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Micrel, Inc.
Case 2
If 29.3V is not high enough to properly forwa r d bias the
LED channel at the ISET current level, the cu rrent amp
loop will drive the bipolar transi stor harder and V(IO) will
drop and the CRV co ntrol loop will increase CR V and the
boost output voltage (V OUT ) will increase.
Internal Dimming Control
In the internal dimming mode, the dimming is d etermined
by the DFS and MODE pins. A n external pulse is tied to
the DRC pin. The duty cycle of the external puls e (pulse at
DRC) is converted to one of 16 levels called Duty Ratio s
(DR) (see Table 2 for DR ratios). It is this inte rnal pulse
(DR) that is used to PWM dim the LEDs.
Figure 3. Internal Dimming Control
External Dimming Control
In external dimming mode, connect the DFS pin to V DD and
apply a PWM dimming pulse to the MODE pin. The
external pulse directly controls the LED curre nt drivers
(see Figure 4).
Figure 4. External Dimming Control
MIC3263
Fau lts
Open LED in Channel
If any LED in a channel fails o pen, the voltage on the
collector of the current amp tran sistor (IO1-IO6) will go
low. The circuitry that monito rs the IO pins will detect less
than 0.5V and turn off base drive to the transistor. A flip-
flop latches the fault condition a nd a power down and
power up sequence is required to r eset that channel.
Without base drive to the transisto r, the channel of LEDs
will turn off and a high impedanc e will be present at the
collector (IO). The othe r five channels will continue
opera ting normally. This fault sequence is identical if up to
three LED channels fail open. If fo ur channels fail open or
short, then the remaining two LE D channels will stay on
and no more faults will be detected.
Short LED in Channel
If any LED in a channel fails shorted, the voltage on
collector of the current amp transi stor (IO1–IO6) will go
high in voltage. If the circuitry th at monitors the current
amp bipolar transistor detects m ore than 7.5V at the
collector (IO), then the base drive to the transistor will turn
off. A flip-flop latches the fault c ondition. A power-down
and power-up sequence is requir ed to reset that channel.
A channel can tolerate a two LED difference before a fault
is detected.
Without bas e drive to the transistor, the channel of LEDs
will turn off and a high impedan ce is present at the
collector (IO). The other five channels will continue
operating normally. This fault sequen ce is identical if more
than one LED channel fails open. If fou r channels fail open
or short, then the remaining two L ED channels will stay on
and no more faults will be detected.
Shorted Cathode (or IO Short)
If the circuitry that moni tors the current amp bipolar
transistor detects less than 0.5V at the collector (IO), then
the base drive to the transistor will turn off. A flip-flop
latches the fault condition. A power-down and power-up
sequence is required to reset that channel.
Without base drive to the transistor, the channel of LEDs
will turn off and a high impedance is present at the
collector (IO). The other five channels will continue
operating normally. This fault sequence is identical if more
than one LED channel fails open. If four channels fail open
or short, then the remaining two LED channels will stay on
and no more faults will be detected.
January 2010
13
M9999-012110
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