Datasheet
Rth @ restart =
1.23V ± (10 PA x R
HYS
)
R17
VCC - 1.23V ± (10 PA x R
HYS
)
- 10 PA
Rth
R17
VCC
Rhys
PGND
THM
AFB
SC
LM3431
SNVS547G –NOVEMBER 2007–REVISED MAY 2013
www.ti.com
Figure 27. THM Circuit with Hysteresis
The THM hysteresis can be determined by calculating the restart threshold as shown below. If R
HYS
is not
installed, calculate Rth@restart using an R
HYS
value of 0Ω.
(33)
Where 10 µA is the THM sink current, and Rth@restart is the thermistor resistance at the restart temperature.
Refer to the manufacturer datasheet to find the restart temperature at the calculated resistance or use the beta
constant to calculate the restart temperature.
During startup (and re-start), the THM monitor is active. Therefore, the thermistor temperature must be below the
restart threshold for the LM3431 to startup.
TSD
If the LM3431 internal junction temperature increases above 160°C TSD is activated. This is a Type 3 fault
condition. Device temperature rise is determined by internal power dissipation primarily in the LG and NDRVx
drivers. The power dissipation can be estimated as follows:
P
D
= P
IQ
+ P
NDRV
+ P
VCC
+ P
G
(34)
P
IQ
= V
IN
x I
Q
Where
• I
Q
is 4.0 mA typically (35)
P
NDRV
= (V
IN
- REFIN - Vbe) x I
NDRV
x D
DIM
x #Strings
Where
• REFIN+Vbe is the NDRV voltage
• I
NDRV
was calculated previously in the NDRV section (36)
For the case of open LEDs, I
NDRV
on the open string will be at the maximum of 15 mA. The LM3431 power
dissipation will be highest in open LED conditions at 100% dimming duty. If NFETs are used for regulation,
P
NDRV
will be a function of dimming frequency and can be calculated as:
P
NDRV_FET
= f dim x Q
g
x V
IN
(37)
P
VCC
= (V
IN
- V
CC
) x I
VCC
Where
•
VCC
is any current being drawn from the VCC pin, such as external op-amp power, or THM voltage divider. (38)
The LG power dissipation, PG is given in the NFET Selection section.
Temperature rise can then be calculated as:
T
RISE
= P
D
x θ
JA
Where
• θ
JA
is typically 32°C/W and varies with pcb copper area (Refer to the PCB Layout section) (39)
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