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LM2795 데이터시트(PDF) 10 Page - Texas Instruments

부품명 LM2795
상세설명  Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control
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Amps
1
10
*
188
BRGT
V
*
+
385
.
0
.
0
SET
R
*
o
MirrorRati
OFFSET
V
+
BRGT
V
385
.
0
*
R
SET
LED =
I
LED =
I
LM2794, LM2795
SNVS168K – MAY 2004 – REVISED MAY 2004
www.ti.com
In addition to RSET, a voltage may be applied to the VBRGT pin to vary the LED current. By adjusting current with
the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied.
Applying a voltage on BRGT between 0 to 3 volts will linearly vary the LED current. Voltages above 3V do not
increase the LED current any further. The voltage on the VBRGT pin is fed into an internal resistor network with a
ratio of 0.385. The resulting voltage is then summed with a measured offset voltage of 0.188V, which comes
from the reference voltage being fed through a resistor network (See Functional Block Diagram). The
brightness control circuitry then uses the summed voltage to control the voltage across RSET. An equation for
approximating the LED current is:
ILED CURRENT SELECTION PROCEDURES
The following procedures illustrate how to set and adjust output current levels. For constant brightness or analog
brightness control, go to “Brightness control using BRGT”. Otherwise refer to “Brightness control using PWM”.
Brightness Control Using PWM
1. Set the BRGT pin to 0V.
2. Determine the maximum desired ILED current. Use the ILED equation to calculate RSET by setting BRGT to 0V
or use Table 3 to select a value for RSET 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 (TON = 100µs) of the device.D ×
(1/f) > TONfMAX = DMIN ÷ TONIf 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 ILED
being half of the programmed LED current. For example, if RSET is set to program 15mA, a 50% duty cycle
will result in an average ILED 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 ILED being half the programmed LED current.
Brightness Control Using BRGT
1. Choose the maximum ILED 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 RSET required or use the ILED equation above to calculate RSET.
3. Use Table 4 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V.
Table 3. RSET Values
LED Current
BRGT
5mA
10mA
15mA
20mA
0.0V
374
187
124
93.1
0.5V
768
383
255
191
1.0V
1.15K
576
383
287
1.5V
1.54K
768
511
383
2.0V
1.91K
953
624
475
2.5V
2.32K
1.15K
768
576
3.0V
2.67K
1.33K
909
665
Table 4. LED Current
RSET Values
BRGT
2.67K
1.33K
909
665
0.0V
0.7mA
1.4mA
2.1mA
2.8mA
10
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Product Folder Links: LM2794 LM2795


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