lt3743 Linear Technology Corporation, lt3743 Datasheet - Page 19

• Manufacturer Part Number: lt3743
• Description: High Current Synchronous Step-down Led Driver With Three-state Control
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Thermal Shutdown
The internal thermal shutdown within the LT3743 engages at
163°C and terminates switching, resets soft-start and shuts
down the PWMGL and PWMGH drivers. When the part has
cooled to 155°C, the internal reset is cleared and soft-start
is allowed to charge once the PWM signal is asserted.
Switching Frequency Synchronization
The nominal switching frequency of the LT3743 is deter-
mined by the resistor from the RT pin to ground and may
be set from 200kHz to 1MHz. The internal oscillator may
also be synchronized to an external clock through the SYNC
pin. The external clock applied to the SYNC pin must have
a logic low below 0.3V and a logic high higher than 1.25V.
The input frequency must be 20% higher than the frequency
determined by the resistor at the RT pin. The duty cycle of
the input signal needs to be greater than 10% and less than
90%. Input signals outside of these specifi ed parameters
will cause erratic switching behavior and subharmonic
oscillations. When synchronizing to an external clock,
please be aware that there will be a fi xed delay from the input
clock edge to the edge of switch. The SYNC pin must be
grounded if the synchronization to an external clock is not
required. When SYNC is grounded, the switching frequency
is determined by the resistor at the RT pin.
Shutdown and UVLO
The LT3743 has an internal UVLO that terminates switching,
resets all synchronous logic, and discharges the soft-start
capacitor for input voltages below 4.2V. The LT3743 also
has a precision shutdown at 1.55V on the EN/UVLO pin.
Partial shutdown occurs at 1.55V and full shutdown is
guaranteed below 0.5V with <2μA I
state. Below 1.5V, an internal current source provides 5.5μA
of pull-down current to allow for programmable UVLO
hysteresis. The following equations determine the voltage
divider resistors for programming the UVLO voltage and
hysteresis as confi gured in Figure 14.
R
R
2
1
=
=
⎛
⎝ ⎜
V
5 5
V
HYST
.
UVLO
1 55
μA
.
V R
– .
1 55
•
2
V
⎞
⎠ ⎟
Q
in the full shutdown
The EN/UVLO pin as an absolute maximum voltage of
6V. To accommodate the largest range of applications,
there is an internal Zener diode that clamps this pin. For
applications where the supply range is greater than 4:1,
size R2 greater than 375k.
LED Current Derating Using the CTRL_T Pin
The LT3743 is designed specifi cally for driving high cur-
rent LEDs. Most high current LEDs require derating the
maximum current based on operating temperature to
prevent damage to the LED. In addition, many applications
have thermal limitations that will require the regulated
current to be reduced based on LED and/or board tem-
perature. To achieve this, the LT3743 uses the CTRL_T
pin to reduce the effective regulated current in the LED
for both the high and low control currents. While CTRL_H
and CTRL_L program the regulated current in the LED,
CTRL_T can be confi gured to reduce this regulated cur-
rent based on the analog voltage at the CTRL_T pin. The
LED/board temperature derating is programmed using a
resistor divider with a temperature dependant resistance
(Figure 15). When the board/LED temperature rises, the
CTRL_T voltage will decrease. To reduce the regulated
current, the CTRL_T voltage must be lower than voltage
at the CTRL_L and CTRL_H pins.
R2
R1
(OPTION A TO D)
Figure 15. LED Current Derating vs Temperature
Using NTC Resistor
V
CTRL_T
REF
LT3743
Figure 14. UVLO Confi guration
R
NTC
LT3743
EN/UVLO
A
V
R
IN
NTC
B
R2
R1
3743 F14
R
X
V
IN
R
NTC
C
LT3743
R
V
R
NTC
19
D
R
3743 F15
V
3743f
R
X