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LM27961TLX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM27961TLX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 13 page  Circuit Description (Continued) Connecting outputs in parallel does not affect internal opera- tion of the LM27961 and has no impact on the Electrical Characteristics and limits previously presented. The avail- able diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to parallel output configurations, just as they do to the standard application circuit on pg1 of the datasheet. SOFT START The LM27961 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start is implemented with a controlled turn-on of the internal voltage reference. During soft start, the current through the LED outputs rise at the rate of the reference voltage ramp. Due to the soft-start circuitry, turn-on time of the LM27961 is ap- proximately 350µs (typ.). THERMAL PROTECTION Internal thermal protection circuitry disables the LM27961 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 POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (P LED) to the power drawn at the input of the part (P IN). With a 1.5x charge pump, the input current is approximately 1.5x the output current (total LED current). For a simple approximation, the current consumed by internal circuitry can be neglected and the efficiency of the LM27961 can be predicted as follows: Neglecting I Q will result in a slightly higher efficiency predic- tion, but this impact will be no more than a few percentage points when several LEDs are driven at full power. ADJUSTING LED BRIGHTNESS (PWM control) Perceived LED brightness can be adjusted using a PWM control signal to turn the LM27961 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. In cases where a PWM signal must be connected to the ENx pins, 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 LM27961 will be shutdown and input current will only be a few micro-amps. This results in a lower time-averaged input current. CAPACITOR SELECTION The LM27961 requires 4 external capacitors for proper op- eration. Surface-mount multi-layer ceramic capacitors are recommended. 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 LM27961 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 LM27961. These capacitors have tight capacitance toler- ance (as good as ±10%) and hold their value over tempera- ture (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 LM27961. 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 LM27961. 20127920 FIGURE 2. One Parallel Connected LED www.national.com 11 |
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