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CS1616-FSZ Fiches technique(PDF) 11 Page - Cirrus Logic |
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CS1616-FSZ Fiches technique(HTML) 11 Page - Cirrus Logic |
11 / 16 page CS1615/16 DS961F1 11 5.6.7 Overcurrent Protection Overcurrent protection (OCP) is implemented by monitoring the voltage across the sense resistor. If this voltage exceeds a threshold VOCP(th) of 1.69V, a fault condition occurs. The IC output is disabled and the controller attempts to restart after approximately one second. 5.6.8 Open Loop Protection Open loop protection (OLP) and sense resistor short protection are implemented by monitoring the voltage across the resistor. If the voltage on pin FBSENSE does not reach the protection threshold VOLP(th) of 200mV, the IC output is disabled, and the controller attempts to restart after approximately one second. 5.7 Overtemperature Protection The CS1615/16 incorporates internal overtemperature protection (iOTP) and the ability to connect an external overtemperature sense circuit for IC protection. Typically, an NTC thermistor is used. 5.7.1 Internal Overtemperature Protection Internal overtemperature protection (iOTP) is activated, and switching is disabled when the die temperature of the devices exceeds 135°C. There is a hysteresis of about 14°C before resuming normal operation. 5.7.2 External Overtemperature Protection The external overtemperature protection (eOTP) pin is used to implement overtemperature protection. A negative temperature coefficient (NTC) thermistor resistive network is connected to pin eOTP, usually in the form of a series combination of a resistor RS and a thermistor RNTC (see Figure 15). The CS1615/16 cyclically samples the resistance connected to pin eOTP. The total resistance on the eOTP pin gives an indication of the temperature and is used in a digital feedback loop to adjust current ICONNECT into the NTC and series resistor RS to maintain a constant reference voltage VCONNECT(th) of 1.25V. Current ICONNECT is generated from a controlled current source with a full-scale current of 80 A. When the loop is in equilibrium, the voltage on the eOTP pin fluctuates around reference voltage VCONNECT(th). A resistance ADC is used to generate current ICONNECT. The ADC output is filtered to suppress noise and compared against a reference that corresponds to 125°C. A second low-pass filter with a time constant of two seconds filters the ADC output and is used to scale down the internal dim level of the system (and hence LED current ILED) if the temperature exceeds 95°C. The large time constant for this filter ensures that the dim scaling does not happen spontaneously and is not noticeable (suppress spurious glitches). The eOTP tracking circuit is designed to function accurately with external capacitance up to 470pF. The tracking range of this resistance ADC is approximately 15.5k to 4M. The series resistor RS is used to adjust the resistance of the NTC to fall within the ADC tracking range, allowing the entire dynamic range of the ADC to be well used. The CS1615/16 recognizes a resistance (RS+RNTC) equal to 20.3k which corresponds to a temperature of 95°C, as the beginning of an overtemperature dimming event and starts reducing the power dissipation. The output current is scaled until the series resistance (RS+RNTC) value reaches 16.6k (125°C). Beyond this temperature, the IC shuts down until the resistance (RS+RNTC) rises above 19.23k. This is not a latched protection state, and the ADC keeps tracking the temperature in this state in order to clear the fault state once the temperature drops below 110°C. When exiting reset, the chip enters startup and the ADC quickly (<5ms) tracks the external temperature to check if it is below the 110°C reference code before the controller is powered up. If this check fails, the chip will wait until this condition becomes true before initializing the rest of the system. For example, a 14k (±1% tolerance) series resistor is required to allow measurements of up to 130°C to be within the eOTP tracking range when a 100k NTC with a Beta of 4275. If the temperature exceeds 95°C, thermistor RNTC is approximately 6.3k and series resistor RS is 14k, so the eOTP pin has a total resistance of 20.3k . The eOTP pin initiates protective dimming action by reducing the power dissipation. At 125°C the thermistor RNTC has 2.6k plus a series resistor RS equal to 14k present a resistance of 16.6k at the eOTP pin reaching the point where a thermal shutdown fault intervenes. The CS1615/16 will continue to monitor pin eOTP and once the series resistor RS plus the thermistor RNTC rises above 19.23k the device will resume power conversion (see Figure 16). If the external overtemperature protection feature is not required, connect the eOTP pin to GND using a 50k -to-500k resistor to disable the eOTP feature. CS1615/16 + - I CONNE CT V CONNE CT (th) Comp_Out eOTP Control eOTP RS CNTC NTC V DD 10 (Optional ) Figure 15. eOTP Functional Diagram Temperature (°C) 125 95 50% 100% 0 25 Figure 16. eOTP Temperature vs. Impedance |
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