MIC3263 Datasheet, PDF (13/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
MIC3263
Case 2
If 29.3V is not high enough to properly forward bias the
LED channel at the ISET current level, the current amp
loop will drive the bipolar transistor harder and V(IO) will
drop and the CRV control loop will increase CRV and the
boost output voltage (V_OUT) will increase.
Internal Dimming Control
In the internal dimming mode, the dimming is determined
by the DFS and MODE pins. An external pulse is tied to
the DRC pin. The duty cycle of the external pulse (pulse at
DRC) is converted to one of 16 levels called Duty Ratios
(DR) (see Table 2 for DR ratios). It is this internal 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 current drivers
(see Figure 4).
Faults
Open LED in Channel
If any LED in a channel fails open, the voltage on the
collector of the current amp transistor (IO1-IO6) will go
low. The circuitry that monitors the IO pins will detect less
than 0.5V and turn off base drive to the transistor. A flip-
flop latches the fault condition and 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 will be present at the
collector (IO). The other five channels will continue
operating normally. This fault sequence is identical if up to
three LED channels fail open. If four channels fail open or
short, then the remaining two LED 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 transistor (IO1–IO6) will go
high in voltage. If the circuitry that monitors the current
amp bipolar transistor detects more than 7.5V 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.
A channel can tolerate a two LED difference before a fault
is detected.
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.
Figure 4. External Dimming Control
Shorted Cathode (or IO Short)
If the circuitry that monitors 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