Thermal Shutdown and Thermal Error Flag

ICJunction

Temperature(T )

J

‘1’

T <T

J

(PTW)

T

T

³

J

(TEF)

T <T

-

T

J

(PTW)

(HYSP)

T

T

³

J

(TEF)

‘0’

XLAT

PTWinSID

(InternalData)

XERR

OUTn

GSCLK

OFF

ON

‘0’

Hi-Z

‘L’

OFF

SCLK

‘0’

TEFinSID

(InternalData)

‘0’

‘1’

T

T

³

J

(PTW)

T

-

<T

T

J

(TEF)

(HYST)

T

T

³

J

(PTW)

‘1’

‘1’

Hi-Z

‘L’

OFF

ON

BLANK

1

4

3

2

4093

4096

4095

4094

1

3

2

Internal Pre-Charge FET

TLC5944

The thermal shutdown (TSD) function turns off all of the constant current outputs on the IC immediately when the

on until the first GSCLK after BLANK goes low while TEF is set to '0'. Figure 32 illustrates the TEF/TSD/XERR

timing sequence.

Figure 32. TEF/TSD/XERR Timing

The internal pre-charge FET can prevent ghosting of multiplexed LED modules. One cause of this phenomenon

is the charging current for parastic capacitance of the constant current output line and driver through the LED.

One of the mechanisms is shown in Figure 33.

In Figure 33, the constant current driver turns LED0-0 on at (1) and off at (2). After LED0-0 is turned off, OUT0

current driver output capacitance, and the board layout capacitance shown as C0-2). After LED0-0 turns off,

SWPMOS0 is turned off and SWNMOS0 is turned on for LINE0, then LINE0 is pulled down to GND. Because

there is a parastic capacitance between LINE0 and OUT0, OUT0 voltage is also pulled down to GND. After that,

SWPMOS1 is turned on for next line (LINE1). When SWPMOS1 turns on, OUT0 voltage is pulled up from the

causing the LED to briefly turn on and creating the ghosting effect of LED1-0.

The TLC5944 has an internal pre-charge FET to prevent ghosting. The power supply of the pre-charge FET must

on and the ghosting is eliminated at (3). The pre-charge FET turns off as soon as BLANK goes low to avoid

Copyright © 2008, Texas Instruments Incorporated

Submit Documentation Feedback

23

Product Folder Link(s): TLC5944