Datasheet

LT3745-1
25
37451f
When the R
TSET
is placed at the T
SET
pin, a current equal
to the current flowing through the R
ISET
passes the
R
TSET
, generating a voltage V
TSET
at the T
SET
pin, which
is calculated as:
V
TSET
= 1.205V •
R
TSET
R
ISET
Then the V
TSET
is compared to an internal proportional-
to-absolute-temperature voltage V
PTAT
,
V
PTAT
= 1.72mV • (T
J
+ 273.15)
where T
J
is the LT3745-1 junction temperature in °C.
When V
PTAT
is higher than V
TSET
, an overtemperature flag
OT = 1 is set. Once the R
TSET
programmed temperature is
exceeded, the part will also gradually derate the nominal
LED current I
LED(NOM)
to limit the total power dissipation
without interrupting its normal operation.
Cascading Devices and Determining Serial Data
Interface Clock
In
a large LCD backlighting or LED display system, multiple
LT3745-1 chips can be easily cascaded to drive all the LED
strings. The minimum serial data interface clock frequency
f
SCKI
for a large display system can be calculated as:
f
SCKI
= N
LT3745-1
• 194 • f
REFRESH
APPLICATIONS INFORMATION
where N
LT3745-1
is the number of LT3745-1 chips and
f
REFRESH
is the refresh rate of the whole system.
Calculating Power Dissipation
The total power dissipation inside the chip can be calcu-
lated as:
P
TOTAL
=
V
IN
• (I
VIN
+
f
SW
• Q
G
)
+
V
CC
• I
VCC
+ GS
n
% • I
LEDn
• V
LEDn
n= 0
15
∑
where I
VIN
is the power input V
IN
quiescent current, I
VCC
is the V
CC
supply current, and V
LEDn
is the LED pin volt-
age for channel n.
From the total power dissipation P
TOTAL
, the junction
temperature T
J
can be calculated as:
T
J
= T
A
+ P
TOTAL
• θ
JA
Keep T
J
below the maximum operating junction tempera-
ture 125°C.