7

LT3439

sn3439 3439fs

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

µs 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

mum slew rate. To determine the maximum slew rate

connect a 50k resistor pot with a 3.4k series resistance to

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

µV 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

µA 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:

APPLICATIO S I FOR ATIO

V

RR

R

V

ON

AB

B

SHDN

=

+









•

SHDN

3439 AI01

VR

V

RR

I

HYST

A

SHDN

AB

SHDN

=

∆

+









•

||

at the turn-on threshold, typically 20

voltage hysteresis seen at the SHDN pin at the turn-on

threshold, typically 35mV.

The resistors can be calculated as follows:

R

VV

V

V

IV

R

VV

V

V

IV

V

A

HYST

SHDN

ON

SHDN

SHDN

SHDN

B

HYST

SHDN

ON

SHDN

SHDN

ON

SHDN

=

∆

()

=

∆

()

()

•–

•

•

•–

•

•–

For example if the turn-on voltage was to be set at 5V with

0.5V of hysteresis:

R

VV

V

mV

AV

k

R

VV

V

mV

AV

V

k

A

B

µ

=

µ

()

=

05

13

5

35

20

1 3

18 27

05

13

5

35

20

5

1 3

642

.• .–

•

•.

.

.• .–

•

•– .

.

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