UM3501_15 Datasheet, PDF (7/11 Pages) Union Semiconductor, Inc. – 600mA, 1.2MHz, Synchronous Step-Down DC-DC Converter

UM3501
Current Mode PWM Control and Current Limit
The UM3501 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 UM3501 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.
Maximum Load Current
The UM3501 will operate with input supply voltage as low as 2.5V, however, the maximum load
current decreases at lower input due to large IR drop on the main switch and synchronous rectifier.
The slope compensation signal reduces the peak inductor current as a function of the duty cycle to
prevent sub-harmonic oscillations at duty cycles greater than 50%. Conversely the current limit
increases as the duty cycle decreases.
Applications Information
Output Voltage Setting
The external resistor divider sets the output voltage.The feedback resistor R2 also sets the
feedback loop bandwidth with the internal compensation capacitor.
Choose R2 around 600kΩ for optimal transient response and feedback leakage current. R1 is then
given by:
R2
R1 
V_{OUT  1}
0.6V
________________________________________________________________________
http://www.union-ic.com Rev.06 Feb.2015
7/11