LM3492QMH/NOPB National Semiconductor, LM3492QMH/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM3492QMH/NOPB
• Description: IC LED DRIVER DIMMING 20-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3492QMH/NOPB

• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Type - Secondary: *
• Mounting Type: Surface Mount
• Topology: Step-Up (Boost)
• Number Of Outputs: 2
• Frequency: 200kHz ~ 1MHz
• Voltage - Supply: 4.5 V ~ 65 V
• Voltage - Output: *
• Package / Case: 20-TSSOP (0.173", 4.40mm Width) Exposed Pad
• Operating Temperature: -40°C ~ 125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

If the voltage on the IOUTn (n = 1, 2) pin is over 24V when
channel n is on, the regulated current of channel n will be
reduced linearly if the voltage further increases (as shown in
Figure 5). The regulated current of another channel is not af-
fected. This over-power protection feature avoids damaging
the current regulator owing to the shorting of many LEDs in
one string.
Output Voltage Feedback
The output voltage is fed back to the FB pin through a feed-
back circuit consists of R
6. The value of C
help feed the AC component of the output voltage back. The
DC component of the output voltage is fed back by R
R
1.05V to 2.5V by DHC. The maximum output voltage of the
boost converter V
Under DHC, the output voltage should be maintained at a
nominal voltage but not the maximum. The nominal output
voltage (V
where V
V
channels 1, 2 of the current regulator). The minimum value of
V
voltage of the FB pin (V
Hence, the equation relating V
(NOM)
Bi-Directional Communication Pin
The COMM pin of the LM3492 is an open-drain bi-directional
I/O pin for interfacing with an external MCU for the following
functions: power-good indication, over-temperature indica-
tion, IOUT over- and under-voltage indications, switching fre-
quency tuning, and channel 1 disabling. Except the power
good indication and the over-temperature alert, all data flow
through the COMM pin is serial and is latched by the falling
edge of the signal applying to the DIM1 pin, even when chan-
nel 1 of the current regulator is disabled. If the DIM1 pin stays
only low or only high, either by an external circuit or letting it
open and pull low internally, data flow will not happen. Figures
7 and 8 show a timing diagram of reading and writing a bit
from and to the LM3492 through the COMM pin.
The COMM pin should be pull-up by an MCU I/O pin which
has pull-up capability or an external resistor R
VCC pin. Otherwise, the voltage of the COMM pin will remain
at zero. The rise time of the output signal of the COMM pin
IOUT,n
IOUT,n
FB2
. The voltage of the FB pin V
V
is as follows:
FIGURE 6. Output Voltage Feedback Circuit
OUT(NOM)
is about 5Ω x I
V
is the voltage of the IOUTn pin, where n is 1, 2 for
LED,n
OUT(MAX)
OUT(NOM)
V
OUT(MAX)
is the forward voltage of LED string n and
= max (V
FB
OUT(MAX)
= V
) is
is recommended to be 10 pF in order to
= 2.5V (1 + R
OUT(NOM)
LED
FB1
. It is recommended that the nominal
LED,n
FB(NOM)
is
, R
FB2
x 2.5V / V
+ V
OUT(MAX)
, and C
) is between 1.05V to 2V.
IOUT,n
FB1
FB
/ R
can be adjusted from
), n = 1, 2
, V
FB
FB(NOM)
FB2
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as shown in Figure
OUT(NOM)
)
COMM
, and V
FB1
to the
and
(3)
(4)
(5)
FB
11
depends on the pull-up power. If the rise time is long
(R
I/O pin is too weak), data may be ready after a longer duration
after the falling edge. In this case, a longer delay between the
falling edge latching and the (input or output) bit is required.
POWER-GOOD INDICATION
Upon startup, the COMM pin reads low. The output voltage
of the boost converter of the LM3492 will rise until the voltage
on the FB pin (V
high to indicate power-good. The power-good indication is in-
dependent of the signal applied on the DIM1 pin.
OVER-TEMPERATURE INDICATION
If the junction temperature of the LM3492 reaches 135°C, the
COMM pin reads low, showing an over-temperature indica-
tion. 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 LM3492 will
not latch off and continue to operate in the presence of the
over-temperature indication.
IOUT UNDER-VOLTAGE INDICATION
The LM3492 gives an IOUTn (n = 1, 2) under-voltage indica-
tion if the voltage of the IOUTn pin when DIMn is high is lower
than its minimum required voltage which can regulate I
COMM
is too large or pull-up power from the connecting MCU
FIGURE 7. Read from the COMM Pin
FIGURE 8. Write to the COMM Pin
FB
) reaches 2.25V, when the COMM pin reads
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LED
,