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ISL6740IBZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6740IBZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 29 page  11 The ISL6740, ISL6741 features a built-in soft start. Soft start is implemented as a clamp on the error voltage input. OTS - The non-inverting input to the over temperature shutdown comparator. The signal input at this pin is compared to an internal threshold of VREF/2. If the voltage at this pin exceeds the threshold, the Fault signal is asserted and the outputs are disabled until the condition clears. There is a nominal 25 µA switched current source used for hysteresis. The amount of hysteresis is adjustable by varying the source impedance of the signal into this pin. OTS may be used to monitor parameters other than temperature, such as voltage. Any signal for which a high out-of-bounds monitor is desired may utilize the OTS comparator. FAULT - The Fault signal is asserted high whenever the outputs, OUTA and OUTB, are disabled. This occurs during an over temperature fault, an input UV fault, a VREF UV fault, or during an over current (ISL6740) or short circuit shutdown fault. Fault can be used to disable synchronous rectifiers whenever the outputs are disabled. Fault is a three-state output and is high impedance during the soft start cycle. Adding a pull-up resistor to VREF or a pull-down resistor to ground determines the state of Fault during soft start. This feature allows the designer to use the Fault signal to enable or disable output synchronous rectifiers during soft start. UV - Undervoltage monitor input pin. A resistor divider between the input source voltage and GND sets the under voltage lock out threshold. The signal is compared to an internal 1.00V reference to detect an under voltage or inhibit condition. CS - This is the input to the current sense comparator(s). The IC has the PWM comparator for peak current mode control (ISL6741) and an over current protection comparator. The over current comparator threshold is set at 0.600V nominal. The CS pin is shorted to GND at the end of each switching cycle. Depending on the current sensing source impedance, a series input resistor may be required due to the delay between the internal clock and the external power switch. This delay may allow an overlap such that the CS signal may be discharged while the current signal is still active. If the current sense source is low impedance it will cause increased power dissipation. ISL6740 - Exceeding the over-current threshold will start a delayed shutdown sequence. Once an over current condition is detected, the soft start charge current source is disabled. The soft start capacitor begins discharging through a 25 µA current source, and if it discharges to less than 4.25V (Sustained Over Current Threshold), a shutdown condition occurs and the OUTA and OUTB outputs are forced low. When the soft start voltage reaches 0.27V (Reset Threshold) a soft start cycle begins. An over current condition must be absent for 50 µs before the delayed shutdown control resets. If the over current condition ceases, and an additional 50 µs period elapses before the shutdown threshold is reached, no shutdown occurs. The SS charging current is re-enabled and the soft start voltage is allowed to recover. ISL6741 - The ISL6741 current mode controller does not shutdown due to an overcurrent condition. The pulse-by- pulse current limit characteristic of peak current mode control limits the output current to acceptable levels. GND - Reference and power ground for all functions on this device. Due to high peak currents and high frequency operation, a low impedance layout is necessary. Ground planes and short traces are highly recommended. OUTA and OUTB - Alternate half cycle output stages. Each output is capable of 0.5A peak currents for driving logic level power MOSFETs or MOSFET drivers. Each output provides very low impedance to overshoot and undershoot. VREF - The 5.00V reference voltage output. +1/-2% tolerance over line, load and operating temperature. Bypass to GND with a 0.047 µF to 2.2µF ceramic capacitor. Capacitors outside of this range may cause oscillation. SS - Connect the soft start timing capacitor between this pin and GND to control the duration of soft start. The value of the capacitor determines the rate of increase of the duty cycle during start up, controls the over current shutdown delay (ISL6740), and the over current and short circuit hiccup restart period. SCSET - Sets the duty cycle threshold that corresponds to a short circuit condition. A resistive divider between RTC and GND or RTD and GND, or a voltage between 0 and 2V may be used to adjust the SCSET threshold. If using a resistor divider from either RTC or RTD, the impedance to GND affects the oscillator timing and should be considered when determining the oscillator timing components. Connecting SCSET to GND disables short circuit shutdown and hiccup. Functional Description Features The ISL6740, ISL6741 PWMs are an excellent choice for low cost bridge and push-pull topologies for applications requiring accurate duty cycle and deadtime control. With its many protection and control features, a highly flexible design with minimal external components is possible. Among its many features are current mode control (ISL6741), adjustable soft start, over current protection, thermal protection, bidirectional synchronization, fault indication, and adjustable frequency. ISL6740, ISL6741 |
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