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FUNCTIONAL DEVICE OPERATION
PROTECTION AND DIAGNOSTIC FEATURES
PROTECTION AND DIAGNOSTIC FEATURES
OVER-CURRENT PROTECTION
The boost converter features Over-current Protection
(OCP). The OCP operates on a cycle by cycle basis.
However, if the OCP condition remains for more than 60 ms
then the boost regulator is latched off and GD asserts high to
shutoff the Q FET. The device can only be restarted by
cycling the power supply.
OVER-VOLTAGE PROTECTION
The 34848 features user programmable Over-voltage
Protection (OVP). The OVP level can be programmed
between 31 and 45 V in 2.0 V steps. When OVP is reached,
the V OUT voltage is clamped at the OVP level, and the 50 ms
timer is started. If the V OUT is clamped at the OVP level for
more than 50 ms, the V OUT voltage is lowered by 4.0 V, the
DHC is turned off, and another 150 ms timer starts After the
150 ms timer has elapsed, the DHC is turned on again.
If V OUT drops below the OVP before the 50 ms timer
expires, normal operation continues. However, if OVP is still
reached after the 50 ms timer expires, the device will repeat
the OVP sequence two more times. If the OVP fault condition
is still present after the third sequence, the Q-FET, LED
drivers, and boost are turned off.
The 34848 uses an internal ADC to measure the voltage
level at the OVP pin and compares it with the configured OVP
level. A resistor divider network needs to be added between
V OUT and ground, with the center tap connected to OVP. The
typical value for the upper divider resistor R UPPER is 243 k
This results in a divider ratio of 23.71 (R UPPER + R LOWER /
R LOWER ), which sets the max OVP value.
The OVP voltage can be programmed from 31 to 45 V.
LED OPEN PROTECTION
The 34848 monitors the LED status at all 8 channels. The
output voltage is continuously maintained at the minimum
voltage possible to drive all LEDs, i.e. the maximum forward
voltage of the 8 strings plus the minimum threshold needed
(0.5 V, typ) for the current sense circuit and the current driver
to regulate.
If an LED fails open, the voltage at the CHx pin for the
effected LED string falls close to ground, and therefore the
correspondent LED channel will be disabled.
LED SHORT PROTECTION
The 34848 also protects against LED short failures. If delta
V FB , |V FB_MAX – V FB_MIN |, is higher than SFDV and PWM
duty of each channels are more than 0%, then this V FB_MAX
channel is disabled.
OVER-TEMPERATURE PROTECTION
The 34848 has an on-chip temperature sensor that
measures die temperature. If the IC temperature exceeds the
OTP threshold, the IC will turn off. While off, the GD pin is set
to high-impedance to turn off the Q-FET. The device turns
back on after recycling the power.
(1%) and 10.7 k (1%) for the lower resistor R LOWER .
34848
Analog Integrated Circuit Device Data
Freescale Semiconductor
25
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