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HV9912NG-G-M934 Datasheet(PDF) 9 Page - Microchip Technology |
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HV9912NG-G-M934 Datasheet(HTML) 9 Page - Microchip Technology |
9 / 20 page 2016 Microchip Technology Inc. DS20005583A-page 9 HV9912 voltage is between the start and stop voltages. Therefore, it is recommended that the input voltage be kept higher than VINSTOP. 3.4 Reference HV9912 includes a 2% accurate 1.25V reference, which can be used as the reference for the output current as well as to set the switch current limit. The reference is buffered so that it can deliver a maximum of 500 µA external current to drive the external circuitry. The reference should be bypassed with at least a 10 nF low-ESR capacitor. Note: To avoid abnormal Startup conditions, the bypass capacitor at the REF pin should not exceed 0.1 µF. 3.5 Oscillator Connecting the resistor between RT and GND will program the time period. In both cases, resistor RT sets the current, which charges an internal oscillator capacitor. The capacitor voltage ramps up linearly. When the voltage increases beyond the internal set voltage, a comparator triggers the set input of the internal SR flip-flop. This starts the next switching cycle. The time period of the oscillator can be computed as shown in Equation 3-4. EQUATION 3-4: TS RT 18pF 3.6 Synchronization The SYNC pin is an input/output (I/O) port to a fault-tolerant peer-to-peer and/or master clock synchronization circuit. For synchronization, the SYNC pins of multiple HV9912-based converters can be connected together and may also be connected to the open drain output of a master clock. When connected in this manner, the oscillators will lock to the device with the highest operating frequency. When synchronizing multiple ICs, it is recommended that the same timing resistor (corresponding to the switching frequency) be used in all the HV9912 circuits. On rare occasions, given the length of the connecting lines for the SYNC pins, a resistor between SYNC and GND may be required to damp any ringing due to parasitic capacitances. It is recommended that the resistor chosen be greater than 300 kΩ. When synchronized in this manner, a permanent High or Low condition on the SYNC pin will result in a loss of synchronization, but the HV9912-based converters will continue to operate at their individually set operating frequencies. Since loss of synchronization will not result in total system failure, the SYNC pin is considered fault tolerant. 3.7 Slope Compensation For Continuous Conduction mode converters operating in the Constant Frequency mode, slope compensation becomes necessary to ensure stability of the Peak Current mode controller if the operating duty cycle is greater than 50%. Choosing a slope compensation which is one half of the down slope of the inductor current ensures that the converter will be stable for all duty cycles. Slope compensation can be programmed by two resistors RSLOPE and RSC. Assuming a down slope of DS (A/µs) for the inductor current, the slope compensation resistors can be computed as illustrated in Equation 3-5. EQUATION 3-5: RSC RSLOPE DS 10 6 TS RCS 10 -------------------------------------------------------------------------- = Where RCS is the current sense resistor which senses the switching FET current Note: The maximum current that can be sourced out of the SC pin is 100 µA. This limits the minimum value of the RSLOPE resistor to 25 kΩ. If the equation for slope compensation produces a RSLOPE less than this value, then RSC would have to be reduced accordingly. It is recommended that RSLOPE be chosen within the range of 25 kΩ to 50 kΩ. 3.8 Current Sense The current sense input of the HV9912 includes a built-in 100 ns (minimum) blanking time to prevent spurious turn-off due to the initial current spike when the FET turns on. The HV9912 includes two high-speed comparators— one is used during normal operation and the other is used to limit the maximum input current during Input Undervoltage or Overload conditions. The IC includes an internal resistor divider network, which steps down the voltage at the COMP pin by a factor of 15. This stepped-down voltage is given to one of the comparators as the current reference. The reference to the other comparator, which acts to limit the maximum inductor current, is given externally. It is recommended that the sense resistor RCS be chosen so as to provide about 250 mV current sense signal. |
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