Datasheet

LT3746
24
3746fa
For more information www.linear.com/3746
Setting Grayscale
Although adjusting the LED current changes its luminous
intensity, or brightness, it will also affect the color match
-
ing between LED channels by shifting the chromaticity
coordinate. The best way to adjust the brightness is to
control the amount of LED on/off time by pulse width
modulation (PWM).
The LT3746 can adjust the brightness for each channel
independently. The 12-bit grayscale PWM dimming results
in 4096 linear brightness steps from 0% to 99.98%. The
brightness level GS
n
% for channel n can be calculated as:
GS
n
% =
GS
n
4096
• 100%
where GS
n
is the nth programmed grayscale setting
(GS
n
= 0 to 4095).
Open/Short LED Fault
The LT3746 has individual LED fault diagnostic circuitry that
detects both open and short LED faults for each channel.
The open LED fault is defined as any LED string is open
or disconnected from the circuit; and the short LED fault
is defined as any LED string is shorted across itself. The
open LED flag is set if the LED pin voltage is lower than
0.1V (typical) during on status with initial 500ns blanking.
The short LED flag is set if the LED pin voltage is higher
than 75% of the LED bus voltage V
OUT
any time. If one
LED channel is shorted across itself, the channel will be
turned off to eliminate unnecessary power dissipation.
The function can also be used to disable LED channels
by connecting their LED pins to the output directly. Both
the open and short LED flags are combined to set the LED
fault bits (S0 to S31) in the status frame to 1.
Thermal Protection
The LT3746 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 LT3746 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 LT3746 chips can be easily cascaded to drive all the
L
E
D strings. The LT3746 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
LT3746
• 386 • f
REFRESH
where N
LT3746
is the number of LT3746 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
31
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