Datasheet
LT3472
8
3472f
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 Forward Diode
Current Voltage Capacitance
Part No. (mA) Drop (V) (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:
V
R
k
V
R
k
POS
NEG
=+
⎛
⎝
⎜
⎞
⎠
⎟
=
⎛
⎝
⎜
⎞
⎠
⎟
125 1
1
50
125
2
50
.•
–. •
Figure 4a. V
SSP
, V
POS
, I
IN
with 100nF on SSP Figure 4b. V
SSP
, V
POS
, I
IN
with SSP Open
Figure 5a. V
SSN
, V
NEG
, I
IN
with 100nF on SSN
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
APPLICATIO S I FOR ATIO
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Figure 5b. V
SSN
, V
NEG
, I
IN
with SSN Open
V
SSP
2V/DIV
V
POS
5V/DIV
I
IN
200mA/DIV
100µs/DIV
3472 FO4b
V
SSN
1V/DIV
V
NEG
5V/DIV
I
IN
100mA/DIV
500µs/DIV
3472 FO5a
V
SSN
2V/DIV
V
NEG
5V/DIV
I
IN
200mA/DIV
100µs/DIV
3472 FO5b
V
SSP
1V/DIV
V
POS
5V/DIV
I
IN
100mA/DIV
1ms/DIV
3472 FO4a