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http://www.union-ic.com Rev.06 Feb.2015

7/11

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

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

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:

1

6

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0

2

1

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V

R

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