FC3470DFN22 datasheet(9/13 Pages) FS

9 / 13 page

Operation Description

Internal Regulator

The FC3470DFN22 is a current mode step down DC/DC converter that provides excellent transient

external compensation components. This device contains internal, low resistance,

high voltage power MOSFET, and operates at a high 600kHz operating frequency to ensure a compact,

high efficiency design with excellent AC and DC performance.

Error Amplifier

The error amplifier compares the FB pin voltage with the internal FB reference (VFB=0.6V) and outputs a

current proportional to the difference between the two. This output current is then used to charge or

discharge the internal compensation network to form the VCOMP voltage, which is used to control the

power MOSFET current. The optimized internal compensation network minimizes the external component

counts and simplifies the control loop design.

Internal Soft-Start

The soft-start is implemented to prevent the converter output voltage from overshooting during startup.

When the chip starts, the internal circuitry generates a soft-start voltage (SS) ramping up from 0V to 0.6V.

When it is lower than the internal reference (VFB), SS overrides VFB so the error amplifier uses SS as the

reference. When SS is higher than VFB, VFB regains control. The SS time is internally fixed to 0.6ms.

Over-Current-Protection and Hiccup

The FC3470DFN22 has cycle-by-cycle current limit function. When current limit is triggered, output

voltage starts to drop until FB is below the internal under-voltage(UV) threshold, typically 42% below

the reference. Once a UV is triggered, the FC3470DFN22 enters hiccup mode to periodically restart

the device. This protection mode is especially useful when the output is dead short to ground. The

average short circuit current is greatly reduced to alleviate the thermal issue and to protect the regulator.

The FC3470DFN22 exits the hiccup mode once the over current condition is removed.

Startup and Shutdown

If both VIN and EN are higher than their appropriate thresholds, the chip starts. The reference block starts

first, generating stable reference voltage and currents, and then the internal regulator is enabled. The

regulator provides stable supply for the remaining circuitries. Three events can shut down the chip: EN

low, VIN low and thermal shutdown. In the shutdown procedure, the signaling path is first blocked to avoid

any fault triggering. The VCOMP voltage and the internal supply rail are then pulled down. The floating

driver is not subject to this shutdown command.

response with no

FC3470DFN22

2019. 11. 29

9/13

Revision No : 0

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