HV9957K7-G Supertex, HV9957K7-G Datasheet - Page 10

• Manufacturer Part Number: HV9957K7-G
• Description: LED Lighting Drivers LED DRIVER 6 CHANNEL W/ FAULT PROTECTION
• Manufacturer: Supertex

Specifications of HV9957K7-G

• Product Category: LED Lighting Drivers
• Rohs: yes
• Input Voltage: 5 V
• Maximum Supply Current: 10 mA
• Maximum Operating Temperature: + 125 C
• Mounting Style: SMD/SMT
• Package / Case: QFN-24
• Minimum Operating Temperature: - 40 C
• Factory Pack Quantity: 3000

solute voltage (REF to GND) above a set threshold, the five
good strings will be misidentified as shorted and disabled.
Another problem exists with a conventional SLP; it can block
activation of OLP. Open strings cause boost voltage to rise
in an attempt to achieve 900mV on the faulty channel. The
channel with the open string remains at ground, eventually
causing VBST to rise until OVP trips. This is the trigger for
the OLP circuit, which then identifiers and disables chan-
nels at 0V. The problem with this approach arises with the
good strings: as VBST rises towards OVP, the good strings
apply a high voltage to the channel outputs. This can cause
SLP to trip before OVP trips, causing good strings to be mis-
interpreted as shorted and disabled while the open string
remains. To reduce the chances of this occurring, the SLP
threshold can be set higher, but this may render SLP use-
less.
The purpose of SLP is to prevent excessive dissipation in
the channel output drivers and the resultant rise in die tem-
perature. If die temperature remains reasonable, the shorted
LED string can continue to operate, albeit at reduced bright-
ness. Note that little power is wasted in the shorted LED.
For these two reasons the primary mechanism for disabling
faulty strings is triggered when die temperature becomes ex-
cessive rather than at an arbitrary voltage level. The excep-
tions are the SLP and OVP limits, which should be high as a
fail safe mechanism rather than as the normal trigger for the
faulty string protection circuit. Once die temperature reaches
the OTP1 threshold, a single string is identified as faulty and
disabled. This should reduce internal power dissipation two
ways. First, it eliminates the power dissipation in the channel
driver with the faulty string. Second, the disconnected string
reduces the load on the boost converter, which in turn allows
the boost switch to run cooler.
When the die temperature falls below the hysteresis tem-
perature, OTP re-arms after 800ms. If the die temperature
has not fallen by the hysteresis temperature, OTP1 is re-
triggered. In addition, to prevent the successive disabling
of channels following an OLP event, the protection circuit is
not re-armed until at least one channel’s voltage has fallen
below the ACH (All Channels High) threshold. When OTP1
resets and ACH goes low, the protection circuit is re-armed
to trigger on another fault, but the previously identified faulty
string remains disabled under the assumption that faulty
strings do not spontaneously repair themselves. Toggling
EN or cycling power resets the faulty strings.
If the disabling of strings does not prevent die temperature
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10
from rising, the second-stage OTP activates, shutting down
the boost converter. OTP2 self-resets when die temperature
falls by the hysteresis temperature, re-enabling the boost
converter but not reconnecting faulty strings.
If a string is fully open, the protection circuit cannot wait
for the temperature to rise to OTP1 level - in the meantime
output voltage would rise to destructive levels. Once OVP
is activated, the channel with the open string is identified
and disconnected. If more than one string is open, OVP will
retrigger and the second open string is disconnected. The
cycle repeats as long as there is an open string on an active
channel.
Once OTP1 is triggered, a fault condition has been estab-
lished and the next step is to determine if a short or long
string is the cause. An ‘outlier’ approach is used. Outliers
being those channels with a voltage significantly different
from the others. The midpoint between the highest and low-
est active channels is compared to the average of the active
channels to determine whether the faulty string is long or
short. The following examples demonstrate the technique.
Shorted String
Partially Open String
CH1
CH1
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CH2
CH2
Partially
open string
CH3
CH3
Shorted string
CH4
CH4
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CH5
CH5
CH6
CH6
Mid > avg,
short string
determined
Mid < avg,
long string
determined
HV9957
V
V
V
V
V
V
CH(mid)
CH(avg)
FB
CH(avg)
CH(mid)
FB