RT6220 datasheet(15/20 Pages) RICHTEK

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RT6220

DS6220-11 July 2020

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Generally, the stability is a serious issue for converter. In

order to achieve better performance on stability and

transient, a feed-forward capacitor, CFF, is added to

increase the noise margin and transient response of loop

control. However, there is a tradeoff of adding a feed-forward

capacitor. An additional dc offset will be generated on

output voltage due to the amplified feedback ripple by feed-

forward compensator. This is not always the case that

every CFF makes the same value of dc offset, it is based

on different pole and zero placement generated by R1, R2

and CFF. For simplicity, a symbol named Vdc,offset is

supposed to be the value of dc offset. This value may

influence the performance (e.g. regulation or peak value

of VOUT) of converter slightly, the suggested CFFis to select

a pair of pole and zero to provide the maximum phase

lead at switching frequency.

OUT,valley

REF

dc,offset

V= 1+







VOUT,valley is the valley of output voltage, and Vdc,offset is

used for describing the additional dc offset on VOUT, the

value is related to the output voltage ripple and CFF.

VOUT

GND

RT6220

CFF (opt.)

Figure 5. The Equivalent Circuit of Feedback Loop

Inductor Selection

Selecting an inductor involves specifying its inductance

and also its required peak current. The exact inductor value

is generally flexible and is ultimately chosen to obtain the

best mix of cost, physical size, and circuit efficiency.

Lower inductor values benefit from reduced size and cost

and they can improve the circuit's transient response.

However, they increase the inductor ripple current and

output voltage ripple and reduce the efficiency due to the

resulting higher peak currents. Conversely, higher inductor

values increase efficiency, but the inductor will either be

physically larger or have higher resistance since more

turns of wire are required and transient response will be

slower since more time is required to change current (up

or down) in the inductor. A good compromise between

size, efficiency, and transient response is to use a ripple

current (

ΔIL) about 20-50% of the desired full output load

current. Calculate the approximate inductor value by

selecting the input and output voltages, the switching

frequency (fSW), the maximum output current (IOUT(MAX))

and estimating a

ΔIL as some percentage of that current.

OUT

V(V

)





 

Once an inductor value is chosen, the ripple current (

ΔIL)

is calculated to determine the required peak inductor

current.

OUT

L(PEAK)

OUT(MAX)

V(V

)

and I













To guarantee the required output current, the inductor

needs a saturation current rating and a thermal rating that

exceeds IL(PEAK). These are minimum requirements. To

maintain control of inductor current in overload and short-

circuit conditions, some applications may desire current

ratings up to the current limit value. However, the IC's

output under-voltage shutdown feature make this

unnecessary for most applications.

For best efficiency, choose an inductor with a low DC

resistance that meets the cost and size requirements.

For low inductor core losses some type of ferrite core is

usually best and a shielded core type, although possibly

larger or more expensive, will probably give fewer EMI

and other noise problems.

Input Capacitor Selection

High quality ceramic input decoupling capacitor, such as

X5R or X7R, with values greater than 20

μF are

recommended for the input capacitor. The X5R and X7R

ceramic capacitors are usually selected for power regulator

capacitors because the dielectric material has less

capacitance variation and more temperature stability.

Voltage rating and current rating are the key parameters

when selecting an input capacitor. Generally, selecting an

input capacitor with voltage rating 1.5 times greater than

the maximum input voltage is a conservatively safe design.

The input capacitor is used to supply the input RMS

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