LM3551 Datasheet, PDF (13/15 Pages) National Semiconductor (TI) – 1A White LED Driver with Flash Timeout Protection

Application Information (Continued)
Component
C_C
R_C
R_FLASH
R_TORCH
C_SS
Value
4.7nF
(Lumiled)
10nF
(Sharp)
10kΩ
(Lumiled)
27kΩ
(Sharp)
User
Determined
User
Determined
User
Determined
Ratings
6.3V X5R or
X7R
Application
Specific
Application
Specific
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
LUMILED LXCL-PWF1
R_T = 5.6Ω, R_F = 2.2Ω
I_TORCH = 200mA, I_FLASH = 700mA
20151215
SHARP GM5BW05340A
R_T = 17Ω, R_F = 6.5Ω
I_TORCH = 75mA, I_FLASH = 250mA
20151216
Note 18: Please refer to the RECOMMENDED MINIMUM COMPONENT
section of the datasheet for more information.
DUAL-MODE CONFIGURATION
R₁ = 29.5kΩ,R₂ = 10kΩ R_F = 3Ω
I_FLASH = 500mA
20151209
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
13
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