LTC1435A Ver la hoja de datos (PDF) - Linear Technology
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LTC1435A Datasheet PDF : 20 Pages
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LTC1435A
APPLICATIONS INFORMATION
The value of COSC is calculated from the desired operating
frequency:
COSC
(pF)
=
1.37(104 )
Frequency (kHz)
–
11
A graph for selecting COSC vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz.
300
250
200
150
100
50
0
0
100 200 300 400 500
OPERATING FREQUENCY (kHz)
1435A F02
Figure 2. Timing Capacitor Value
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic trade-
off, the effect of inductor value on ripple current and low
current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current ∆IL decreases with higher induc-
tance or frequency and increases with higher VIN or VOUT:
∆IL
=
1
(f)(L)
VOUT
1
–
VOUT
VIN
Accepting larger values of ∆IL allows the use of low induc-
tances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ∆IL = 0.4(IMAX). Remember, the maximum
∆IL occurs at the maximum input voltage.
The inductor value also has an effect on low current opera-
tion. The transition to low current operation begins when
the inductor current reaches zero while the bottom MOSFET
is on. Lower inductor values (higher ∆IL) will cause this to
occur at higher 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 decrease.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and VOUT.
60
VOUT = 5.0V
VOUT = 3.3V
50
VOUT ≤ 2.5V
40
30
20
10
0
0 50 100 150 200 250 300
OPERATING FREQUENCY (kHz)
1435A F03
Figure 3. Recommended Inductor Values
For low duty cycle, high frequency applications where the
required minimum on-time,
( )( ) tON(MIN) =
VOUT
VIN(MAX )
,
f
is less than 350ns, there may be further restrictions on the
inductance to ensure proper operation. See Minimum On-
Time Considerations section for more details.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot afford
the core loss found in low cost powdered iron cores, forc-
ing the use of more expensive ferrite, molypermalloy or Kool
Mµ® cores. Actual core loss is independent of core size for
Kool Mµ is a registered trademark of Magnetics, Inc.
8
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