RT8016L datasheet(8/13 Pages) RICHTEK

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RT8016L

DS8016L-02 March 2011

www.richtek.com

Application Information

The basic RT8016L application circuit is shown in Typical

Application Circuit. External component selection is

determined by the maximum load current and begins with

the selection of the inductor value and operating frequency

followed by CIN and COUT.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current ΔIL increases with higher VIN and decreases

with higher inductance:

OUT

x 1

F x L

⎡⎤

Δ=

−

⎢⎥

⎣⎦

Having a lower ripple current reduces the ESR losses in

the output capacitors and the output voltage ripple. Highest

efficiency operation is achieved at low frequency with small

ripple current. This, however, requires a large inductor. A

reasonable starting point for selecting the ripple current

is ΔIL = 0.4(IMAX). The largest ripple current occurs at the

highest VIN. To guarantee that the ripple current stays

below a specified maximum, the inductor value should be

chosen according to the following equation :

OUT

L(MAX)

IN(MAX)

x 1

f x ΔIV

⎡⎤

⎡

⎤

=−

⎢⎥

⎢

⎥

⎢⎥

⎢

⎥

⎣⎦

⎣

⎦

Inductor Core Selection

Once the value for L is known, the type of inductor can be

selected. High efficiency converters generally cannot afford

the core loss found in low cost powdered iron cores, forcing

the use of more expensive ferrite or mollypermalloy cores.

Actual core loss is independent of core size for a fixed

inductor value but it is very dependent on the inductance

selected. As the inductance increases, core losses

decrease. Unfortunately, increased inductance requires

more turns of wire and therefore copper losses will

increase. Ferrite designs have very low core losses and

are preferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard”, which means that

inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage ripple.

Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy materials

are small and don't radiate energy but generally cost more

than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price vs. size requirements and

any radiated field/EMI requirements.

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the

trapezoidal current at the source of the top MOSFET. To

prevent large ripple voltage, a low ESR input capacitor

sized for the maximum RMS current should be used. RMS

current is given by :

OUT

RMS

OUT(MAX)

OUT

=−

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. Note that ripple current ratings from

capacitor manufacturers are often based on only 2000

hours of life which makes it advisable to further derate the

capacitor, or 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.

The selection of COUT is determined by the effective series

resistance (ESR) that is required to minimize voltage ripple

and load step transients, as well as the amount of bulk

capacitance that is necessary 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, ΔVOUT, is determined by :

OUT

VI ESR

8fC

⎡⎤

Δ≤ Δ

⎢⎥

⎣⎦

The output ripple is highest at maximum input voltage

since ΔILincreases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

RMS current handling requirements. Dry tantalum, special

polymer, aluminum electrolytic and ceramic capacitors are

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