RT9178

9

DS9178-16 April 2008

www.richtek.com

Tantalum:

Solid tantalum capacitors are recommended for use on

the output because their typical ESR is very close to the

ideal value required for loop compensation. They also work

well as input capacitors if selected to meet the ESR

requirements previously listed.

Tantalums also have good temperature stability: a good

quality tantalum will typically show a capacitance value

that varies less than 10 to 15% across the full temperature

range of 125

°C to −40°C. ESR will vary only about 2X

going from the high to low temperature limits.

The increasing ESR at lower temperatures can cause

oscillations when marginal quality capacitors are used (if

the ESR of the capacitor is near the upper limit of the

stability range at room temperature).

Aluminum:

This capacitor type offers the most capacitance for the

money. The disadvantages are that they are larger in

physical size, not widely available in surface mount, and

have poorAC performance (especially at higher frequencies)

due to higher ESR and ESL.

Compared by size, the ESR of an aluminum electrolytic

is higher than either Tantalum or ceramic, and it also varies

greatly with temperature. A typical aluminum electrolytic

can exhibit an ESR increase of as much as 50X when

going from 25

°C down to −40°C.

It should also be noted that many aluminum electrolytics

only specify impedance at a frequency of 120Hz, which

indicates they have poor high frequency performance.

Only aluminum electrolytics that have an impedance

specified at a higher frequency (between 20kHz and

100kHz) should be used for the device. Derating must be

applied to the manufacturer's ESR specification, since it

is typically only valid at room temperature.

Any applications using aluminum electrolytics should be

thoroughly tested at the lowest ambient operating

temperature where ESR is maximum.

Thermal Considerations

The RT9178 series can deliver a current of up to 200mA

over the full operating junction temperature range. However,

the maximum output current must be derated at higher

ambient temperature to ensure the junction temperature

does not exceed 125

°C.

With all possible conditions, the junction temperature must

be within the range specified under operating conditions.

Power dissipation can be calculated based on the output

current and the voltage drop across regulator.

The final operating junction temperature for any set of

conditions can be estimated by the following thermal

equation:

the die (125

temperature. The junction to ambient thermal resistance

(

footprint is 250

website in which “Recommended Footprints for Soldering

Surface Mount Packages” for detail.

PCB Layout

Good board layout practices must be used or instability

can be induced because of ground loops and voltage drops.

The input and output capacitors MUST be directly

connected to the input, output, and ground pins of the

device using traces which have no other currents flowing

through them.

The regulator ground pin should be connected to the

external circuit ground so that the regulator and its

capacitors have a “single point ground”.