LTC1436A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1436A' PDF : 28 Pages View PDF
LTC1436A
LTC1436-PLL-A/LTC1437A
APPLICATIONS INFORMATION
( ) ( ) 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. When using Figure 2 for
synchronizable applications, choose COSC correspond-
ing to a frequency approximately 30% below your center
frequency. (See Phase-Locked Loop and Frequency Syn-
chronization.)
300
VPLLLPF = 0V
250
200
150
100
50
0
0
100 200 300 400 500
OPERATING FREQUENCY (kHz)
1436 F02
Figure 2. Timing Capacitor Value
( )( ) ∆IL =
1
fL
VOUT1 −
VVOIUNT
Accepting larger values of ∆IL allows the use of low
inductances, 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
operation. 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 (TGS pin open),
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 = 5V
50
VOUT = 3.3V
VOUT ≤ 2.5V
40
30
20
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:
10
0
0 50 100 150 200 250 300
OPERATING FREQUENCY (kHz)
1436 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.
12
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