Datasheet

LT3743
18
3743fd
APPLICATIONS INFORMATION
Voltage Regulation and Overvoltage Protection
The LT3743 uses the FB pin to regulate the output to a
maximum voltage and to provide a high speed overvoltage
lockout to avoid high voltage conditions that may damage
expensive high current LEDs. The regulated output voltage
is programmed using a resistor divider from the output
and ground (Figure 12). When the output voltage exceeds
130% of the regulated voltage level (1.3
V at the FB pin),
the internal open-LED flag is set, terminating switching
and forcing both PWMGL and PWMGH signals high. The
regulated output voltage must be greater than 2V and is
set by the equation:
V
OUT
=1V 1+
R2
R1
The internal power consumption of the LT3743 is de-
termined by the switching frequency, V
IN
, and the gate
charge, Q
G
of the external switching MOSFETs selected.
The 4mm × 5mm QFN package has a θ
JA
of 35°C/W. The
following equation calculates the maximum switching
frequency to avoid current limit and thermal shutdown at
a given ambient operating temperature, T
A
:
f
S
≤
163°C– T
A
(
)
35°C/W
(
)
• V
IN
– 5V
(
)
• Q
GHG
+Q
GLG
(
)
f
S
≤
60mA
Q
GLG
+Q
GHG
(
)
Since the regulated output current flowing into the LED
may be very large, the switching frequency needs to be
carefully considered. Higher switching frequencies will
reduce the large size of high saturation current inductors,
while reducing efficiency and increasing power loss within
the LT3743.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
1 40.2
0.750 53.6
0.5 82.5
0.3 143
0.2 221
Figure 12. Output Voltage Regulation and Overvoltage Protection
Feedback Connections
LT3743
R2
V
OUT
R1
3743 F12
FB
Soft-Start
Unlike conventional voltage regulators, the LT3743 utilizes
the soft-start function to control the regulated inductor
current. The charging current is 5.5µA and reduces the
regulated current for both the high and low regulated
current states. The SS pin is latched in a discharge state
until the first PWM pulse and is reset by UVLO and thermal
shutdown.
Programming Switching Frequency
The LT3743 has an
operational switching frequency
range between 200kHz and 1MHz. This frequency is
programmed with an external resistor from the RT pin to
ground. Do not leave this pin open under any condition.
The RT pin is also current limited to 60µA. See Table 4
and Figure
13 for resistor values and the corresponding
switching frequencies.
R
T
(kΩ)
0
FREQUENCY (MHz)
0.4
0.8
1.2
0.2
0.6
1.0
100 200 300 400
3743 F13
500500 150 250 350 450
Figure 13. Frequency vs R
T
Resistance