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Micrel, Inc.
MIC3263
OVP
An open LED in a channel will not trigger an OVP. OVP
monitors the boost output voltage. If an open o ccurs on the
load (all channels open) an OVP fault will trigger a n over-
voltage condition. When the OVP triggers, it turns off the
boost and starts an OVP cycle. If one, tw o, o r three
C o ndition
1 L ED Shorts
2 L EDs Shor t
in Same
Fault
NO
NO
Monitor
IO > 1.2V
1.2 < IO < 7.5
Result
All Channels
On
All Channels
On
channels open, they will not trigger an OVP . Fou r open
Channel
channels will trigger an OVP fault and will cycl e on an d off
at about 2Hz as long as there are four open chan nels. If
one of the LED channels is reconnected (not open), then
operation returns to no rmal for those three chann els th at
are reconnected with ou t having to go through a po wer on
reset.
M o re Than 2
LEDs Short
in Same
C h annels
1 LED Opens
in C hannel 1
YES
YES
IO > 7.5V
IO < 0.5V
1 Channel
Off; 5
Channels On
1 Channel
Off; 5
In t he even t of a load o pening (four or more ch annels
Channels On
open) the following will occur:
1. VIO will drop below 1V
2 . Charge pump will raise CRV during each DR pulse
on-time
3. CRV will increase to 2.4V
4. When CRV reaches 2.4V the bo ost output
maximum voltage will be; V OUT_MA X = 2.4*
(R2+R2)/R1.
2 or 3
C h annels
Op en LEDs
4 o r More
Ch annels
Open
All Channels
Open
YES
YES
YES
IO < 0.5V
IO < 0.5V
OVP
Threshold
Exceeded
3 Channels
Off; 3
Channels On
4 Channels
Off; 2
Channels On
OVP
Triggered
5. Feedback V OVP will reach 2.4V and the OVP
comparator will trip and turn off the boos ter.
6. With the booster off, V OUT and V OVP will discharge.
When feedback reduces to 1.7V the booster is
turned back on.
V OUT Shorted
YES Curren t Limit
Exceed ed
Table 1. Fault Summary
Output
Current is
Limited
7. The OVP circuit will switch 1.8V onto CR V
8. If the load is still open the cycle will co ntinue.
Power-O n Sequence
V IN needs t o be present before PWM pulses are applied to
the DRC pin. Some channels may not turn on if the power
up sequence isn’t followed. This is because the circuits
that monitor the IO pins may see transients during the turn
on-time and may interpret voltage spikes during turn on as
a fault, preventing that channel from turning on. When a
channel is off, its IO pin is at high impedance.
It is best to follow the sequence:
1. V IN
2. PWM dimming at DRC
3. Enable high
January 2010
14
M9999-012110
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