RT8015 datasheet(11/15 Pages) RICHTEK

11 / 15 page

RT8015

DS8015-03

March 2011

www.richtek.com

Figure 2. Setting the Output Voltage











 +

REF

OUT

Using Ceramic Input and Output Capacitors

Higher values, 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

and the power is supplied by a wall adapter through long

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

input, VIN. 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.

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation :

where VREF equals to 0.8V typical.

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 2.

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 :

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,

∆VOUT, is determined by :

The output ripple is highest at 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 requirements. Dry tantalum, special

polymer, aluminum electrolytic and ceramic capacitors are

all available in surface mount packages. Special polymer

capacitors offer very low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

highest capacitance density but it is important to only

OUT

OUT(MAX)

RMS

−









∆

≤

∆

OUT

8fC

ESR

use types that have been surge tested for use in switching

power supplies. Aluminum electrolytic capacitors have

significantly higher ESR but can be used in cost-sensitive

applications provided that consideration is given to ripple

current ratings 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.

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 sim ilar

characteristics. The choice of which style inductor to use

mainly depends on the price vs. size requirements and

any radiated field/EMI requirements.

RT8015

GND

VOUT

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