UD38251

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

4 of 7

www.unisonic.com.tw

QW-R115-007.B



FUNCTION DESCRIPTIONS

The UTC UD38251 is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages

The UTC UD38251 uses current-mode control to regulate the output voltage. The output voltage is measured at

FB through a resistive voltage divider and amplified through the internal Tran conductance error amplifier. The

voltage at the COMP pin is compared to the switch current measured internally to control the output voltage.

The converter uses internal N-Channel MOSFET switches to step-down the input voltage to the regulated output

voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor

connected between SW and BS is needed to drive the high side gate. The boost capacitor is charged from the

internal 5V rail when SW is low.

comparator is tripped and the COMP pin is discharged to GND, forcing the high-side switch off.



APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider

divides the output voltage down to the feedback voltage by the ratio.

Thus the output voltage is:

R2

R2

R1

V

V

FB

OUT







Inductor Selection

The inductor is required to supply constant current to the output load while being driven by the switched input

voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However,

the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A

good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be

approximately 30% of the maximum switch current limit.

<28V

<35V

Inductor

33uH

22uH

The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI

requirements.