LT3461A_15 Datasheet, PDF (5/8 Pages) Linear Technology – 1.3MHz/3MHz Step-Up DC/DC Converters with Integrated Schottky in ThinSOT

LT3461/LT3461A
APPLICATIO S I FOR ATIO
Inrush Current
The LT3461 has a built-in Schottky diode. When supply
voltage is applied to the V_IN pin, the voltage difference
between V_IN and V_OUT generates inrush current flowing
from input through the inductor and the Schottky diode to
charge the output capacitor. The maximum nonrepetitive
surge current the Schottky diode in the LT3461 can
sustain is 1.5A. The selection of inductor and capacitor
value should ensure the peak of the inrush current to be
below 1.5A. In addition, turn-on of the LT3461 should be
delayed until the inrush current is less than the maximum
current limit. The peak inrush current can be calculated as
follows:
⎛
⎞
I_P
=
V_IN –
L
C
0.6
–1
• exp
⎜
⎜−
⎜
⎝⎜
2
π
⎟
⎟
L
C
⎟
− 1_⎠⎟
where L is the inductance, r is the resistance of the
inductor and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
V_IN (V)
5
L (µH)
4.7
5
10
C (µF)
1
1
I_P (A)
1.1
0.9
Thermal Considerations
Significant power dissipation can occur on the LT3461
and LT3461A, particularly at high input voltage. Device
load, voltage drops in the power path components, and
switching losses are the major contributors. It is impor-
tant to measure device power dissipation in an application
to ensure that the LT3461 does not exceed the absolute
maximum operating junction temperature of 125°C over
the operating ambient temperature range. Generally, for
supply voltages below 5V the integrated current limit
function provides adequate protection for nonfault condi-
tions. For supply voltages above 5V, Figures 3a and 3b
show the recommended operating region of the LT3461
and LT3461A, respectively. These graphs are based
on 250mW on-chip dissipation. Improvement of these
numbers can be expected if the LT3461 is supplied from a
separate low voltage rail.
160
V_IN = 5V
V_IN = 8V
120
V_IN = 12V
V_IN >15V
80
40
0
6
14
22
30
38
V_OUT (V)
3461 F01a
Figure 3a. LT3461 Operating Region
160
V_IN = 5V
120
V_IN = 8V
V_IN = 12V
V_IN >15V
80
40
0
6
14
22
30
38
V_OUT (V)
3461 F01b
Figure 3b. LT3461A Operating Region
Switching Frequency
The key difference between the LT3461 and LT3461A is the
faster switching frequency of the LT3461A. At 3MHz, the
LT3461A switches at twice the rate of the LT3461. The
higher switching frequency of the LT3461A allows physi-
cally smaller inductors and capacitors to be used in a given
application, but with a slight decrease in efficiency and
maximum output current when compared to the LT3461.
Generally if efficiency and maximum output current are
crucial, or a high output voltage is being generated, the
LT3461 should be used. If application size and cost are
more important, the LT3461A will be the better choice.
3461af
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