Ver2.1

20

LD1024

16-Channel, Constant-Current LED Driver with LED Open Detection

LED OPEN DETECTION (LOD) AND PRE-THERMAL WARNING (PTW)

The LED open detection (LOD) circuit checks the voltage of each active (that is, on) constant current sink output

(OUT0 through OUT15) to detect open LEDs and LEDs shorted to GND while OEB is low. The LOD bits in the

status information data register (SID) are set to '1' if the voltage of the corresponding OUTn pin is less than the

LED open detection threshold (VLOD = 0.3 V, typ). The status information data can be read from the DOUT pin.

To avoid false detection of open LEDs, the LED driver design must ensure that the constant-current sink output

voltage is greater than 0.3 V when the outputs are on. Also, the output on-time must be greater than 16 times CLK

signal toggle after falling Edge of STROBE signal.

The PTW function indicates that the IC junction temperature is too high. The PTW bit in the SID data is set to '1'

while the IC junction temperature exceeds the temperature threshold (T(PTW) = +138 °C, typ). If the IC junction

temperature decreases below the temperature of T(PTW), the SID data are set depending on the LOD function.

The constant current outputs are not forced off during PTW conditions, so the controller should take appropriate

action (such as reducing the duty cycle of effected channels).

The SID data latched in the latch are transferred into the on/off shift register with a rising edge

of LAT. SID can be shifted out from DOUT with rising edges of CLK. The data in the on/off control shift register

are replaced with the LOD and PTW data at the rising edge of LAT. Therefore, STROBE should be input only once

after the shift data are updated to avoid the on/off control data latch information from being replaced with LOD

and PTW data in the shift register. A timing diagram for LOD, PTW, and SID is shown in Figure 14.

Figure 14. LOD/PTW/SID Timing

OEB

OUTn

OUTn OFF

OUTn OFF

GND

when

OUTn is on, then the LOD circuit reports error information to

the

LOD data latch and the error information is set as ‘1’ to the

bit

that corresponds with the error OUTn in the LOD data latch.

LOD circuit needs 1us to detect LED

open correctly as maximum.

LOD Circuit Data

(Internal)

No Error Information

Latest Error Information From LOD

Circuit

No Error Information

LOD and PTW data are always copied into SID data latch while OEB

PTW Circuit Data

(Internal)

Normal Temperature

Normal Temperature

PTW Error

Previous LOD and PTW Data

No Error Information

Latest Error Information From LOD and PTW

Circuit

LOD and PTW data of from before

OEB goes high are held in the

SID data latch at the rising edge of OEB

SID Data Latch

(Internal)

1

2

3

14

15

16

CLK

STROBE