LT8302 datasheet(13/20 Pages) LINER

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LTM8068

8068fb

For more information www.linear.com/LTM8068

APPLICATIONS INFORMATION

VOUT2 Post Regulator Bypass Capacitance and Low

Noise Performance

The VOUT2 linear regulator may be used with the addition

of a 0.01μF bypass capacitor from VOUT to the BYP pin

to lower output voltage noise. A good quality low leak-

age capacitor, such as a X5R or X7R ceramic, is recom-

mended. This capacitor will bypass the reference of the

regulator, lowering the output voltage noise to as low as

20µVRMS. Using a bypass capacitor has the added benefit

of improving transient response.

Safety Rated Capacitors

Some applications require safety rated capacitors, which

are high voltage capacitors that are specifically designed

and rated for AC operation and high voltage surges. These

capacitors are often certified to safety standards such

as UL 60950, IEC 60950 and others. In the case of the

LTM8068, a common application of a safety rated capaci-

tor would be to connect it from GND to VOUTN. To provide

maximum flexibility, the LTM8068 does not include any

components between GND and VOUTN. Any safety capaci-

tors must be added externally.

The specific capacitor and circuit configuration for any

application depends upon the safety requirements of the

system into which the LTM8068 is being designed. Table

2 provides a list of possible capacitors and their manufac-

turers. The application of a capacitor from GND to VOUTN

may also reduce the high frequency output noise on the

output.

Table 2. Safety Rated Capacitors

MANUFACTURER PART NUMBER

DESCRIPTION

Murata

Electronics

GA343DR7GD472KW01L

4700pF, 250V AC, X7R,

4.5mm × 3.2mm

Capacitor

Johanson

Dielectrics

302R29W471KV3E-****-SC 470pF, 250V AC, X7R,

4.5mm × 2mm

Capacitor

Syfer Technology 1808JA250102JCTSP

100pF, 250V AC, C0G,

1808 Capacitor

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8068. The LTM8068 is neverthe-

less a switching power supply, and care must be taken to

minimize electrical noise to ensure proper operation. Even

with the high level of integration, you may fail to achieve

specified operation with a haphazard or poor layout. See

Figure 1 for a suggested layout. Ensure that the grounding

and heat sinking are acceptable.

A few rules to keep in mind are:

1. Place the RFB1 and RFB2 resistors as close as possible

to their respective pins.

2. Place the CIN capacitor as close as possible to the VIN

and GND connections of the LTM8068.

3. Place the COUT1 capacitor as close as possible to

VOUT1 and VOUTN. Likewise, place the COUT2 capaci-

tor as close as possible to VOUT2 and VOUTN.

4. Place the CIN and COUT capacitors such that their

ground current flow directly adjacent or underneath

the LTM8068.

8068 F01

RUN

FB2

BYP

FB1

LTM8058

COUT2

COUT1

VOUTN

VOUT2

VIN

VOUT1

CIN

THERMAL/INTERCONNECT VIAS

Figure 1. Layout Showing Suggested External

Components, Planes and Thermal Vias

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