Datasheet

LT3745
25
3745f
Thermal Protection
The LT3745 has two over temperature thresholds: one
is the fixed internal thermal shutdown and the other one
is programmed by a resistor, R
TSET
, between the T
SET
pin and ground. When the junction temperature exceeds
165°C, the part will enter thermal shutdown mode, shut
down serial data interface, turn off LED channels, and stop
switching. After the junction temperature drops below
155°C, the part will initiate a new soft-start.
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 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.
applicaTions inForMaTion
Cascading Devices and Determining Serial Data
Interface Clock
In a large LCD backlighting or LED display system, mul-
tiple LT3745 chips can be easily cascaded to drive all the
LED strings. The LT3745 adopts a 6-wire topology, which
balances the internal clock skew and matches the external
trace capacitance with an easy PCB layout.
The minimum serial data interface clock frequency f
SCKI
for a large display system can be calculated as:
f
SCKI
= N
LT3745
• 194 • f
REFRESH
where N
LT3745
is the number of LT3745 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.