LED7708 STMicroelectronics, LED7708 Datasheet - Page 44

• Manufacturer Part Number: LED7708
• Description: LED Lighting Drivers 16 Ch 85mA LED 3.6 to 36V 250kHz
• Manufacturer: STMicroelectronics

Specifications of LED7708

• Rohs: yes

Recommended operating conditions
8.7.2
8.7.3
8.7.4
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Equation 16
Once the voltage at the VFB pin has crossed the related threshold, the LED7708 shuts
down, the CFP (critical fault protection) flag of the DEVCFG0 register is asserted, the XFLT
pin is set low and the condition is latched.
Normal operation is restored by toggling the DEN bit of the DEVCFG0 register or by
performing a POR.
Overtemperature alert and thermal protection
The LED7708 implements a two-level thermal protection. When the junction temperature
rises above 130 °C (typ.), the overtemperature alert flag (OTAF) is asserted and the XFLT
pin goes low. The OTAF flag is cleared as soon as the chip temperature drops below 125 °C
(typ.). This bit is used by the host controller to enable output voltage regulation if temporarily
disabled.
If the temperature of the chip crosses 150 °C, the device shuts down. Automatic restart
occurs as soon as the temperature drops to 130 °C.
Overcurrent protection
Overcurrent occurring in the power MOSFET is managed by skipping subsequent switching
cycles until the faulty condition is present. The XFLT pin is not affected. See also
Section 8.3.3
LED short-circuit and open channel faults
The faults that can occur to the current generators section are basically the following:
●
●
The LED7708 manages these faulty conditions following a particular algorithm.
When a channel opens, its voltage drop falls to zero: the output voltage regulation circuitry
tags it as “probably faulty” and increases the output voltage. If the “open” condition is
temporary, the voltage drop across the generator is self-recovered and the output voltage is
corrected. In case of a real open channel, the regulation DAC reaches the maximum value
(DAC overflow) and the OVFW bit of the DEVCFG0 register is asserted.
The LED7708 can automatically disconnect the open channels if required. Therefore, if the
open channels auto-disconnection (OCAD) bit of the DEVCFG0 register is asserted, the
faulty channel is suddenly turned off, excluded from the regulation loop and the output
voltage is quickly adjusted to the optimal level. During the open channel detection (i.e. when
the regulation DAC is rolling up), the LED short-circuit detection circuitry is masked to avoid
false detection due to the high output voltage.
If the OCAD bit is cleared, the output voltage is kept to the maximum value and the fault
management is demanded to the host controller (XFLT pin low).
In case of LED short-circuit, the voltage across the related channel increases. The LED7708
compares the drop across each channel with a fixed threshold in order to detect this
one or more open (floating) channels
excessive voltage drop (shorted LEDs).
for details.
Doc ID 022698 Rev 1
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LED7708