LM3632A Datasheet, PDF (15/64 Pages) Texas Instruments – Single-Chip Backlight With Bias Power and 1.5-A Flash LED Driver

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LM3632A
SNVSA63A – APRIL 2015 – REVISED SEPTEMBER 2015
7.3 Features Description
7.3.1 Backlight
The backlight is enabled if the BL_EN bit (bit[0] in reg[0x0A]) is set to ‘1’, at least one of the backlight sink
outputs is enabled (bit[3] and/or bit[4] in reg[0x0A]), and the brightness value is different than 0. When the
brightness value is 0 or the BL_EN bit is ‘0’, the backlight is disabled.
7.3.1.1 Brightness Control
Brightness can be controlled either by the I²C brightness register or a combination of the external PWM control
and the I²C brightness register. The backlight truth table is shown in Table 1.
When controlling brightness through I²C, registers 0x04 and 0x05 are used. Registers 0x04 and 0x05 hold the
11-bit brightness data. Register 0x04 contains the 3 LSBs, and register 0x05 contains the 8 MSBs. The LED
current transitions to the new level after a write is done to register 0x05.
When controlling brightness through I²C, setting the brightness value to '0' shuts down the backlight. When
controlling the brightness with PWM input, if PWM input is low for a certain period of time (25 ms typ.), the
backlight shuts down. When using the combination of a PWM input and the I²C register, either option shuts down
the backlight.
EN PIN
0
1
1
1
1
1
1
1
1
BL_EN
0x0A[0]
X
0
1
1
1
1
1
1
1
Table 1. Backlight Truth Table
BLED1_EN
0x0A[4]
X
X
0
1
0
1
1
0
1
BLED2_EN
0x0A[3]
X
X
0
0
1
1
0
1
1
PWM_EN
0x09[6]
X
X
X
0
0
0
1
1
1
ACTION
Shutdown
Standby
Bias enable
BLED1 ramp to target current
BLED1 & BLED2 ramp-to-target current
BLED1 & BLED2 ramp-to-target current
BLED1 ramp to (target current × PWM duty cycle)
BLED1 & BLED2 ramp to (target current × PWM
duty cycle)
BLED1 & BLED2 ramp to (target current × PWM
duty cycle)
7.3.1.1.1 LED Current with PWM Disabled
When LED brightness is controlled from the I²C brightness registers, the 11-bit brightness data directly controls
the LED current in BLED1 and BLED2. LED mapping can be selected as either linear or exponential. When this
mode is selected, setting the PWM input to 0 does not disable the backlight.
With exponential mapping the 11-bit code-to-current response is approximated by the equation:
I_LED = 50 µA × 1.003040572^{I2C BRGT CODE} (for codes > 0)
(1)
Equation 1 is valid for I²C brightness codes between 1 and 2047. Code 0 disables the backlight. The Code-to-
LED current response realizes a 0.304% change in LED current per LSB of brightness code.
Figure 22 and Figure 23 detail the exponential response of the LED current vs. brightness code. Figure 22 shows
the response on a linear Y axis while Figure 23 shows the response on a log Y axis to show the low current
levels at the lower codes.
Copyright © 2015, Texas Instruments Incorporated
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