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PM6670 Datasheet(PDF) 31 Page - STMicroelectronics |
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PM6670 Datasheet(HTML) 31 Page - STMicroelectronics |
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31 / 53 page  PM6670 Device description 31/53 Referring to Figure 36, the RDSon sensing technique allows high efficiency performance without the need for an external sensing resistor. The on-resistance of the MOSFET is affected by temperature drift and nominal value spread of the parameter itself; this must be considered during the RILIM setting resistor design. It must be taken into account that the current limit circuit actually regulates the inductor valley current. This means that RILIM must be calculated to set a limit threshold given by the maximum DC output current plus half of the inductor ripple current: Equation 22 The PM6670 provides also a fixed negative current limit to prevent excessive reverse inductor current when the switching section sinks current from the load in forced-PWM (3rd quadrant working conditions). This negative current limit threshold is measured between PHASE and PGND pins, comparing the drop magnitude on PHASE pin with an internal 120mV fixed threshold. 7.1.6 POR, UVLO and Soft Start The PM6670 automatically performs an internal startup sequence during the rising phase of the analog supply of the device (AVCC). The switching controller remains in a stand-by state until AVCC crosses the upper UVLO threshold (4.2V typ.), keeping active the internal discharge MOSFETs (only if AVCC > 1V). The soft-start allows a gradual increase of the internal current limit threshold during start-up reducing the input/output surge currents. At the beginning of start-up, the PM6670 current limit is set to 25% of nominal value and the Under Voltage Protection is disabled. Then, the current limit threshold is sequentially brought to 100% in four steps of approximately 750 µs (Figure 37). Figure 37. Soft-start waveforms DSon ILIM CL R R A 100 I ⋅ µ = S5 Current limit threshold Switching output Time |
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Similar Description - PM6670 |
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