lt3741 Linear Technology Corporation, lt3741 Datasheet - Page 16

• Manufacturer Part Number: lt3741
• Description: High Power, Constant Current, Constant Voltage, Step-down Controller
• Manufacturer: Linear Technology Corporation

LT3741
applicaTions inFormaTion
Output Current Regulation
To adjust the regulated load current, an analog voltage is
applied to the CTRL1 pin. Figure 5 shows the regulated
voltage across the sense resistor for control voltages up
to 2V. Figure 6 shows the CTRL1 voltage created by a volt-
age divider from V
divider, please be aware that the V
to 500µA. Above 1.5V, the control voltage has no effect on
the regulated inductor current.
Voltage Regulation and Overvoltage Protection
The LT3741 uses the FB pin to regulate the output voltage
and to provide a high speed overvoltage lockout to avoid
high voltage conditions. The regulated output voltage
is programmed using a resistor divider from the output
and ground (Figure 7). When the output voltage exceeds
125% of the regulated voltage level (1.5V at the FB pin),
the internal overvoltage flag is set, terminating switching.
The regulated output voltage must be greater than 1.5V
and is set by the equation:
Programming Switching Frequency
The LT3741 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

V
OUT
=
Figure 5. Sense Voltage vs CTRL Voltage
1 21
60
50
40
30
20
10
0
.
0
V

 
1
REF
+
0.5
R
R
to ground. When sizing the resistor
2
1

 
V
CTRL
1.0
(V)
REF
1.5
pin is current limited
3741 F05
2.0
Figure 7. Output Voltage Regulation and Overvoltage Protection
Feedback Connections
not leave this pin open under any condition. The RT pin
is also current limited to 60µA. See Table 4 and Figure 8
for resistor values and the corresponding switching
frequencies.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz)
Figure 6. Analog Control of Inductor Current
1.2
1.0
0.8
0.6
0.4
0.2
0.750
0
Figure 8. Frequency vs R
0.5
0.3
0.2
0.1
1
0
50
100
LT3741
150
LT3741
200
FB
CTRL1
R
V
T
250
REF
(k )
3741 F06
300
R2
R1
3741 F07
350
T
Resistance
R2
R1
400
V
OUT
R
450
3743 F08
T
40.2
53.6
82.5
143
221
453
(kΩ)
500
3741f