LT3439 LINER [Linear Technology], LT3439 Datasheet - Page 7

• Manufacturer Part Number: LT3439
• Description: Slew Rate Controlled Ultralow Noise1A Isolated DC/DC Transformer Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Oscillator SYNC
The oscillator can be synchronized to an external clock.
Set the RC timing components for an oscillator frequency
10% below the desired sync frequency.
It is recommended that the SYNC pin be driven with a
square wave that has an amplitude greater than 2V, a pulse
width greater than 1 s and a rise time less than 500ns. The
rising edge of the sync waveform triggers the change in
the state of the outputs.
Slew Rate Setting
Setting the LT3439 maximum slew rate is easy. The
external resistor to ground on the R
mum slew rate. To determine the maximum slew rate
connect a 50k resistor pot with a 3.4k series resistance to
the R
increase the pot until the noise level meets your require-
ments. Note that slower slewing waveforms will lower the
power supply efficiency. Consult Linear Technology Appli-
cation Note 70, “A Monolithic Switching Regulator with
100 V Output Noise” for recommended noise measure-
ment techniques.
Shutdown
The SHDN pin is used to shut down the part. Grounding
this pin will disable all internal circuitry.
Increasing the SHDN voltage above the turn-on threshold,
approximately 1.3V, will enable the part. At the turn-on
threshold approximately 20 A of current is sourced out of
the pin. This current, in conjunction with the Thevenin
resistance on the pin, sets up the amount of hysteresis.
This allows the user to set the turn-on voltage of the supply
and the start-up hysteresis with a resistor divider. The
hysteresis can be used to prevent the part from shutting
down due to input voltage sag from an initial high current
draw. When the SHDN pin is greater than 2.1V, the
hysteresis current is reduced to zero.
In addition to the current hysteresis, there is also approxi-
mately 35mV of voltage hysteresis on the SHDN pin.
If a resistor divider is used to set the turn on threshold the
resistors are determined by the following equations:
SL
pin. Start at the lowest resistance setting and
U
U
W
SL
pin sets the maxi-
U
V
V
V
voltage. I
at the turn-on threshold, typically 20 A. V
voltage hysteresis seen at the SHDN pin at the turn-on
threshold, typically 35mV.
The resistors can be calculated as follows:
For example if the turn-on voltage was to be set at 5V with
0.5V of hysteresis:
The nearest 1% values would be 18.2k and 6.49k.
A resistor in series with the SHDN pin could further change
hysteresis without changing the turn-on voltage.
Thermal Considerations
Decreasing the noise by lowering the slew rate of the
output switches does not come for free. Lower slew rates
ON
SHDN
HYST
V
R
R
R
R
V
V
IN
ON
is the input voltage at which the supply will turn on and
HYST
B
B
A
A
is the actual hysteresis voltage seen at the input
is the SHDN pin turn-on threshold, typically 1.3V.
R
A
SHDN
0 5
V
0 5
V
HYST
.
R
HYST
.
R
20
A
V
V
A
R
3439 AI01
R
I
is the current hysteresis sourced by the IC
SHDN
B
• .
• .
B
SHDN
•
20
•
1 3
R
•
1 3
A
V
V
B
I
R
•
SHDN
SHDN
SHDN
V
V
V
•
A
A
5
SHDN
•
||
–
–
V
• .
V
V
R
1 3
5
ON
5
SHDN
– .
•
–
B
–
V
V
V
1 3
V
V
–
V
•
SHDN
•
ON
ON
35
35
V
I
V
SHDN
SHDN
mV
mV
•
•
V
V
SHDN
SHDN
6 42
18 27
.
.
k
LT3439
k
SHDN
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is the
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