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| 型号： | LM2795EVAL |
| 厂商： | National Semiconductor |
| 文件页数： | 12/23页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION LM2795 |
| 标准包装： | 1 |
| 电流 - 输出 / 通道： | 80mA |
| 输出及类型： | 4，非隔离 |
| 输出电压： | 4V |
| 特点： | 充电泵 |
| 输入电压： | 2.7 ~ 5.5 V |
| 已供物品： | 板 |
| 已用 IC / 零件： | LM2795 |
| 相关产品： | LM2795TLX-ND - IC LED DRVR WHT BCKLT 14USMD LM27953TLX-ND - IC LED DRVR WHT BCKLT 18USMD LM27952SDX-ND - IC LED DRIVR WHITE BCKLGT 14-LLP LM27951SDX-ND - IC LED DRIVR WHITE BCKLGT 14-LLP LM2795BLDKR-ND - IC LED DRVR WHT BCKLT 14USMD LM27953TLDKR-ND - IC LED DRVR WHT BCKLT 18USMD LM27952SDDKR-ND - IC LED DRIVR WHITE BCKLGT 14-LLP LM27951SDDKR-ND - IC LED DRIVR WHITE BCKLGT 14-LLP LM27952SDCT-ND - IC LED DRIVR WHITE BCKLGT 14-LLP LM27951SDCT-ND - IC LED DRIVR WHITE BCKLGT 14-LLP 更多... |
SNVS168L – JANUARY 2002 – REVISED MAY 2013
2. Determine the maximum desired I LED current. Use the I LED equation to calculate R SET by setting BRGT to 0V
or use Table 3 to select a value for R SET when BRGT equals 0V.
3. Brightness control can be implemented by pulsing a signal at the SD pin. LED brightness is proportional to
the duty cycle (D) of the PWM signal. For linear brightness control over the full duty cycle adjustment range,
the PWM frequency (f) should be limited to accommodate the turn-on time (T ON = 100μs) of the device.
D × (1/f) > T ON
f MAX = D MIN ÷ T ON
If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the LM2794, zero duty
cycle will turn off the LEDs and a 50% duty cycle will result in an average I LED being half of the programmed
LED current. For example, if R SET is set to program 15mA, a 50% duty cycle will result in an average I LED of
7.5mA. For the LM2795 however, 100% duty cycle will turn off the LEDs and a 50% duty cycle will result in
an average I LED being half the programmed LED current.
Brightness Control Using BRGT
1. Choose the maximum I LED desired and determine the max voltage to be applied to the BRGT pin. For
constant brightness, set BRGT to a fixed voltage between 0V to 3V.
2. Use Table 3 to determine the value of R SET required or use the I LED equation above to calculate R SET .
3. Use Table 4 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V.
Table 3. R SET Values
LED Current
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
5mA
374 ?
768 ?
1.15K ?
1.54K ?
1.91K ?
2.32K ?
2.67K ?
10mA
187 ?
383 ?
576 ?
768 ?
953 ?
1.15K ?
1.33K ?
15mA
124 ?
255 ?
383 ?
511 ?
624 ?
768 ?
909 ?
20mA
93.1 ?
191 ?
287 ?
383 ?
475 ?
576 ?
665 ?
Table 4. LED Current
R SET Values
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
2.67K ?
0.7mA
1.4mA
2.1mA
2.9mA
3.6mA
4.3mA
5.0mA
1.33K ?
1.4mA
2.9mA
4.3mA
5.8mA
7.2mA
8.7mA
10.1mA
909 ?
2.1mA
4.2mA
6.3mA
8.4mA
10.5mA
12.7mA
14.8mA
665 ?
2.8mA
5.7mA
8.6mA
11.5mA
14.4mA
17.3mA
20.2mA
CHARGE PUMP OUTPUT (P OUT )
The LM2794/5 charge pump is an unregulated switched capacitor converter with a gain of 1.5. The voltage at the
output of the pump (the P OUT pin) is nominally 1.5 × V IN . This rail can be used to deliver additional current to
other circuitry. Figure 23 shows how to connect additional LEDs to P OUT . A ballast resistor sets the current
through each LED, and LED current matching is dependent on the LED forward voltage matching. Because of
this, LEDs driven by P OUT are recommended for functions where brightness matching is not critical, such as
keypad backlighting.
Since P OUT is unregulated, driving LEDs directly off P OUT is usually practical only with a fixed input voltage. If the
input voltage is not fixed (Li-Ion battery, for example), using a linear regulator between the P OUT pin and the
LEDs is recommended. Texas Instruments LP3985-4.5V low-dropout linear regulator is a good choice for such
an application.
12
Product Folder Links: LM2794 LM2795
Copyright ? 2002–2013, Texas Instruments Incorporated
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