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LT3999 Datasheet(PDF) 9 Page - Linear Technology

Part No. LT3999
Description  Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3999 Datasheet(HTML) 9 Page - Linear Technology

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LT3999
9
3999fa
For more information www.linear.com/LT3999
APPLICATIONS INFORMATION
Soft-Start and Current Limit
The LT3999 soft-start ramps the peak switch current over
a time programmed by either a capacitor or a resistor and
capacitor on the ILIM/SS pin.
Whenprogrammingthesoft-starttimewithacapacitoronly
the soft-start time is calculated with the following formula:
tSS (ms) = CSS • 80
where CSS is in µF.
Thecurrentlimitdefaultstotheinternallysetvaluebecause
there is no resistor on the pin.
When programming the soft-start time with a resistor
and capacitor on the ILIM/SS pin the soft-start time is
calculated with the following formula:
τ = RC
where 3τ will be 95% of the maximum current.
The cycle-by-cycle current limit of the LT3999 is set with
a resistor on the ILIM/SS pin. Use the following formula
to calculate the value of the resistor:
RILIM (kΩ) = ILIM • 86.4
OVLO/DC and UVLO
The UVLO pin has a precision voltage threshold with
hysteresis to enable the LT3999. The pin is typically con-
nected to VIN through a resistor divider; however, it can
be directly connected to VIN.
The OVLO/DC pin has a precision voltage threshold with
hysteresis to disable the LT3999 switching operation. The
pin is typically connected to VIN through a resistor divider.
The OVLO/DC pin can be directly connected to GND to
disable the function. It is possible to use two separate
resistor divider strings for OVLO/DC and UVLO pins or
combine them together and use one resistor divider string
to drive both pins. See Figure 1.
Resistors are chosen by first selecting RB. Then calculate
RA with the following formula:
RA =RB
VTH
1.25V
–1


where VTH is the VIN referred voltage at which the supply
is enabled (UVLO) or disabled (OVLO/DC).
Transformer Design
Table 3 lists recommended center tapped transformers
for a variety of input voltage, output voltage and power
combinations. These transformers will yield slightly high
output voltages so that they can accommodate an LDO
regulator on the output.
If your application is not listed, the LTC Applications group
is available to assist in the choice and/or the design of the
transformer. In the design/selection of the transformer
the following characteristics are critical and should be
considered:
Table 3. Recommended Center Tapped Transformers
NOMINAL
INPUT
VOLTAGE (V)
NOMINAL
OUTPUT
VOLTAGE (V)
OUTPUT
POWER (W)
PART NUMBER
5
5
5
Coilcraft PA6383
5
12
1
Coilcraft PA6381
5
12
3
Cooper Bussmann
CTX02-19064
12
12
10
Coilcraft PA6384
24
24
20
Cooper Bussmann
CTX02-19061
VIN
UVLO
RA2
RA1
RA
RB
RB
3999 F01
OVLO/DC
VIN
UVLO
OR
OVLO/DC
Figure 1. Precision UVLO and OVLO Resistor Divider


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