UM3502QA Datasheet, PDF (8/13 Pages) Union Semiconductor, Inc. – Low-Profile, 600mA, Synchronous Step-Down Converter with Integrated Inductor

UM3502QA
Function Description
Integrated Inductor
The UM3502QA utilizes a low loss, multilayer inductor. The DCR of the integrated inductor is
180mΩ and the inductor is about 2.2µH. The use of an internal inductor localizes the noise
associated with the output loop currents. The proprietary integrated inductor construction reduces
the area of the converter’s large current loop that can reduce the radiated noise coupled into the
traces of the circuit board. Furthermore, the package layout is optimized to reduce the electrical
path length for the AC ripple currents that are a major source of radiated emissions from DC-DC
converters. The integrated inductor significantly reduces parasitic effects that can harm loop
stability, and makes layout very simple. All these lead to lower output noise and fewer influences
on the input power.
Current Mode PWM Control and Current Limit
The UM3502QA uses constant frequency, current mode step-down architecture. Both the main
(P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. From the
block diagram, a comparator ICOMP is used to realize current limit protection. Lossless current
sensing converts the peak current signal to a voltage to sum in with the internal slope
compensation. This summed signal is compared to the error amplifier output to provide a peak
current control command for the PWM. The cycle-by-cycle current limit is set at 1200mA
(typical). During normal operation, the internal top power MOSFET is turned on each cycle when
the oscillator sets the RS latch, and turned off when the current comparator ICOMP, resets the RS
latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output
of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn, causes the EA amplifier’s output
voltage to increase until the average inductor current matches the new load current. While the top
MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to
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 UM3502QA 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 Nov.2015
8/13