LM3551 datasheet(13/15 Pages) NSC

13 / 15 page

Application Information (Continued)

Component

Value

Ratings

4.7nF

(Lumiled)

6.3V X5R or

X7R

10nF

(Sharp)

10k

Ω

(Lumiled)

27k

Ω

(Sharp)

FLASH

User

Determined

Application

Specific

TORCH

User

Determined

Application

Specific

User

Determined

6.3V X5R or

X7R

Torch and Flash Resistor ratings are dependent upon the

current through each resistor. The minimum ratings will vary

depending upon the current selected on an applicaiton by

application basis. Power Rating Minimum = (Desired Cur-

rent)

2 x Resistor Value. See the CURRENT SET EQUA-

TIONS section to determine torch and flash currents.

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM3551/2

when the junction temperature exceeds +140˚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 +120˚C

(typ.). It is important that the board layout provide good

thermal conduction to keep the junction temperature within

the specified operating ratings.

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 3.0mm. The main advantage of this

exposed DAP is to offer lower thermal resistance when it is

soldered to the thermal land pad on the PCB. For PCB

layout, National highly recommends a 1:1 ratio between the

package 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 mount-

ing LLP packages, please refer to National Semiconductor

Application Note AN-1187.

Application Examples

TYPICAL CONFIGURATIONS

20151215

LUMILED LXCL-PWF1

T = 5.6

Ω,R

F = 2.2

Ω

TORCH = 200mA, IFLASH = 700mA

20151216

SHARP GM5BW05340A

T =17

Ω,R

F = 6.5

Ω

TORCH = 75mA, IFLASH = 250mA

Note 18: Please refer to the RECOMMENDED MINIMUM COMPONENT

section of the datasheet for more information.

DUAL-MODE CONFIGURATION

20151209

1 = 29.5k

Ω,R

2 = 10k

Ω R

F =3

Ω

FLASH = 500mA

Using the Dual-Mode configuration with either the LM3551/2,

a 5V, high current rail (approx. 700mA total) can be created

while still allowing for a high flash with a true load discon-

nect. R1 and R2 setup the +5V following the equation: V

OUT

= 1.265 x (1 + R1/R2) . When the LM3551/2 is on, and the

T/F pin is low (logic ’0’), the part will provide a regulated

output voltage that can be used to provide a voltage rail

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