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ISL6721AVZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6721AVZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 22 page  11 FN9110.6 March 5, 2008 The minimum amount of capacitance to place at the SLOPE pin is calculated in Equation 6: where tON is the On time and VSLOPE is the amount of voltage to be added as slope compensation to the current feedback signal. In general, the amount of slope compensation added is 2 to 3 times the minimum required. Example: Assume the inductor current signal presented at the ISENSE pin decreases 125mV during the Off period, and: Switching Frequency, fsw = 250kHz Duty Cycle, D = 60% tON = D/fsw = 0.6/250E3 = 2.4µs tOFF = (1 - D)/fsw = 1.6µs Determine the downslope: Downslope = 0.125V/1.6µs = 78mV/µs. Now determine the amount of voltage that must be added to the current sense signal by the end of the On time. Therefore, An appropriate slope compensation capacitance for this example would be 1/2 to 1/3 the calculated value, or between 68pF and 33pF. Overvoltage and Undervoltage Monitor The OV and UV signals are inputs to a window comparator used to monitor the input voltage level to the converter. If the voltage falls outside of the user designated operating range, a shutdown fault occurs. For OV faults, the supply current, ICC, is reduced to 200µA for ~295ms at which time recovery is attempted. If the fault is cleared, a soft-start cycle begins. Otherwise another shutdown cycle occurs. A UV condition also results in a shutdown fault, but the device does not enter the low power mode and no restart delay occurs when the fault clears. A resistor divider between VIN and LGND to each input determines the operational thresholds. The UV threshold has a fixed hysteresis of 75mV nominal. Overcurrent Operation The overcurrent threshold level is set by the voltage applied at the ISET pin. Setting the overcurrent level may be accomplished by using a resistor divider network from VREF to LGND. The ISET threshold should be set at a level that corresponds to the desired peak output inductor current plus the additive effects of slope compensation. Overcurrent delayed shutdown is enabled once the soft-start cycle is complete. If an overcurrent condition is detected, the soft-start charging current source is disabled and the discharging current source is enabled. The soft-start capacitor is discharged at a rate of 40µA. At the same time, a 50µs retriggerable one-shot timer is activated amd it remains active for 50µs after the overcurrent condition stops. The soft-start discharge cycle cannot be reset until the one- shot timer becomes inactive. If the soft-start capacitor discharges by more than 0.125V to 4.375V, the output is disabled and the soft-start capacitor is discharged. The output remains disabled and ICC drops to 200µA for approximately 295ms. A new soft-start cycle is then initiated. The shutdown and restart behavior of the OC protection is often referred to as hic-cup operation due to its repetitive start-up and shutdown characteristic. If the overcurrent condition ceases at least 50µs prior to the soft-start voltage reaching 4.375V, the soft-start charging and discharging currents revert to normal operation and the soft-start voltage is allowed to recover. Hiccup OC protection may be defeated by setting ISET to a voltage that exceeds the Error Amplifier current control voltage, or about 1.5V. Leading Edge Blanking The initial 100ns of the current feedback signal input at ISENSE is removed by the leading edge blanking circuitry. The blanking period begins when the GATE output leading edge exceeds 3.0V. Leading edge blanking prevents current spikes from parasitic elements in the power supply from causing false trips of the PWM comparator and the overcurrent comparator. Fault Conditions A Fault condition occurs if VREF falls below 4.65V, the OV input exceeds 2.50V, the UV input falls below 1.45V, or the junction temperature of the die exceeds ~+130°C. When a Fault is detected the GATE output is disabled and the soft-start capacitor is quickly discharged. When the Fault condition clears and the soft-start voltage is below the reset threshold, a soft-start cycle begins. Time Downslope Current Sense Signal TIME DOWNSLOPE CURRENT SENSE SIGNAL FIGURE 5. C SLOPE 4.24 6 – ×10 t ON V SLOPE ----------------------- • = F (EQ. 6) V SLOPE 1 2 --- 0.078 2.4 • • 94mV == (EQ. 7) C SLOPE MIN () 4.24 6 – ×10 2.4 6 – ×10 0.094 ----------------------- • 110pF ≈ = (EQ. 8) ISL6721 |
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