LT3472 Просмотр технического описания (PDF) - Linear Technology
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LT3472 Datasheet PDF : 12 Pages
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LT3472
APPLICATIO S I FOR ATIO
and larger capacitance, which can cause significant switch-
ing losses at 1.1MHz switching frequency. Some recom-
mended Schottky diodes are listed in Table 4.
Table 4. Recommended Schottky Diodes
Forward
Current
Part No. (mA)
Forward
Voltage
Drop (V)
Diode
Capacitance
(pF) Manufacturer
CMDSH-3 100 0.58 @100mA 7 @ 10V Central Semiconductor
CMDSH2-3 200 0.49 @ 200mA 15 @ 10V (631) 435-1110
www.centralsemi.com
Setting the Output Voltages
The LT3472 has an accurate feedback resistor of 50k for
each channel. Only one resistor is needed to set the output
voltage for each channel. The output voltage can be set
according to the following formulas:
VPOS
=
1.25 • ⎛⎝⎜1+
R1
50k
⎞⎠⎟
VNEG
=
–1.25
•
⎛
⎝⎜
R2 ⎞
50k ⎠⎟
In order to maintain accuracy, high precision resistors are
preferred (1% is recommended).
Soft-Start
The LT3472 has independent soft-start control for each
channel. As shown in Figure 1, the SSP and SSN pins have
an internal resistor of 50k pulling up to 1.25V, respec-
tively. By connecting a capacitor from the SSP or SSN pin
to ground, the ramp of each output can be programmed
individually. If SSP or SSN is open or pull higher than
1.25V, the corresponding output will ramp up quickly. The
waveforms with and without soft-start for the Boost
channel are shown in Figure 4.
The waveforms with and without soft-start for the negative
channel are shown in Figure 5.
Start Sequencing
The LT3472 has internal sequencing circuitry that inhibits
the negative channel from operating until feedback voltage
of the step-up channel reaches about 1.1V, ensuring that
VSSP
1V/DIV
VSSP
2V/DIV
VPOS
5V/DIV
IIN
100mA/DIV
1ms/DIV
3472 FO4a
Figure 4a. VSSP, VPOS, IIN with 100nF on SSP
VPOS
5V/DIV
IIN
200mA/DIV
100µs/DIV
3472 FO4b
Figure 4b. VSSP, VPOS, IIN with SSP Open
VSSN
1V/DIV
VNEG
5V/DIV
IIN
100mA/DIV
500µs/DIV
3472 FO5a
Figure 5a. VSSN, VNEG, IIN with 100nF on SSN
8
VSSN
2V/DIV
VNEG
5V/DIV
IIN
200mA/DIV
100µs/DIV
3472 FO5b
Figure 5b. VSSN, VNEG, IIN with SSN Open
3472f
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