LM3509SDX National Semiconductor Corporation, LM3509SDX Datasheet - Page 18

• Manufacturer Part Number: LM3509SDX
• Description: High Efficiency Boost For White Led S And/or Oled Displays With Dual Current Sinks And I2c Compatible Brightness Control
• Manufacturer: National Semiconductor Corporation

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BRIGHTNESS RATE OF CHANGE DESCRIPTION
RMP0 and RMP1 control the rate of change of the LED cur-
rent I
or BSUB. There are 4 user programmable LED current rates
of change settings for the LM3509 (see Table 5).
TABLE 5. Rate of Change Bits
For example, if R
contents of BMAIN set to 0x1F (I
contents of BMAIN are changed to 0x00 resulting in (I
0mA). With RMP0 =1 and RMP1 = 1 (52ms/step), I
change from 20mA to 0mA in 31 steps with 52ms elapsing
between steps, excluding the step from 0x1F to 0x1E, result-
ing in a full scale current change in 1560ms. The total time to
transition from one brightness code to another is:
The following 3 additional examples detail possible scenarios
when using the brightness register in conjunction with the rate
of change bits and the enable bits.
Example 1:
Step 1: Write to BMAIN a value corresponding to I
mA.
TABLE 6. GPIO Register Function
SHUTDOWN AND OUTPUT ISOLATION
The LM3509 provides a true shutdown for either MAIN or
SUB/FB when configured as a White LED bias supply. Write
a 0 to ENM (bit 1) of the General Purpose register to turn off
the MAIN current sink and force MAIN high impedance. Write
a 0 to ENS (bit 2) of the General Purpose register to turn off
Bits 7 – 3
MAIN
RMP0
X
X
X
0
0
1
1
and I
SUB/FB
SET
Logic Output
Data (Bit 2)
Logic Input
= 12kΩ then I
in response to changes in BMAIN and /
X
RMP1
0
1
0
1
MAIN
LED_MAX
Mode (Bit 1)
= 20mA), suppose the
Change Rate
(t
51µs/step
13ms/step
26ms/step
52ms/step
X
STEP
0
1
= 20mA. With the
FIGURE 10. GPIO Register Description
)
MAIN
MAIN
Enable GPIO (Bit 0)
MAIN
= 20-
will
=
0
1
1
18
Step 2: Write 1 to ENM (turning on MAIN)
Step 3: I
RMP1. (RMP0 and RMP1 bits set the duration spent at one
brightness code before incrementing to the next).
Step 4: ENM is set to 0 before 20mA is reached, thus the LED
current fades off at a rate given by RMP0 and RMP1 without
I
Example 2:
Step 1: ENM is 1, and BMAIN has been programmed with
code 0x01. This results in a small current into MAIN.
Step 2: BMAIN is programmed with 0x1F (full scale current).
This causes I
ed by RMP0 and RMP1.
Step 3: Before I
with 0x09. I
Step 4: When I
down to the current corresponding to 0x09 at a rate set by
RMP0 and RMP1.
Example 3:
Step 1: Write to BMAIN a value corresponding to I
mA.
Step 2: Write a 1 to both RMP0 and RMP1.
Step 3: Write 1 to ENM (turning on MAIN).
Step 4: I
RMP1. (RMP0 and RMP1 bits set the duration spent at one
brightness code before incrementing to the next).
Step 5: After 1.04s I
(0.16875 × 20mA = 3.375mA). Simultaneously, RMP0 and
RMP1 are both programmed with 0.
Step 6: I
at a new ramp rate of 51µs/step.
the SUB/FB current sink and force SUB/FB high impedance.
Writing a 1 to ENM or ENS turns on the MAIN and SUB/FB
current sinks respectively. When in shutdown the leakage
current into MAIN or SUB/FB is typically 3.6µA. See Typical
Performance Plots for start-up responses of the LM3509 us-
ing the ENM and ENS bits in White LED and OLED modes.
MAIN
going up to 20mA.
RESET/GPIO is configured as an active low reset
input. This is the default power on state.
RESET/GPIO is configured as a logic input. The logic
level applied to RESET/GPIO can be read via bit 2 of
the GPIO register.
RESET/GPIO is configured as a logic output. A 0 in
bit 2 forces RESET/GPIO low. A 1 in bit 2 forces
RESET/GPIO high impedance.
MAIN
MAIN
MAIN
MAIN
ramps toward 20mA with a rate set by RMP0 and
MAIN
continues ramping from 3.375mA to 20mA, but
ramps to 20mA with a rate set by RMP0 and
MAIN
MAIN
will continue to ramp to full scale.
to ramp toward full-scale at the rate select-
reaches full-scale BMAIN is programmed
MAIN
has reached full-scale value it will ramp
has ramped to 16.875% of I
Function
30004346
MAIN
LED_MAX
= 20-