lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 12

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

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
APPLICATIONS INFORMATION
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
consideration when selecting the intended frequency of
operation. Higher switching frequency increases power
dissipation and lowers effi ciency. 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.
LT3686A
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
12
T
. Minimum on-time and edge loss must be taken into
500mA/DIV
100mA/DIV
10V/DIV
IN
V
V
– V
OUT
SW
AC
I
L
OUT
V
V
L = 6.8μH
C
I
OUT
IN
OUT
OUT
= 35V
= 1.2A
)/L • 100ns
= 3.3V
= 22μF
2μs/DIV
3686A F05
Table 1. R
FREQUENCY (MHz)
Figure 6b. Suggested Inductance vs Frequency
2.5
2.3
1.5
1.3
0.9
0.7
0.5
0.3
2.1
1.9
1.7
1.1
T
vs Frequency
300
250
200
150
100
50
40
35
30
25
20
15
10
Figure 6a. Switching Frequency vs R
0
5
0
0.25
0
0.5
3.3V
0.75
OUT
5V
FREQUENCY (MHz)
FREQUENCY (MHz)
R
OUT
T
9.53
15.8
20.0
25.5
31.6
40.2
52.3
69.8
12.1
97.6
150
280
1
1.25
(kΩ)
1.5
1.75
12V
OUT
MIN SYNC FREQUENCY
2
3686A F06b
3686A F06a
2.25
2.5
0.363
0.874
0.615
2.30
1.99
1.70
1.42
1.14
N/A
N/A
N/A
N/A
T
3686af