LM27966 datasheet(12/13 Pages) NSC

12 / 13 page

Application Information (Continued)

J =TA +(PDISS x

JA)

The junction temperature rating takes precedence over the

ambient temperature rating. The LM27966 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maxi-

mum operating rating of 100˚C. The maximum ambient tem-

perature rating must be derated in applications where high

power dissipation and/or poor thermal resistance causes the

junction temperature to exceed 100˚C.

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM27966

when the junction temperature exceeds 170˚C (typ.). This

feature protects the device from being damaged by high die

temperatures that might otherwise result from excessive

power dissipation. The device will recover and operate nor-

mally when the junction temperature falls below 165˚C (typ.).

It is important that the board layout provide good thermal

conduction to keep the junction temperature within the speci-

fied operating ratings.

CAPACITOR SELECTION

The LM27966 requires 4 external capacitors for proper op-

eration (C

1 =C2 = 1µF, CIN =COUT = 1µF). Surface-mount

multi-layer ceramic capacitors are recommended. These ca-

pacitors are small, inexpensive and have very low equivalent

series resistance (ESR <20m

Ω typ.). Tantalum capacitors,

OS-CON capacitors, and aluminum electrolytic capacitors

are not recommended for use with the LM27966 due to their

high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R

temperature characteristic are preferred for use with the

LM27966. These capacitors have tight capacitance toler-

ance (as good as ±10%) and hold their value over tempera-

ture (X7R: ±15% over -55˚C to 125˚C; X5R: ±15% over

-55˚C to 85˚C).

Capacitors with Y5V or Z5U temperature characteristic are

generally not recommended for use with the LM27966. Ca-

pacitors with these temperature characteristics typically

have wide capacitance tolerance (+80%, -20%) and vary

significantly over temperature (Y5V: +22%, -82% over -30˚C

to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C

range). Under some conditions, a nominal 1µF Y5V or Z5U

capacitor could have a capacitance of only 0.1µF. Such

detrimental deviation is likely to cause Y5V and Z5U capaci-

tors to fail to meet the minimum capacitance requirements of

the LM27966.

The minimum voltage rating acceptable for all capacitors is

6.3V. The recommended voltage rating for the input and

output capacitors is 10V to account for DC bias capacitance

losses.

PCB LAYOUT CONSIDERATIONS

The LLP is a leadframe based Chip Scale Package (CSP)

with very good thermal properties. This package has an

exposed DAP (die attach pad) at the center of the package

measuring 2.6mm x 2.5mm. The main advantage of this

exposed DAP is to offer lower thermal resistance when it is

soldered to the thermal land on the PCB. For PCB layout,

National highly recommends a 1:1 ratio between the pack-

age and the PCB thermal land. To further enhance thermal

conductivity, the PCB thermal land may include vias to a

ground plane. For more detailed instructions on mounting

LLP packages, please refer to National Semiconductor Ap-

plication Note AN-1187.

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