UC3750L-XX-AF5-R datasheet(6/8 Pages) UTC

6 / 8 page

UC3750

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 8

www.unisonic.com.tw

QW-R502-360.b

DESCRIPTION INFORMATION(Cont.)

8. Inductor Value Calculation

Selecting the proper inductance must take an inductor’s physical size, transient respond and power conversion

requirements into consideration. The fact is that lower inductor value can make the cost, PC board space smaller

and can provide faster transient response while resulting in higher ripple current and core losses. The loading

current, IOUT = 0.5A and the inductor ripple current, IL−RIPPLE(P−P) is designed to be less than 40% of the load current,

i.e. 0.5A x 40% = 0.2A. The relationship between the inductor value and inductor ripple current is as followed:

)

(

RIPPLE

)

(

OUT

)

(

Where RDS (ON) is the ON resistance of the external P-channel MOSFET.

9. P-Channel Power MOSFET Selection

The operation of UTC UC3750 must be used with an external P-Channel power MOSFET. The main keys for

the power MOSFET are the gate threshold, VGS, the “ON” resistance, RDS (ON) and its total gate charge, QT. Low input

voltage operation need a low gate threshold device that can work down to the minimum input voltage level. RDS (ON)

determines the conduction losses for each switching cycle, the lower the ON resistance, the higher and the efficiency

can be achieved. A power MOSFET with lower gate charge can give lower switching losses but the fast transient can

cause unwanted EMI to the system. Compromise is required during the design stage.

10. Flywheel Diode Selection

The flywheel diode is turned on and carries load current during the off time. The average diode current depends

on the P-Channel switch duty cycle. At high input voltages, the diode conducts most of the time. In case of VIN

approaches VOUT, the diode conducts only a small fraction of the cycle. While the output terminals are shorted, the

diode will subject to its highest stress. Under this condition, the diode must be able to safely handle the peak current

circulating in the loop. So, it is important to select a flywheel diode that can meet the diode peak current and average

power dissipation requirements. Under normal conditions, the average current conducted by the flywheel diode is

given by:

OUT

Where ID is the average diode current and VF is the forward diode voltage drop.

A fast switching diode must also be used to optimize efficiency. Schottky diodes are ideal for low forward drop

and fast switching times.

11. Input and Output Capacitor Selection (CIN and COUT)

In continuous mode operation, the source current of the P-Channel MOSFET is a square wave of duty cycle

(VOUT +VF)/VIN. We must select a low ESR input capacitor that can support the maximum RMS input current to

prevent large input voltage transients. The maximum RMS input current, can be estimated by the equation below:

OUT

)

MAX

(

RMS

)

(

≈

IRMS (MAX) has a maximum value at VIN = 2VOUT, where IRMS (MAX) = IOUT/2. As a general practice, this simple

worst-case condition is used for design. Selecting of the output capacitor, the required effective series resistance

(ESR) of the capacitor decide the COUT and the capacitance will be adequate for filtering unless the ESR requirement

is met. The output voltage ripple, VRIPPLE is approximated by:

)

(ESR+

OUT

OSC

IPPLE

RIPPLE

≈

Where FOSC is the switching frequency and ESR is the effective series resistance of the output capacitor. We

can see from equation 4 that the output voltage ripple contributed by two parts. For a better performance, Low ESR

tantalum capacitors should be used. The major contributor is the capacitor ESR for most. Ordinary

aluminum-electrolytic capacitors have high ESR and should be avoided while higher quality Low ESR

aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Surface-mount tantalum capacitors are

better and provide neat and compact solution for space sensitive applications.

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