LT3504IUFD データシートの表示（PDF） - Linear Technology
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LT3504IUFD Datasheet PDF : 24 Pages
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LT3504
applications information
Frequency Selection
The maximum frequency that the LT3504 can be pro-
grammed to is 2.5MHz. The minimum frequency is 250kHz.
The switching frequency can be programmed in two ways.
The first method is by tying a 1% resistor (RT) from the
RT/SYNC pin to ground. Table 1 can be used to select the
value of RT. The second method is to synchronize (sync)
the internal oscillator to an external clock. The external
clock must have a minimum amplitude from 0V to 1.6V
and a minimum pulse-width of 50ns.
Table 1. RT/SYNC Pin Resistance to Program Oscillator
Frequency
FREQUENCY (MHz)
RT/SYNC PIN RESISTANCE (kΩ)
0.20
140
0.3
82.5
0.4
56.2
0.5
43.2
0.6
34.8
0.7
28.0
0.8
23.7
0.9
20.5
1.0
18.2
1.1
16.9
1.2
14.7
1.3
13.0
1.4
11.5
1.5
10.7
1.6
9.76
1.7
8.66
1.8
8.06
1.9
7.32
2.0
6.81
2.1
6.34
2.2
6.04
2.3
5.62
2.4
5.36
2.5
4.99
In certain applications, the LT3504 may be required to be
alive and switching for a period of time before it begins
to receive a sync signal. If the sync signal is in a high
impedance state when it is inactive then the solution is to
simply tie an RT resistor from the RT/SYNC pin to ground
(Figure 2). The sync signal should be capable of driving the
RT resistor. If the sync signal is in a low impedance state
or an unknown state when it is inactive, then the solution
is to tie the RT resistor from the RT/SYNC pin to ground
and then to drive the RT/SYNC pin with the sync signal
through a 1nF capacitor as shown in Figure 3.
PORT
LT3504
RT/SYNC
GND
RT
3504 F02
Figure 2. Driving the RT/SYNC Pin From
a Port That Is in a High Impedance State
When it Is Inactive
PORT
1nF
LT3504
RT/SYNC
GND
RT
3504 F03
Figure 3. Driving the RT/SYNC Pin from
a Port That Is in a Low Impedance State
When it Is Inactive
BOOST Regulator and SKY Pin Considerations
The on-chip boost regulator generates the SKY voltage
to be 4.85V above VIN. The SKY voltage is the source of
drive current for the buck regulators which is used to fully
saturate the power switch. The boost regulator requires
two external components: an inductor and a capacitor.
A good first choice for an inductor is given by:
L
=
20.5µH
f
where f is in MHz.
Thus, for a 250kHz programmed switching frequency,
a good first choice for an inductor value is 82µH. For a
2.5MHz programmed switching frequency, a good first
For more information www.linear.com/LT3504
3504fa
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