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LT3745
APPLICATIONS INFORMATION
V TSET = 1.205V ?
ThermalProtection
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:
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.
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
15
? I VCC + ∑ GS n % ? I LEDn ? V LEDn
n = 0
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.
3745f
25
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