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RT9746 Scheda tecnica(PDF) 8 Page - Richtek Technology Corporation |
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RT9746 Scheda tecnica(HTML) 8 Page - Richtek Technology Corporation |
8 / 10 page RT9746 8 DS9746-00 March 2016 www.richtek.com © Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information Power Up The RT9746 has a threshold of 2.7V power on reset (POR) with a built-in hysteresis of 100mV. Before the input voltage reaches the POR threshold, the RT9746 is off. When the input voltage is over the POR threshold, the VOUT of RT9746 will delay for 32ms which includes soft-start time of 8ms. The 32ms delay allows the transient at the input during a hot insertion of the power supply to settle down before the IC starts to operate. During the soft-start transition, the RT9746 slowly turns on the internal MOSFET to reduce the inrush current. Over Temperature Protection (OTP) The RT9746 monitors its internal temperature to prevent thermal failures. The chip turns off the MOSFET when the junction temperature reaches 160 °C. The IC will resume after the junction temperature is cooled down 20 °C. Input Over-Voltage Protection The RT9746 provides input over-voltage protection via internal or external resistor to set OVP level. If OVPFB is connected to GND, the RT9746 uses the internal OVP level setting. If an external resistor-divider is connected to OVPFB and VOVPFB exceeds the VOVP_SEL voltage, the RT9746 will adopt external OVP level setting. The OVP level VOVP is set as below. OVP REF_EXT R2 V = V x 1+ R1 When the input volatge exceeds the OVP level, the RT9746 will turn off internal MOSFET around 0.18us to prevent the high input volatge from damaging the end system. When the input volatge returns to normal operation volatge range with hysteresis (internal 150mV, external 100mV), the RT9746 will turn on the MOSFET to re-enable output. Over-Current Protection (OCP) The RT9746 monitors the output current to prevent the output short or the charging of the battery with an excessive current. The RT9746 has a built-in 180 μs delay time to prevent any transient noise triggering the OCP. If the OCP situation keeps for 180 μs, the internal MOSFET will be turned off. Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TAis the ambient temperature, and θJAis the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125 °C. The junction to ambient thermal resistance, θJA, is layout dependent. For WL-CSP-12B 1.92x1.27 (BSC) package, the thermal resistance, θJA, is 59.6°C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA= 25 °C can be calculated by the following formula : PD(MAX) = (125 °C − 25°C) / (59.6°C/W) = 1.67W for WL-CSP-12B 1.92x1.27 (BSC) package The maximum power dissipation depends on the operating ambient temperature for fixed TJ(MAX) and thermal resistance, θJA. The derating curve in Figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. |
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