Datasheet

LT3479
11
3479fc
Increasing switching frequency reduces output voltage
ripple but also reduces effi ciency. The user should set
the frequency for the maximum tolerable output voltage
ripple. Figure 8 shows a reduction in effi ciency of about
4% between 1MHz and 2MHz operation in a typical
application.
Inrush Current Protection
The LT3479 features a novel inductor current sensing
circuit that protects the LT3479 during hot plugging and
short circuits. An internal resistor in series with the ex-
ternal inductor senses the inductor current at all times.
When it exceeds 5A, a soft-start cycle is initiated. Figure 9
APPLICATIONS INFORMATION
shows an output overload with inrush current protection
disabled. Notice that soft-start remains high, and that
the inductor current does not return to zero. Figure 10
illustrates the benefi ts of inrush current protection. The
output short initiates a new soft start cycle reducing the
inductor current. After the fault has passed, the inductor
current slowly returns to its equilibrium value. To ensure
bond wire integrity, the inductor current should not exceed
8A for more than 10ms.
Bypassing the 8.5mΩ inductor current sense resistor
disables inrush current protection. Connect the inductor
supply trace and bypass capacitor to the L pin and leave
the V
S
pin open to disable this feature.
Figure 7. Switching Frequency
R
T
(kΩ)
0
1.5
1.0
0.5
3.5
3.0
2.5
2.0
3479 F07
SWITCH FREQUENCY (MHz)
0.1
100
10
Figure 8. Effi ciency vs Switching Frequency
I
OUT
(A)
0
EFFICIENCY (%)
70
80
1MHz
0.8
3479 F08
60
50
0.2
0.4
0.6
90
65
75
55
85
2MHz
20μs/DIV
V
SW
10V/DIV
I
L
4V/DIV
V
SS
2V/DIV
V
OUT
20V/DIV
3479 F09
Figure 9. Output Overload with
Inrush Current Protection Enabled
20μs/DIV
V
SW
10V/DIV
I
L
4V/DIV
V
SS
2V/DIV
V
OUT
20V/DIV
3479 F10
Figure 10. Output Overload with
Inrush Current Protection Disabled
Figure 11. Circuit for Output Overload
LT3479
BOOST
REGULATOR
V
IN
0.5Ω
3479 F11
V
OUT