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FDS6670AS Datasheet(PDF) 12 Page - Richtek Technology Corporation |
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FDS6670AS Datasheet(HTML) 12 Page - Richtek Technology Corporation |
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12 / 18 page  12 DS8298-01 November 2011 www.richtek.com RT8298 Copyright 2011 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. © Figure 6. Startup Sequence Using External Sync Clock Soft-Start The RT8298 provides soft-start function. The soft-start function is used to prevent large inrush current while converter is being powered-up. An internal current source (6nA) charges an internal capacitor (15pF) to build a soft- start ramp voltage. The VFB voltage will track the internal ramp voltage during soft-start interval. The typical soft- start time is calculated as follows : Figure 6 shows the synchronization operation in startup period. When the EN/SYNC is triggered by an external clock, the RT8298 enters soft-start phase and the output voltage starts to rise. When VFB is lower than 0.7V, the oscillation frequency will be proportional to the feedback voltage. With higher VFB, the switching frequency is relatively higher. After startup period about 3.5ms, the IC operates with the same frequency as the external clock. Figure 7. Hiccup Mode Under Voltage Protection SS (0.808V 15pF) t = = 2ms 6nA × Operating Frequency and Synchronization The internal oscillator runs at 600kHz (typ.) when the EN/ SYNC pin is at logic-high level (>2V). If the EN pin is pulled to low-level for 10 μs above, the IC will shut down. The RT8298 can be synchronized with an external clock ranging from 300kHz to 1.5MHz applied to the EN/SYNC pin. The external clock duty cycle must be from 10% to 90%. Over Temperature Protection The RT8298 features an Over Temperature Protection (OTP) circuitry to prevent from overheating due to excessive power dissipation. The OTP will shut down switching operation when junction temperature exceeds 150 °C. Once the junction temperature cools down by approximately 20 °C, the converter will resume operation. To maintain continuous operation, the maximum junction temperature should be lower than 125 °C. Under Voltage Protection For the RT8298, it provides Hiccup Mode Under Voltage Protection (UVP). When the VFB voltage drops below 0.4V, the UVP function will be triggered to shut down switching operation. If the UV condition remains for a period, the RT8298 will retry every 2ms. When the UV condition is removed, the converter will resume operation. The UVP is disabled during soft-start period. EN/SYNC VFB CLK Foldback 600kHz External CLK 3.5ms (Start-up period) 10µs Duty Cycle Limitation The RT8298 has a maximum duty cycle 90%. The minimum input voltage is determined by the maximum duty cycle and its minimum operating voltage 4.5V. The voltage drops of high side MOSFET and low side MOSFET also must be considered for the minimum input voltage. The minimum duty cycle can be calculated by the following equation : Duty Cycle(min) = fSW x tON(min) Time (2.5ms/Div) Hiccup Mode VOUT (1V/Div) IL (5A/Div) VIN = 12V, IOUT = Short |
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