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ISL6334D Datasheet(PDF) 20 Page - Renesas Technology Corp |
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ISL6334D Datasheet(HTML) 20 Page - Renesas Technology Corp |
20 / 28 page ISL6334D FN6802 Rev 4.00 Page 20 of 28 Apr 29, 2016 Overcurrent Protection The ISL6334D has two levels of overcurrent protection. Each phase is protected from a sustained overcurrent condition by limiting its peak current, while the combined phase currents are protected on an instantaneous basis. In instantaneous protection mode, ISL6334D utilizes the sensed average current IAVG to detect an overcurrent condition. See “Current Sensing” on page 13 for more details on how the average current is measured. The average current is continually compared with a constant 105µA reference current, as shown in Figure 13. Once the average current exceeds the reference current, a comparator triggers the converter to shutdown. The current out of the IMON pin is equal to the sensed average current IAVG. With a resistor from IMON to GND, the voltage at IMON will be proportional to the sensed average current and the resistor value. The ISL6334D continuously monitors the voltage at IMON pin. If the voltage at IMON pin is higher than 1.11V, a comparator triggers the overcurrent shutdown. By increasing the resistor between IMON and GND, the overcurrent protection threshold can be adjusted to be less than 105µA. For example, the overcurrent threshold for the sensed average current IAVG can be set to 95µA by using a 11.8kΩ resistor from IMON to GND. At the beginning of overcurrent shutdown, the controller places all PWM signals in a high-impedance state within 20ns, commanding the Intersil MOSFET driver ICs to turn off both upper and lower MOSFETs. The system remains in this state a period of 4096 switching cycles. If the controller is still enabled at the end of this wait period, it will attempt a soft-start. If the fault remains, the trip-retry cycles will continue indefinitely (as shown in Figure 14) until either controller is disabled or the fault is cleared. Note that the energy delivered during trip-retry cycling is much less than during full-load operation, so there is no thermal hazard during this kind of operation. For the individual channel overcurrent protection, ISL6334D continuously compares the sensed current signal of each channel with the 129µA reference current. If one channel current exceeds the reference current, ISL6334D will pull the PWM signal of this channel to low for the rest of the switching cycle. This PWM signal can be turned on next cycle if the sensed channel current is less than the 129µA reference current. The peak current limit of individual channel will not trigger the converter to shutdown. Thermal Monitoring (VR_HOT/VR_FAN) There are two thermal signals to indicate the temperature status of the voltage regulator: VR_HOT and VR_FAN. Both VR_FAN and VR_HOT pins are open-drain outputs, and external pull-up resistors are required. Those signals are valid only after the controller is enabled. The VR_FAN signal indicates that the temperature of the voltage regulator is high and more cooling airflow is needed. The VR_HOT signal can be used to inform the system that the temperature of the voltage regulator is too high and the CPU should reduce its power consumption. The VR_HOT signal may be tied to the CPU’s PROC_HOT signal. The diagram of thermal monitoring function block is shown in Figure 15 on page 21. One NTC resistor should be placed close to the power stage of the voltage regulator to sense the operational temperature, and one pull-up resistor is needed to form the voltage divider for the TM pin. As the temperature of the power stage increases, the resistance of the NTC will reduce, resulting in the reduced voltage at the TM pin. Figure 16 on page 21 shows the TM voltage over the temperature for a typical design with a recommended 6.8kΩ NTC (P/N: NTHS0805N02N6801 from Vishay) and 1kΩ resistor RTM1. We recommend using those resistors for accurate temperature compensation. There are two comparators with hysteresis to compare the TM pin voltage to the fixed thresholds for VR_FAN and VR_HOT signals respectively. The VR_FAN signal is set to high when the TM voltage is lower than 39.1% of VCC voltage, and is pulled to GND when the TM voltage increases to above 45.1% of VCC FIGURE 13. VR_RDY AND PROTECTION CIRCUITRY - + VID + 0.175V VDIFF - + 105µA IAVG DAC OV OC UV VR_RDY 50% SOFT-START, FAULT AND CONTROL LOGIC - + OC IMON 1.11V 0A 0V 2ms/DIV OUTPUT CURRENT FIGURE 14. OVERCURRENT BEHAVIOR IN HICCUP MODE fSW = 500kHz OUTPUT VOLTAGE |
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