mp1010bef MPS, mp1010bef Datasheet - Page 7

• Manufacturer Part Number: mp1010bef
• Description: Cold Cathode Fluorescent Lamp Driver
• Manufacturer: MPS

Fault Protection
Open Lamp: The OL Pin (#18) is used to detect
whether an open lamp condition has occurred.
A capacitor divider (Cs1 and Cs2) is used to
feedback the lamp voltage to OL with a DC bias
of V
OL pin is typically at 5V DC with an AC swing of
less than 4V in amplitude. If an open lamp
condition exists, the AC voltage on the OL line
will swing below zero volts. When that occurs,
the IC regulates the OL voltage to 10V p-p and
a 1µA current source will inject into the FT pin.
If the voltage at the FT pin exceeds 1.2V, then
the chip will shut down.
Accordingly, the open lamp voltage can be set
using the capacitor divider.
Cs1 must be rated at 3KV or above. Its value is
typically between 10pF to 22pF.
Where V
lamp striking voltage in RMS value.
The value of Rs is typically 100kΩ. It is not
critical as long as the resistance is much larger
than the impedance of Cs2.
Excessive Secondary Current (Shorted Lamp
and UL safety specs): The FB pin (#19) is used
to detect whether excessive secondary current
has occurred. During normal operation the peak
FB voltage is below 1.2V. If a fault condition
occurs that increases the secondary current,
then the voltage at FB will be greater than 1.2V.
When that occurs, a 120µA current source will
inject into the FT pin. If the voltage at the FT pin
exceeds 1.2V, then the chip will shut down.
The following is the design reference for the
secondary winding current.
Sensing network: Rsfb, Risb, Csfb and Cisb.
The recommended value is 3.9kΩ to 10kΩ
(typically 4.7kΩ).
MP1010B Rev. 2.2
9/24/2007
REF
through Rs. During normal operation the
STRIKE_MAX
Csfb
Cs
Rsfb
2
<
=
>
0
Cs
1
7 .
0
is the required maximum
2 .
1
1
7 .
mA
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
×
2 .
V
mA
V
V
×
3
KHz
STRIKE
2
5 .
π
=
V
. 1
=
k 7
_
93
MAX
Ω
nF
MP1010B – COLD CATHODE FLUORESCENT LAMP DRIVER
© 2007 MPS. All Rights Reserved.
www.MonolithicPower.com
Select a capacitor with less than 93nF
capacitance for Csfb and make the FB pin
voltage peak value around 0.7V in normal
operation.
The 0.7mA DC and 0.7mA/KHz AC current
values are taken from the UL60950 safety
requirement.
Cisb and Risb make up a high-pass filter,
wherein the corner frequency should be
between 1KHz and 2KHz so as to minimize the
attenuation of the AC signal.
The recommended value for Risb is 100kΩ.
A typical value for Cisb is 1.2nF.
Fault Timer: The timing for the fault timer will
depend on the sourcing current, as described
above, and the capacitor on the FT pin. The
user can program the time for the voltage to
rise before the chip detects a “real” fault. When
a fault is triggered, then the internal drive
voltage (V
Rft, Cft2 and Cft1 form the fault timer circuit.
The recommended value for Rft is 100kΩ.
For open lamp time:
For secondary short turn off time:
A typical value for Cft2 is 1µF. Cft1 should be
much smaller than Cft2, which makes the short
circuit
recommended value for Cft1 is between 1nF
and 10nF.
If the protection speed for the secondary short
is not critical, the fault timer circuit can be
simplified as a single 1µF capacitor connected
to the FT pin.
protection
1
( T
Cisb
6 .
Dr
open
) will collapse to 0V.
nF
( T
>
short
=
2
_
2
π
lamp
π
×
)
×
Risb
=
Risb
speed
)
Cft
1
=
1
×
( 1
Cft
×
120
2
µ
1
KHz
) F
KHz
( 2
µ
µ
×
1
very
A
) F
µ
1
=
A
2 .
>
×
0
V
Cisb
1
8 .
2 .
nF
fast.
V
The
7