LM3633 datasheet(21/58 Pages) TI2

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IHIGH = ILED_FULLSCALE x

255

x BREGH_X

(44 -

)

IHIGH = ILED_FULLSCALE x 0.85

BREGH_X + 1

5.8181818

ILOW = ILED_FULLSCALE x

255

x BREGL_X

(44 -

)

ILOW = ILED_FULLSCALE x 0.85

BREGL_X + 1

5.8181818

LM3633

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SNVS867 – JUNE 2014

7.4.3.6 Low-Level Brightness

The LED current low brightness level (ILOW) is the LED current set point that the pattern rests at during the tLOW

period. This level is set via the Pattern Generator Low-Level Brightness registers (BREGL_C to BREGL_H). The

brightness level has 8 bits of programmability. ILOW is a function of the Control Bank full-scale current setting and

the code in the Pattern Generator Low-Level Brightness registers.

For exponential mapping ILOW is:

(7)

For linear mapping ILOW is:

(8)

BREGL_X is the Pattern Generator Low-Level Brightness Register setting for the specific Control Bank.

7.4.3.7 High-Level Brightness

The LED current high brightness level (IHIGH) is the LED current set point that the pattern rests at during the tHIGH

period. This high-current level is set via the Control Banks Brightness Register (BREGH_C to BREGH_H). The

brightness level has 8 bits of programmability. IHIGH is a function of the Control Bank full-scale current setting and

the code in the Control Banks Brightness Register, prior to the Mapping Mode selected.

For exponential mapping IHIGH is:

(9)

For linear mapping IHIGH is:

(10)

BREGH_X is the Control Banks Brightness Register setting for the specific Control Bank.

7.4.4 Fault Flags/Protection Features

The LM3633 contains both an LED open-fault and LED short-fault detection. These fault detections are designed

to be used in production-level testing and not during normal operation. For the fault flags to operate they must be

enabled via the LED Fault Enable Register (see Table 35). The Open LED String (HVLED), Shorted LED String

(HVLED), Open LED (LVLED), and Shorted LED (LVLED) sections detail proper procedure for reading back

open and short faults in both the high-voltage LED and low-voltage LED strings.

7.4.4.1 Open LED String (HVLED)

An open LED string is detected when the voltage at the input to any active high-voltage current sink has fallen

below 200 mV, and the boost output voltage has hit the OVP threshold. This test assumes that the HVLED string

being detected for an open is connected to the LM3633 boost output (COUT+) (see Table 31). For an HVLED

string not connected to the LM3633 boost output voltage, but connected to another voltage source, the boost

output does not trigger the OVP flag. In this case an open LED string is not detected.

The procedure for detecting an open fault in the HVLED current sinks (provided they are connected to the boost

output voltage) is:

•

Apply power to the LM3633

•

Enable Open Fault (Register 0xB4, bit [0] = 1)

•

Assign HVLED1, HVLED2 and HVLED3 to Bank A (Register 0x10, Bits [2:0] = (0, 0, 0)

•

Set the start-up ramp times to the fastest setting (Register 0x12 = 0x00)

•

Set Bank A full-scale current to 20.2 mA (Register 0x20 = 0x13)

•

Configure HVLED1, HVLED2, and HVLED3 for LED string anode connected to COUT (Register 0x28, bits[2:0]

= (1,1,1))

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