Datasheet

LT3686
12
3686fc
The part is robust enough to survive prolonged opera-
tion under these conditions as long as the peak inductor
current does not exceed 2A. Inductor current saturation
and junction temperature may further limit performance
during this operating regime.
Frequency Selection
The maximum frequency that the LT3686 can be pro-
grammed to is 2.5MHz. The minimum frequency that the
LT3686 can be programmed to is 300kHz. The switching
frequency is programmed by tying a 1% resistor from the
RT pin to ground. Table 1 can be used to select the value
of R
T
. Minimum on-time and edge loss must be taken into
consideration when selecting the intended frequency of
operation. Higher switching frequency increases power
dissipation and lowers efficiency. Finite transistor bandwidth
limits the speed at which the power switch can be turned
on and off, effectively setting the minimum on-time of the
LT3686. For a given output voltage, the minimum on-time
determines the maximum input voltage to remain in con-
tinuous mode operation outlined in the Minimum On Time
section of the data sheet. Finite transition time results in
a small amount of power dissipation each time the power
switch turns on and off (edge loss). Edge loss increases
with frequency, switch current, and input voltage.
ApplicAtions inForMAtion
inductor current is therefore peak current plus minimum
switch delay:
1.7A + (V
IN
– V
OUT
)/L•100ns
Figure 5. Pulse Skip with Large Load Current Will Be Limited by
the DA Valley Current Limit. Notice the Flat Inductor Valley
Current and Reduced Switching Frequency
V
IN
= 35V
V
OUT
= 3.3V
L = 6.8µH
C
OUT
= 22µF
I
OUT
= 1.2A
V
SW
10V/DIV
I
L
500mA/DIV
V
OUT
100mA/DIV
AC
2µs/DIV
3686 F05
Table 1. R
T
vs Frequency
FREQUENCY (MHz) R
T
(kΩ)
2.5 9.53
2.3 12.1
2.1 15.4
1.9 20
1.7 25.5
1.5 31.6
1.3 40.2
1.1 52.3
0.9 69.8
0.7 97.6
0.5 150
0.3 280
FREQUENCY (MHz)
50
100
R
T
(kΩ)
200
250
300
150
0 0.5 1 1.5 2 2.5
3686 F06a
0
FREQUENCY (MHz)
5
10
INDUCTANCE (µH)
20
25
40
30
35
15
0.25 0.75
5V
OUT
1.25 1.75 2.25
3686 F06b
0
12V
OUT
3.3V
OUT
Figure 6a. Switching Frequency vs R
T
Figure 6b. Suggested Inductance vs Frequency