RT6908 datasheet(19/22 Pages) RICHTEK

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RT6908

DS6908-01

March 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

VCORE Buck External MOSFET Selection

The VCORE buck controller drives two external N-MOSFETs

as the switch. There are some considerations to choose

the external MOSFET. It includes MOSFET drain to source

voltage stress, on-resistance, total gate charge

characteristics and power dissipation for thermal

performance.

Buck Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current,

ΔIL, will increase with higher

VIN and decrease with higher inductance, as shown in

below equation :

⎛⎞ ⎛

⎞

Δ× −

⎜⎟ ⎜

⎟

⎝⎠ ⎝

⎠

OUT

OSC

I =

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. High frequency with small ripple current can achieve

the highest efficiency operation. However, it requires a

large inductor to achieve this goal. For the ripple current

selection, the value of IL(MAX) = 0.4 is a reasonable starting

point. The largest ripple current occurs at the highest VIN.

To guarantee that the ripple current stays below the

specified maximum, the inductor value should be chosen

according to the following equation :

⎛⎞ ⎛

⎞

×−

⎜⎟ ⎜

⎟

×Δ

⎝⎠ ⎝

⎠

OUT

OSC

L(MAX)

IN(MAX)

L =

Buck Input Capacitor Selection

The input capacitance, CIN, is needed to filter the

trapezoidal current at the source of the high-side MOSFET.

To prevent large ripple current, a low ESR input capacitor

sized for the maximum RMS current should be used. The

RMS current is given by :

××

−

OUT

RMS

OUT(MAX)

OUT

= I

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT / 2. This simple worst-case condition is commonly

used for design because even significant deviations do

not offer much relief. Choose a capacitor rated at a higher

temperature than required. Several capacitors may also

be paralleled to meet size or height requirements in the

design. For the input capacitor, a 10

μF low ESR ceramic

capacitor is recommended.

Buck Output Capacitor Selection

The selection of COUT is determined by the required ESR

to minimize voltage ripple. Moreover, the amount of bulk

capacitance is also a key for COUT selection to ensure

that the control loop is stable. Loop stability can be

checked by viewing the load transient response as

described in a later section. The output ripple, VOUT, is

determined by :

⎛⎞

ΔΔ ×⎜⎟

××

⎝⎠

OUT

OSC

OUT

= I

ESR +

The output ripple will be highest at the maximum input

voltage since IL increases with input voltage. Multiple

capacitors placed in parallel may be needed to meet the

ESR and RMS current handling requirement. Suitable

candidates such as dry tantalum, special polymer,

aluminum electrolytic and ceramic capacitors are all

available in surface mount packages. Special polymer

capacitors offer very low ESR value. However, it provides

lower capacitance density than other types. Although

tantalum capacitors have the highest capacitance density,

it is important to only use types that pass the surge test

for use in switching power supplies. Aluminum electrolytic

capacitors have significantly higher ESR. However, it can

be used in cost-sensitive applications requiring high ripple

current rating and long term reliability. Ceramic capacitors

have excellent low ESR characteristics but can have a

high voltage coefficient and audible piezoelectric effects.

The high Q of ceramic capacitors with trace inductance

can also lead to significant ringing. Nevertheless, higher

value, lower cost ceramic capacitors are now becoming

available in smaller case sizes. Their high ripple current,

high voltage rating and low ESR make them ideal for

switching regulator applications. However, care must be

taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input,

VIN, and the power is supplied by a wall adapter through

long wires, a load step at the output can induce ringing at

the input. At best, this ringing can couple to the output

and be mistaken as loop instability. At worst, a sudden

inrush of current through the long wires can potentially

cause a voltage spike at VIN large enough to damage the

part.

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