Datasheet LT3439 (Analog Devices) - 7

LT3439
U U W U APPLICATIO S I FOR ATIO Oscillator SYNC
R R  V A B = + V ON The oscillator can be synchronized to an external clock.  R SHDN B  • Set the RC timing components for an oscillator frequency 10% below the desired sync frequency. RA V SHDN IN It is recommended that the SYNC pin be driven with a RB square wave that has an amplitude greater than 2V, a pulse 3439 AI01 width greater than 1µs and a rise time less than 500ns. The V rising edge of the sync waveform triggers the change in ON is the input voltage at which the supply will turn on and V the state of the outputs. SHDN is the SHDN pin turn-on threshold, typically 1.3V.  ∆ 
Slew Rate Setting
V V = R SHDN • +I HYST A  R ||R SHDN Setting the LT3439 maximum slew rate is easy. The A B external resistor to ground on the RSL pin sets the maxi- VHYST is the actual hysteresis voltage seen at the input mum slew rate. To determine the maximum slew rate voltage. ISHDN is the current hysteresis sourced by the IC connect a 50k resistor pot with a 3.4k series resistance to at the turn-on threshold, typically 20µA. ∆VSHDN is the the RSL pin. Start at the lowest resistance setting and voltage hysteresis seen at the SHDN pin at the turn-on increase the pot until the noise level meets your require- threshold, typically 35mV. ments. Note that slower slewing waveforms will lower the power supply efficiency. Consult Linear Technology Appli- The resistors can be calculated as follows: cation Note 70, “A Monolithic Switching Regulator with (V • V – V • V ∆ ) 100µV Output Noise” for recommended noise measure- HYST R SHDN ON SHDN A = ment techniques. I • V SHDN SHDN (V • V – V • V ∆ )
Shutdown
HYST R SHDN ON SHDN B = I • (V – V ) The SHDN pin is used to shut down the part. Grounding SHDN ON SHDN this pin will disable all internal circuitry. For example if the turn-on voltage was to be set at 5V with Increasing the SHDN voltage above the turn-on threshold, 0.5V of hysteresis: approximately 1.3V, will enable the part. At the turn-on threshold approximately 20µA of current is sourced out of (0 5.V •1 3.V – 5V •3 m 5 V) R = = 18. k 27 the pin. This current, in conjunction with the Thevenin A 20 A µ • 1 3 . V resistance on the pin, sets up the amount of hysteresis. (0 5. •1 3. – 5 •35 ) This allows the user to set the turn-on voltage of the supply V V V mV R = 6 4 . k B 2 and the start-up hysteresis with a resistor divider. The 20 A µ • (5V – 1 3.V) = hysteresis can be used to prevent the part from shutting down due to input voltage sag from an initial high current The nearest 1% values would be 18.2k and 6.49k. draw. When the SHDN pin is greater than 2.1V, the A resistor in series with the SHDN pin could further change hysteresis current is reduced to zero. hysteresis without changing the turn-on voltage. In addition to the current hysteresis, there is also approxi- mately 35mV of voltage hysteresis on the SHDN pin.
Thermal Considerations
If a resistor divider is used to set the turn on threshold the Decreasing the noise by lowering the slew rate of the resistors are determined by the following equations: output switches does not come for free. Lower slew rates sn3439 3439fs 7