lt3743 Linear Technology Corporation, lt3743 Datasheet - Page 18

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

LT3743
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 (1.3V at the FB pin),
the internal open-LED fl ag 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:
Figure 12. Output Voltage Regulation and Overvoltage Protection
Feedback Connections
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 fi rst 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.
18
V
OUT
=
1
V
⎛
⎝ ⎜
1
+
R
R
2
1
LT3743
⎞
⎠ ⎟
FB
R2
R1
3743 F12
V
OUT
The internal power consumption of the LT3743 is de-
termined by the switching frequency, V
charge, Q
The 4mm × 5mm QFN package has a θ
following equation calculates the maximum switching
frequency to avoid current limit and thermal shutdown at
a given ambient operating temperature, T
Since the regulated output current fl owing 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 effi ciency and increasing power loss within
the LT3743.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz)
f
f
S
S S
≤
≤
(
(
35
Q
G
GLG
1.2
1.0
0.8
0.6
0.4
0.2
°
Figure 13. Frequency vs R
0.750
0
of the external switching MOSFETs selected.
C W
60
0.5
0.3
0.2
0.1
0
1
/
+
mA
50
Q
) (
•
100
GHG
(
V
150
163
IN
)
–
200
°
5
R
C T
T
250
V
(kΩ)
–
)
300
•
A
(
Q
)
350
GHG
T
400
Resistance
+
450
JA
R
3743 F13
Q
T
IN
142
218
450
40
54
83
(kΩ)
A
of 35°C/W. The
GLG
500
, and the gate
:
)
3743f