x θ JA) The junction temperature rating takes precedence over the ambient temperature rating. The LM2751 may be operated outside the ambient temperature rating, so long as the junc- tion temperature of the device does not exceed the maxi- mum operating rating of 115˚C. The maximum ambient tem- perature rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junction temperature to exceed 115˚C. www.national.com 10 |
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