LTC3403EDD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3403EDD Datasheet PDF : 16 Pages
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U
OPERATIO (Refer to Functional Diagram)
1200
1000
VOUT = 1.8V
800
VOUT = 1.5V
600
VOUT = 2.5V
400
200
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
3403 F02
Figure 2. Maximum Output Current vs Input Voltage
LTC3403
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles > 40%. However, the LTC3403 uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
APPLICATIO S I FOR ATIO
The basic LTC3403 application circuit is shown in Fig-
ure␣ 1. External component selection is driven by the load
requirement and begins with the selection of L followed by
CIN and COUT.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 4.7µH. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. As Equation 1 shows, a greater difference be-
tween VIN and VOUT produces a larger ripple current.
Where these voltages are subject to change, the highest
VIN and lowest VOUT will determine the maximum ripple
current. A reasonable starting point for setting ripple
current is IL = 240mA (40% of the maximum load, 600mA).
∆IL
=
1
(f)(L)
VOUT
1–
VOUT
VIN
(1)
At output voltages below 0.6V, the switching frequency
decreases linearly to a minimum of approximately 700kHz.
This places the maximum ripple current (in forced con-
tinuous mode) at the highest input voltage and the lowest
output voltage. In practice, the resulting ouput ripple
voltage is 10mV to 15mV using the components specified
in Figure 1.
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor should be enough for most applications (600mA
+ 120mA). For better efficiency, choose a low DC-resis-
tance inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher IL) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy but generally
cost more than powdered iron core inductors with similar
electrical characteristics. The choice of which style induc-
tor to use often depends more on the price versus size
requirements and any radiated field/EMI requirements
than on what the LTC3403 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3403 applications.
3403f
9
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