Datasheet

LT3686A
12
3686afa
to keep the inductor valley current to 1.7A (Figure 5). Peak
inductor current is therefore peak current plus minimum
switch delay:
1.7A + (V
IN
– V
OUT
)/L•100ns
ApplicAtions inForMAtion
Table 1. R
T
vs Frequency
FREQUENCY (MHz) R
T
(kΩ) MIN SYNC FREQUENCY (MHz)
2.5 9.53 N/A
2.3 12.1 N/A
2.1 15.4 N/A
1.9 20.0 N/A
1.8 22.6 2.50
1.7 25.5 2.30
1.5 31.6 1.99
1.3 40.2 1.70
1.1 52.3 1.42
0.9 69.8 1.14
0.7 97.6 0.874
0.5 150 0.615
0.3 280 0.363
FREQUENCY (MHz)
50
100
R
T
(kΩ)
200
250
300
150
0 0.5 1 1.5 2 2.5
3686A 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
3686A F06b
0
12V
OUT
3.3V
OUT
Figure 6a. Switching Frequency vs R
T
Figure 6b. Suggested Inductance vs Frequency
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
3686A F05
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 LT3686A can be pro-
grammed to is 2.5MHz. The minimum frequency that the
LT3686A 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 band-
width limits the speed at which the power switch can be
turned on and off, effectively setting the minimum on-time
of the LT3686A. For a given output voltage, the minimum
on-time determines the maximum input voltage to remain
in continuous 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.