Datasheet

LT3514
12
3514fa
For more information www.linear.com/LT3514
The three step-down converters in the LT3514 are referred
to as channels 1, 3, and 4, while the boost converter is
referred to as channel 5. There is no channel 2. This nam
-
ing convention is intended to maintain consistency and
limited pin compatibility with the LT3504, a four channel
step-down converter. Essentially, two 1A converters (chan
-
nels 2 and 3) of the LT3504 were combined to make the
2A converter (channel 3) of the LT3514.
FB Resistor Network
The output voltage is programmed with a resistor divider
connected from the output and the FB pin. Choose the 1%
resistor according to:
R1= R2 •
V
OUT
0.8V
− 1






A good value for R2 is 10.2kΩ, R2 should not exceed
20kΩ to avoid bias current error.
Input Voltage Range
The input voltage range for LT3514 applications depends
on the output voltage and on the absolute maximum rat
-
ing of the V
IN
pin.
The minimum input voltage to regulate the output gener-
ally has to be at least 400mV greater than the greatest
programmed output voltage. The only exception is when
the largest programmed output voltage is less than 2.8V
.
In this case the minimum input voltage is 3.2V
.
The absolute maximum input voltage of the LT3514 is
40V and the part will regulate output voltages as long
as the input voltage remains less than or equal to 40V.
However for constant-frequency operation (no pulse-
skipping) the maximum input voltage is determined by
the minimum on-time of the LT3514 and the programmed
switching frequency. The minimum on-time is the shortest
period of time that it takes the switch to turn on and off.
Therefore the maximum input voltage to operate without
pulse-skipping is:
V
IN(PS)
= [ (V
OUT
+ V
D
)/(f
SW
• t
ON(MIN)
) ] + V
SW
– V
D
APPLICATIONS INFORMATION
where:
• V
IN(PS)
is the maximum input voltage to operate in
constant frequency operation without skipping pulses.
• V
OUT
is the programmed output voltage
• V
SW
is the switch voltage drop, at I
OUT1,4
= 1A,
V
SW1,4
= 0.4V, at I
OUT3
= 2A, V
SW3
= 0.4V.
• V
D
is the catch diode forward voltage drop, for an
appropriately sized diode, V
D
= 0.4V
• f
SW
is the programmed switching frequency
• t
ON(MIN)
is the minimum on-time, worst-case over
temperature = 110ns (at T = 125°C)
At input voltages that exceed V
IN(PS)
the part will continue
to regulate the output voltage up to 40V. However the
part will skip pulses (see Figure 1) resulting in unwanted
harmonics, increased output voltage ripple, and increased
Figure 1a. The LT3514 Operating in Constant-Frequency
Operation (Below V
IN(PS)
), V
IN
= 26.5V, V
OUT
= 3.3V,
f
SW
= 2MHz, t
ON(MIN)
= 74ns at T = 25°C
Figure 1b. The LT3514 Operating in Pulse-Skipping
Mode (Above V
IN(PS)
), V
IN
= 27V, V
OUT
= 3.3V, f
SW
=
2MHz, t
ON(MIN)
= 74ns at T = 25°C
2µs/DIV
3514 F01a
I
L
0.5A/DIV
V
SW
10V/DIV
2µs/DIV
3514 F01b
I
L
0.5A/DIV
V
SW
10V/DIV