Datasheet

LT3504
10
3504fa
For more information www.linear.com/LT3504
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 10kΩ, R2 should not exceed 20kΩ
to avoid bias current error.
Input Voltage Range
The input voltage range for LT3504 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 LT3504 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 LT3504 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
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
OUT
= 1A, V
SW
= 0.4V
• V
D
is the catch diode forward voltage drop, for an ap-
propriately 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)
applicaTions inForMaTion
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
peak inductor current. Provided that the inductor does not
saturate and that the switch current remains below 2A,
operation above V
IN(PS)
is safe and will not damage the
part. For a more detailed discussion on minimum on-time
and pulse-skipping, refer to the Applications Information
section of the LT3505 data sheet.
Avoid starting up the LT3504 at input voltages greater
than 36V, as the LT3504 must simultaneously conduct
maximum currents at high V
IN
. The maximum operating
junction temperature of 125°C may be exceeded due to
the high instantaneous power dissipation.
Figure 1a: The LT3504 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 LT3504 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
3504 F01a
I
L
0.5A/DIV
V
SW
10V/DIV
2µs/DIV
3504 F01b
I
L
0.5A/DIV
V
SW
10V/DIV