UD052012

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LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R115-039.a



APPLICATION INFORMATION (Cont.)

Where FOSC is the switching frequency, L is the inductance value, VIN is the input voltage, ESR is the equivalent

series resistance value of the output capacitor, ESL is the equivalent series inductance value of the output capacitor

and the COUT is the output capacitor.

Low ESR capacitors are preferred to use. Ceramic, tantalum or low ESR electrolytic capacitors can be used

depending on the output ripple requirements. When using the ceramic capacitors, the ESL component is usually

negligible.

It is important to use the proper method to eliminate high frequency noise when measuring the output ripple.

Inductor Selection

The output inductor is used for storing energy and filtering output ripple current. But the trade-off condition often

happens between maximum energy storage and the physical size of the inductor. The first consideration for selecting

the output inductor is to make sure that the inductance is large enough to keep the converter in the continuous

current mode. That will lower ripple current and result in lower output ripple voltage. The ∆IL is inductor peak-to-peak

ripple current:

1

×

×

OUT

OUT

L

IN

OSC

V

V

I

V

FL











A good compromise value between size and efficiency is to set the peak-to-peak inductor ripple current ∆IL equal to

30% of the maximum load current. But setting the peak-to-peak inductor ripple current ∆IL between 20%~50% of the

maximum load current is also acceptable. Then the inductance can be calculated with the following equation:

∆IL=0.3×IOUT(MAX)



×

××

OUT

IN

OUT

IN

OSC

L

V

VV

L

VF

I







To guarantee sufficient output current, peak inductor current must be lower than the UD052012 high-side MOSFET

current limit.

()

2

L

PEAK

OUT MAX

I

II





Feedforward Capacitor Selection

Internal compensation function allows users saving time in design and saving cost by reducing the number of

external components. The use of a feedforward capacitor C3 in the feedback network is recommended to improve

transient response or higher phase margin.

UTC

UD052012

VOUT

FB

R1

R2

C3

For optimizing the feedforward capacitor, knowing the cross frequency is the first thing. The cross frequency (or

the converter bandwidth) can be determined by using a network analyzer. When getting the cross frequency with no

feedforward capacitor identified, the value of feedforward capacitor C3 can be calculated with the following equation:

11

11

3

21

12

CROSS

C

FR

RR

















Where FCROSS is the cross frequency.

To reduce transient ripple, the feedforward capacitor value can be increased to push the cross frequency to higher

region. Although this can improve transient response, it also decreases phase margin and causes more ringing. In

the other hand, if more phase margin is desired, the feedforward capacitor value can be decreased to push the cross

frequency to lower region. In general, the feedforward capacitor range is between 10pF to 330pF.