Datasheet SD8942 (Shouding) - 5

SD8942 FUNCTIONAL DESCRIPTION
Internal Regulator
The SD8942 is a current mode step down DC/DC compensation network minimizes the external converter that provides excellent transient response component counts and simplifies the control loop with no extra external compensation components. design. This device contains an internal, low resistance, high voltage power MOSFET, and operates at a high
Internal Soft-Start
600K operating frequency to ensure a compact, high efficiency design with excellent AC and DC The soft-start is implemented to prevent the performance. converter output voltage from overshooting during startup. When the chip starts, the internal circuitry
Error Amplifier
generates a soft-start voltage (SS) ramping up from 0V to 0.6V. When it is lower than the internal The error amplifier compares the FB pin voltage with reference (REF), SS overrides REF so the error the internal FB reference (VFB) and outputs a amplifier uses SS as the reference. When SS is current proportional to the dif erence between the higher than REF, REF regains control. The SS time two. This output current is then used to charge or is internally fixed to 1 ms. discharge the internal compensation network to form the COMP voltage, which is used to control the power MOSFET current. The optimized internal
Over-Current-Prote ction and Hiccup Startup and Shutdown
The SD 8942 has cycle-by-cycle over current lim it If both VIN and EN are higher than their appropriate when the inductor current peak value exceeds the thresholds, the chip starts. The r eference block set current limit threshold. Meanwhile, output voltage starts first, generating stable reference voltage and starts to drop until FB is below the Under-Voltage currents, and then the internal regulator is enabled. (UV) threshold, typically 30% below the reference. The regulator provides stable supply for the Onc e a UV is triggered, the SD8942 enters hiccup remaining circuitries. Three events can shut down mode to periodically restart the part. This protection the chip: EN low, VIN low and thermal shutdown. I n mode is es pecially useful when the output is the shutdown procedure, the signaling pat h is first dead-short to ground. The average short circuit blocked to avoid any fault triggering. The COMP current is greatly reduced to alleviate the thermal voltage and the internal supply rail are then pulled issue and to protect the regulator. The SD8942 exits down. The floating driver is not subject to this the hiccup mode once the over current condit ion is shutdown command. removed. www.shouding.net 5