LT3686 LINER [Linear Technology], LT3686 Datasheet - Page 12

• Manufacturer Part Number: LT3686
• Manufacturer: LINER [Linear Technology]

ApplicAtions inForMAtion
LT3686
inductor current is therefore peak current plus minimum
switch delay:
1.7A + (V
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
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.
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

T
. Minimum on-time and edge loss must be taken into
500mA/DIV
100mA/DIV
IN
10V/DIV
V
– V
V
OUT
SW
AC
I
L
OUT
V
V
L = 6.8µH
C
I
OUT
IN
OUT
OUT
= 35V
)/L • 100ns
= 1.2A
= 3.3V
= 22µF
2µs/DIV
3686 F05
Table 1. R
FREQUENCY (MHz)
Figure 6b. Suggested Inductance vs Frequency
T
vs Frequency
300
250
200
150
100
50
40
35
30
25
20
15
10
0
Figure 6a. Switching Frequency vs R
5
0
0.25
2.5
2.3
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5
0.3
0
0.5
3.3V
0.75
OUT
5V
FREQUENCY (MHz)
FREQUENCY (MHz)
OUT
1
1.25
1.5
1.75
12V
OUT
2
2.25
R
3686 F06a
3686 F06b
12.38
25.32
31.95
40.85
151.6
280.8
T
9.51
15.8
52.7
69.9
98.6
20
(kΩ)
2.5
T
3686fa