LM3492HC Datasheet, PDF (11/17 Pages) Texas Instruments – Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator

30170527
FIGURE 5. Over-power Protection
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_FB1, R_FB2, and C_FB as shown in Figure
6. The value of C_FB is recommended to be 10 pF. in order to
help feed the AC component of the output voltage back. The
DC component of the output voltage is fed back by R_FB1 and
R_FB2. The voltage of the FB pin V_FB can be adjusted by DHC.
When V_FB reaches V_FB-OVP, the maximum output voltage of
the boost converter V_OUT(MAX) reaches its maximum, and it is
calculated as follows:
V_OUT(MAX) = 2.88V (1 + R_FB1/ R_FB2)
(3)
Under DHC, the output voltage should be maintained at a
nominal voltage but not the maximum. The nominal output
voltage (V_OUT(NOM)) is
V_OUT(NOM) = max (V_LED,n + V_IOUT,n), n = 1, 2
(4)
where V_LED,n is the forward voltage of LED string n and
V_IOUT,n is the voltage of the IOUTn pin, where n is 1, 2 for
channels 1, 2 of the current regulator). The minimum value of
V_IOUT,n is about 5Ω x I_LED. It is recommended that the nominal
voltage of the FB pin (V_FB(NOM)) is between 1.05V to 2V.
Hence, the equation relating V_OUT(MAX), V_OUT(NOM), and V_FB
(NOM) ^{is as follows:}
V_OUT(MAX) = V_OUT(NOM) x 2.88V / V_FB(NOM)
(5)
30170506
FIGURE 6. Output Voltage Feedback Circuit
Over-Voltage Protection
When V_FB is higher than the FB pin over-voltage protection
(OVP) threshold V_FB-OVP (typically 2.76V and maximum
2.88V), the on-period of the integrated MOSFET stop imme-
diately, and the MOSFET keeps off until V_FB falls back below
below 2.545V (typical hysteresis 0.215V).
An alternative method to implement OVP is to directly monitor
V_OUT instead of V_FB. An external circuit as shown in Figure
7 is required. Current is injected to the ILIM pin to drive the
LM3492HC to the current limit mode once V_OUT is higher than
the avalanche voltage of the zener diode D_OVP plus 0.7V, the
typical voltage on the ILIM pin. In this case, a maximum limit
of V_OUT is imposed. However, at the maximum limit of V_OUT,
V_FB should be higher than 2.25V to avoid affecting the startup
of the LM3492HC.
30170530
FIGURE 7. External OVP circuit
Bi-Directional Communication Pin
The COMM pin of the LM3492HC is an open-drain bi-direc-
tional I/O pin for interfacing with an external MCU for the
following functions: power-good indication, over-temperature
indication, IOUT over- and under-voltage indications, switch-
ing frequency 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
channel 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 occur.
Figure 8 and Figure 9 show a timing diagram of reading and
writing a bit from and to the LM3492HC 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_COMM to the
VCC pin. Otherwise, the voltage of the COMM pin will remain
at zero. The rise time of the output signal of the COMM pin
depends on the pull-up power. If the rise time is long
(R_COMM is too large or pull-up power from the connecting MCU
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.
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