LM3492MH/NOPB National Semiconductor, LM3492MH/NOPB Datasheet - Page 12

• Manufacturer Part Number: LM3492MH/NOPB
• Description: IC LED DVR DIM 2CH 20-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3492MH/NOPB

• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: Step-Up (Boost)
• Number Of Outputs: 2
• Frequency: 200kHz ~ 1MHz
• Voltage - Supply: 4.5 V ~ 65 V
• 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
• Voltage - Output: -

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and the voltage of the CDHC pin reaches its maximum, and
these conditions last for 508 consecutively dimming signals
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 LM3492 will not latch
off and continue to operate in the presence of the IOUT under-
voltage indication.
IOUT OVER-VOLTAGE INDICATION
The LM3492 gives an IOUTn (n = 1, 2) over-voltage indication
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.
Except powering up the LM3492 at a very low dimming ratio
such that V
enough to reduce V
be regarded as a short fault of the LED string n. A bit pattern
(refer to table 1) can be read from the COMM pin. The
LM3492 will not latch off and continue to operate in the pres-
ence of the IOUT over-voltage indication.
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 LM3492 is shutdown;
ii) the LM3492 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
LM3492 can be tuned down 20% or 40%, or resume normal
by writing commands (refer to table 2) to the COMM pin. This
IOUT1 under-voltage
IOUT2 under-voltage
IOUT1 over-voltage
IOUT2 over-voltage
Indication
indication
indication
indication
indication
OUT
TABLE 1. COMM Bit Patterns
maintains at a maximum and DHC is not fast
OUT
, the IOUT over-voltage indication can
COMM Bit Pattern
0001
0011
0101
0111
SW
) of the
12
helps avoid interfering some sensitive devices, for example
radios, working nearby the LM3492. Upon reset, f
LM3492 will resume normal by default. If the over-tempera-
ture indication or any COMM bit pattern has already present-
ed, no command can be written to the LM3492.
CHANNEL 1 DISABLE
After power good, channel 1 of the current regulator can be
disabled by writing a command (refer to table 2) to the COMM
pin. If LED string 1 is malfunctioning, channel 1 can be dis-
abled and the signal applied on the DIM1 pin can serve as
only a clock signal for the data flow of the COMM pin. Channel
1 is by default enabled after reset. If the over-temperature in-
dication or any COMM bit pattern has already presented, no
command can be written to the LM3492.
Application Information
EXTERNAL COMPONENTS
The following procedures are to design an LED driver using
the LM3492 with an input voltage ranged from 9V to 24V and
two LED strings consists of 10 LEDs each with a forward volt-
age of 3.8V for each LED when running at 200 mA. The output
power is 15.2W. The switching frequency f
be 300 kHz.
R
with 10 LEDs is 38V, and the minimum voltage of the IOUTn
pin (n = 1, 2) is 1V when I
39V. Since V
1.05V to 2V during operation, design V
when V
sign R
kΩ, and a standard resistor value of 402 kΩ is selected. C
is selected to be 10 pF as recommended.
L
to peak inductor current ripple (I
conduction mode (CCM) operation, the average inductor cur-
rent I
I
Also,
If V
LED string is turned on (since the two channels of the LM3492
are individually dimmable), I
be calculated that I
47 µH. On the other hand, from (6), I
V
strings are turned on together. Hence I
I
From (7), t
state peak inductor current I
L1
LR
1
IN
FB1
: The main parameter affected by the inductor is the peak
equals to the input current I
IN
is
is minimum, which is 9V in this example, and the two LED
f
f
SW
SW
, R
L1
is maximum, which is 24V in this example, and only one
f
Channel 1 disable
SW
FB2
should be larger than half of I
OUT(NOM)
tune down by 20%
tune down by 40%
FB2
resume normal
Command
to be 16.2 kΩ, from (3), R
, and C
on
I
IN
is 2.56 µs. From (9), I
FB(NOM)
t
= (V
on
is 39V. From (5), V
I
L1(PEAK)
L
I
= (1 – V
LR
FB
TABLE 2. Commands
1
IN(MIN)
OUT(NOM)
= (V
: The nominal voltage of the LED string
= (V
is recommended to be ranged from
IN
, t
= I
IN
LED
IN
on
x t
IN
x t
L1
/V
, and L
L1(PEAK)
x 2I
on
is 200 mA. Hence, V
is minimum. From (6)-(8), it can
OUT
on
+ I
IN
) / 2I
LR
) / L
. Hence,
LED
LR
) / f
). To maintain a continuous
Command Bit Pattern
1
FB1
1111011101110111
1111000100010001
1111001100110011
1111010101010101
OUT(MAX)
IN
/ 2
LR
1
) / V
are 0.325A, 1.28 µs, and
is
LR
SW
. For a boost converter,
is calculated to be 405
IN(MAX)
IN
is 0.49A. The steady
IN
FB(NOM)
is maximum when
SW
is 65V. Also, de-
is 1.73A. Then,
is designed to
to be 1.5V
OUT(NOM)
SW
of the
(10)
(6)
(7)
(8)
(9)
FB
is