UM3506QA Datasheet, PDF (7/9 Pages) Union Semiconductor, Inc. – Synchronous Step-Down Converter

UM3506QA
reverse, as indicated by the current reversal comparator I_ZERO, or the beginning of the next clock
cycle.
When the output is shorted to ground, the inductor current may exceed the maximum inductor
peak current if not allowed enough time to decay. To prevent the inductor current from running
away, the bottom N-channel MOSFET is allowed to stay on for more than one cycle, thereby
allowing the inductor current time to decay.
Pulse Skipping Mode Operation
At very light loads, the UM3506QA automatically enters Pulse Skipping Mode. In the Pulse
Skipping Mode, the inductor current may reach zero or reverse on each pulse. The PWM control
loop will automatically skip pulses to maintain output regulation. The bottom MOSFET is turned
off by the current reversal comparator, I_ZERO, and the switch voltage will ring. This is
discontinuous mode operation, and is normal behavior for the switching regulator.
Enable
The EN pin provides a means to shut down the converter or enable normal operation. A logic low
will disable the converter and cause it to shut down. A logic high will enable the converter into
normal operation. In shutdown mode, the device quiescent current will be less than 1μA. The EN
pin must not be left floating.
Thermal Shutdown
When excessive power is dissipated in the chip, the junction temperature rises. Once the junction
temperature exceeds the thermal shutdown temperature, the thermal shutdown circuit turns off the
converter output voltage thus allowing the device to cool. When the junction temperature
decreases by 25°C, the device will go through the normal startup process.
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http://www.union-ic.com Rev.01 Oct.2016
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