LM3492HC Datasheet, PDF (12/17 Pages) Texas Instruments – Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator

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FIGURE 8. Read from the COMM Pin
FIGURE 9. Write to the COMM Pin
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POWER-GOOD INDICATION
Upon startup, the COMM pin reads low. The output voltage
of the boost converter of the LM3492HC will rise until the volt-
age on the FB pin (V_FB) reaches 2.25V, when the COMM pin
reads high to indicate power-good. The power-good indica-
tion is independent of the signal applied on the DIM1 pin.
OVER-TEMPERATURE INDICATION
If the junction temperature of the LM3492HC reaches 135°C,
the COMM pin reads low, showing an over-temperature indi-
cation. External MCU should consider to turn off or reduce the
brightness of the LED strings in order to prevent over-tem-
perature. The over-temperature indication is independent of
the signal applied on the DIM1 pin. The COMM pin reads high
if the junction temperature falls below 120°C. The LM3492HC
will not latch off and continue to operate in the presence of
the over-temperature indication.
IOUT UNDER-VOLTAGE INDICATION
The LM3492HC gives an IOUTn (n = 1, 2) under-voltage in-
dication if the voltage of the IOUTn pin when DIMn is high is
lower than its minimum required voltage which can regulate
I_LED, and the voltage of the CDHC pin reaches its maximum,
and these conditions last for 508 consecutively dimming sig-
nals applied on the DIMn pin. This means that the current of
the LED string n does not reach its regulated value. In most
case, the IOUT under-voltage indication can be regarded as
an open fault of the LED string n. A bit pattern (refer to table
1) can be read from the COMM pin. The LM3492HC will not
latch off and continue to operate in the presence of the IOUT
under-voltage indication.
IOUT OVER-VOLTAGE INDICATION
The LM3492HC gives an IOUTn (n = 1, 2) over-voltage indi-
cation if the voltage of the IOUTn pin when DIMn is high is
higher than a threshold of typically 6.5V, and the condition
lasts for 508 consecutively dimming signals applied on the
DIMn pin. The IOUT over-voltage indication can be regarded
as a short fault of the LED string n except the following 2 cas-
es: i) powering up the LM3492HC at a very low dimming ratio
such that V_OUT maintains at a maximum and DHC is not fast
enough to reduce V_OUT; ii) under DHC over-ridding. A bit pat-
tern (refer to table 1) can be read from the COMM pin. The
LM3492HC will not latch off and continue to operate in the
presence of the IOUT over-voltage indication.
TABLE 1. COMM Bit Patterns
Indication
COMM Bit Pattern
IOUT1 over-voltage
indication
0001
IOUT2 over-voltage
indication
0011
IOUT1 under-voltage
indication
0101
IOUT2 under-voltage
indication
0111
COMM BIT PATTERN
Table 1 summarized all COMM bit patterns of IOUT over- and
under-voltage indications. A bit pattern can be read from the
COMM pin continuously even the condition causing IOUT
over- and under-voltage disappeared. An existing COMM bit
pattern will be clear if one of the following condition occurs:
i) the LM3492HC is shutdown;
ii) the LM3492HC is disabled by pulling the EN pin low;
iii) the over-temperature indication is appearing.
To clear the COMM bit pattern without affecting the operation
of the boost converter and the current regulator, a reset signal
which pulls the EN pin low for about 200 ns can be applied.
In this case, the COMM pin will not give bit pattern any more
unless a condition causing IOUT over- or under-voltage ap-
pears and lasts for 508 consecutively dimming signals.
If an external MCU is used to read the COMM bit pattern, it is
recommended that a reset signal should be send to clear the
COMM bit pattern after the bit pattern is read. This can avoid
that the existing COMM bit pattern is overwritten by another
pattern appeared in a later time.
In case of over-temperature, the COMM pin will be pulled low
to give an over-temperature indication disregard of any exist-
ing COMM bit pattern. After that the over-temperature indi-
cation disappears, the COMM bit pattern appeared before the
over-temperature indication will appear again.
SWITCHING FREQUENCY TUNING
After power good, the switching frequency (f_SW) of the
LM3492HC can be tuned down 20% or 40%, or resume nor-
mal by writing commands (refer to table 2) to the COMM pin.
This helps avoid interfering some sensitive devices, for ex-
ample radios, working nearby the LM3492HC. Upon reset,
f_SW of the LM3492HC will resume normal by default. If the
over-temperature indication or any COMM bit pattern has al-
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