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LM2796 датащи(PDF) 9 Page - Texas Instruments

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номер детали LM2796
подробное описание детали  LM2796 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost
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LM2796 датащи(HTML) 9 Page - Texas Instruments

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LM2796
www.ti.com
SNVS273A – MAY 2004 – REVISED MAY 2013
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. Frequencies 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 connection 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 capacitance, series resistors and/or ferrite
beads may provide benefits.
If the current and voltage spikes can be tolerated, connecting 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 operation. 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 aluminum 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.
Capacitors 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 capacitors to fail to meet the minimum
capacitance requirements of the LM2796.
The minimum recommended voltage rating for these capacitors is 10V.
DSBGA MOUNTING
The LM2796 is an 18-bump DSBGA with a bump size of approximately 300 micron diameter. The DSBGA
package requires specific mounting techniques detailed in Texas Instruments' Application Note 1112 (Literature
Number SNVA009).
Copyright © 2004–2013, Texas Instruments Incorporated
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