UCC36351

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

4 of 7

www.unisonic.com.tw

QW-R115-012.B



FUNCTION DESCRIPTIONS

The UTC UCC36351 is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages

The UTC UCC36351 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 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

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:

2

R

2

R

1

R

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.

VIN

<28V

<35V

Inductor

33uH

33uH

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

requirements.

Output Short-Circuit protection

auto restart function can be started that restart the regulator cycle by cycle.

(Retry time 2mS, Shutdown regulator time 80mS).

Output Cable Resistance Compensation

To compensate for resistive voltage drop across the charger's output cable, the UTC UCC36351 integrates a

simple, user-programmable cable voltage drop compensation using the impedance at the FB pin. Use the curve in

Figure 1 to choose the proper feedback resistance values for cable compensation. R1 is the high side resistor of

voltage divider.

)

uA

4

(

I

1

R

)

2

R

/

1

R

1

(

V

V

FB

FB

OUT

×

+

+

×

=