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LM2796TLX Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM2796TLX Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 8 page  Circuit Description (Continued) LED outputs rise at the rate of the reference voltage ramp. Due to the soft-start circuitry, turn-on time of the LM2796 is approximately 100µs (typ.). Thermal Protection Internal thermal protection circuitry disables the LM2796 when the junction temperature exceeds 160˚C (typ.). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device will recover and operate nor- mally when the junction temperature falls below 120˚C (typ.). It is important that the board layout provides good thermal conduction. This will help to keep the junction temperature within specified operating ratings. Applications Information ADJUSTING LED BRIGHTNESS (PWM control) Perceived LED brightness can be adjusted using a PWM control signal to turn the LM2796 current sources 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, perceived LED brightness will be about half as bright as compared to when the LEDs are driven continuously with 15mA. A PWM signal thus provides brightness (dimming) control for the solution. 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 interference with internal current driver circuitry. The preferred method for applying a PWM signal to adjust brightness is to keep the master EN voltage ON continuously and to apply the PWM signal(s) to the current source enable pin(s): ENA and/or ENB. The benefit of this type of connec- tion can be best understood with a contrary example. When a PWM signal is connected to the master enable (EN) pin, the charge pump repeatedly turns on and off. Every time the charge pump turns on, there is an inrush of current as capacitances, both internal and external, are recharged. This inrush current results in a current and voltage spike at the input of the part. By only applying the PWM signal to ENA/ ENB, the charge pump stays on continuously and much lower input noise results. In cases where a PWM signal must be connected to the EN pin, measures can be taken to reduce the magnitude of the charge-pump turn-on voltage spikes. More input capaci- tance, series resistors and/or ferrite beads may provide ben- efits. If the current and voltage spikes can be tolerated, connect- ing the PWM signal to the EN pin does provide a benefit: lower supply current when the PWM signal is active. When the PWM signal is low, the LM2796 will be shutdown and input current will only be a few micro-amps. This results in a lower time-averaged input current than the prior suggestion, where EN is kept on continuously. CAPACITOR SELECTION The LM2796 requires 4 external capacitors for proper opera- tion. Surface-mount multi-layer ceramic capacitors are rec- ommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR <20mW typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors are not recommended for use with the LM2796 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2796. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over -55˚C to 125˚C; X5R: ±15% over -55˚C to 85˚C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2796. Ca- pacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capaci- tors to fail to meet the minimum capacitance requirements of the LM2796. The minimum recommended voltage rating for these capaci- tors is 10V. MICRO SMD MOUNTING The LM2796 is an 18-bump micro SMD with a bump size of approximately 300 micron diameter. The micro SMD pack- age requires specific mounting techniques detailed in Na- tional Semiconductor Application Note 1112 (AN-1112). www.national.com 7 |
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