Datasheet LT3435 (Analog Devices) - 10

LT3435
U U W U APPLICATIO S I FOR ATIO
and Soft-Start Current graphs in Typical Performance
INPUT CAPACITOR
Characteristics). Step-down regulators draw current from the input supply Frequency foldback is done to control power dissipation in in pulses. The rise and fall times of these pulses are very both the IC and in the external diode and inductor during fast. The input capacitor is required to reduce the voltage short-circuit conditions. A shorted output requires the ripple this causes at the input of LT3435 and force the switching regulator to operate at very low duty cycles. As switching current into a tight local loop, thereby minimiz- a result the average current through the diode and induc- ing EMI. The RMS ripple current can be calculated from: tor is equal to the short-circuit current limit of the switch (typically 4.7A for the LT3435). Minimum switch on time I I OUT = V V – V ( ) limitations would prevent the switcher from attaining a RIPPLE RMS ( ) OUT IN OUT VIN sufficiently low duty cycle if switching frequency were maintained at 500kHz, so frequency is reduced by about Ceramic capacitors are ideal for input bypassing. At 500kHz 4:1 when the FB pin voltage drops below 0.4V (see switching frequency input capacitor values in the range of Frequency Foldback graph). In addition, if the current in 4.7µF to 20µF are suitable for most applications. If opera- the switch exceeds 1.5 times the current limitations speci- tion is required close to the minimum input required by the fied by the V LT3435 a larger value may be required. This is to prevent C pin, due to minimum switch on time, the LT3435 will skip the next switch cycle. As the feedback excessive ripple causing dips below the minimum operat- voltage rises, the switching frequency increases to 500kHz ing voltage resulting in erratic operation. with 0.95V on the FB pin. During frequency foldback, Input voltage transients caused by input voltage steps or external syncronization is disabled to prevent interference by hot plugging the LT3435 to a pre-powered source such with foldback operation. Frequency foldback does not as a wall adapter can exceed maximum VIN ratings. The affect operation during normal load conditions. sudden application of input voltage will cause a large In addition to lowering switching frequency the soft-start surge of current in the input leads that will store energy in ramp rate is also affected by the feedback voltage. Large the parasitic inductance of the leads. This energy will capacitive loads or high input voltages can cause a high cause the input voltage to swing above the DC level of input input current surge during start-up. The soft-start func- power source and it may exceed the maximum voltage tion reduces input current surge by regulating switch rating of the input capacitor and LT3435. All input voltage current via the V transient sequences should be observed at the V C pin to maintain a constant voltage ramp IN pin of rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from the LT3435 to ensure that absolute maximum voltage the C ratings are not violated. SS pin to the output determines the maximum output dV/dt. When the feedback voltage is below 0.4V, the VC pin The easiest way to suppress input voltage transients is to will rise, resulting in an increase in switch current and add a small aluminum electrolytic capacitor in parallel with output voltage. If the dV/dt of the output causes the current the low ESR input capacitor. The selected capacitor needs through the CSS capacitor to exceed ICSS the VC voltage is to have the right amount of ESR to critically damp the reduced resulting in a constant dV/dt at the output. As the resonant circuit formed by the input lead inductance and feedback voltage increases ICSS increases, resulting in an the input capacitor. The typical values of ESR will fall in the increased dV/dt until the soft-start function is defeated range of 0.5Ω to 2Ω and capacitance will fall in the range with 0.9V present at the FB pin. The soft-start function of 5µF to 50µF. does not affect operation during normal load conditions. However, if a momentary short (brown out condition) is If tantalum capacitors are used, values in the 22µF to present at the output which causes the FB voltage to drop 470µF range are generally needed to minimize ESR and below 0.9V, the soft-start circuitry will become active. meet ripple current and surge ratings. Care should be 3435fa 10