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SI9976DY-T1 Datasheet(PDF) 5 Page - Vishay Siliconix |
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SI9976DY-T1 Datasheet(HTML) 5 Page - Vishay Siliconix |
5 / 6 page Si9976 Vishay Siliconix Document Number: 70016 S-40757—Rev. F, 19-Apr-04 www.vishay.com 5 DETAILED DESCRIPTION Power On Conditioning Bootstrap-type floating supplies require that the bootstrap capacitor be charged at power on. In the case of the Si9976, this is accomplished by pulsing the IN line low with the EN line held high, thus turning on the low-side MOSFET and providing the charging path for the capacitor. Operating Voltage: 20 to 40 V The Si9976 is intended to be powered by a single power supply within the range of 20 to 40 V and is designed to drive a totem pole pair of NMOS power transistors such as those within the Si9955. The power transistors must be powered by the same power supply as this driver. In addition to the high-voltage power supply (20 to 40 V), the Si9976 must have a power supply connected to the VCC terminal, if a fault output signal is desired. This power supply provides operating voltage for the fault output and allows the high output voltage level to be compatible with system logic that monitors the fault condition. The value of this power supply must be within the range of 4.5 to 16.5 V to ensure functionality of the output. Internal fault circuitry, which is used for shorted-load protection, is not affected by this power supply. Cross-Conduction Protection The high-side power transistor can only be turned on after a fixed time delay following the return to ground of the low-side power transistor’s gate. The low-side transistor can only be turned on after a fixed time delay following the high-side transistor turn-off signal. Undervoltage Lockout During power up, both power transistors are held off until the internal regulated power supply, VDD, is approximately one Vbe from the final value, nominally 16 V. After power up, the undervoltage lockout circuitry continues to monitor VDD. If an undervoltage condition occurs, both the high-side and low-side transistors will be turned off and the fault output will be set high. When the undervoltage condition no longer exists, normal function will resume automatically. Separate voltage sensing of the bootstrap capacitor voltage allows a turn-on signal to be sent to the high-side drive circuit if either the bootstrap capacitor has full voltage, or the load voltage is high (driven high by an inductive load or shorted high). The voltage sensing circuit will allow the high-side power transistor to turn on if an on signal is present and the voltage on the bootstrap capacitor rises from undervoltage to operating voltage. Short Circuit Protection This device is intended to be used only in a half-bridge which drives inductive loads. A shorted load is presumed if the load voltage does not make the intended transition within an allotted time. Separate timing is provided for the two transitions. A longer time is allowed for the high-side to turn on (300 ns vs. 200 ns) since the propagation delays are longer. Excessive capacitive loading can be interpreted as a short. The value of capacitance that is needed to produce the indication of a short depends on the load driving capability of the power transistors. ESD Protection Electrostatic discharge protection devices are between VDD and GND, VCC and GND, and from terminals IN, EN, G2, and FAULT to both VDD and GND. V+, CAP, S1, and G1 are not ESD protected. Fault Feedback Detection of a shorted load sets a latch which turns off both the high-side and the low-side power transistors. If VCC is present, a one level will be present on the FAULT output. To reset the system, the enable input, EN, must be lowered to a logic zero and then raised to a logic one. The logic level of the input, IN, will determine which power transistor will be turned on first after reset. An undervoltage condition on VDD is not latched, but causes a one level on the FAULT output, if VCC is present. Static (dc) Operation All components of a charge pump, except the holding (bootstrap) capacitor, are included in the circuit. This charge pump will provide current that is sufficient to overcome any leakage currents which would reduce the enhancement voltage of the high-side power transistor while it is on. This allows the high-side power transistor to be on continuously. When the low-side power transistor is turned on, additional charge is restored to the bootstrap capacitor, if needed. The maximum switching speed of the system at 50% duty cycle is limited by the on time of the low-side power transistor. During this time, the bootstrap capacitor charge must be restored. However, if the duty cycle is skewed so that the on time of the high-side power transistor is long enough for the charge pump to completely restore the charge lost during switching, then the on time of the low-side power transistor is not restricted. |
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