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LT3430

sn3430 3430is

APPLICATIO S I FOR ATIO

the switching frequency. If the gain of the error amplifier

is high enough at the switching frequency, output ripple

voltage (although smaller for a ceramic output capacitor)

may still affect the proper operation of the regulator. A

filter capacitor CF in parallel with the RC/CC network is

suggested to control possible ripple at the VC pin. An “All

Ceramic” solution is possible for the LT3430 by choosing

the correct compensation components for the given

application.

Example: For VIN = 8V to 40V, VOUT = 5V at 2A, the LT3430

can be stabilized, provide good transient response and

maintain very low output ripple voltage using the follow-

ing component values: (refer to the first page of this data

sheet for component references) C3 = 4.7

µF, RC = 3.3k,

CC = 22nF, CF = 220pF and C1 = 100µF. See Application

Note 19 for further detail on techniques for proper loop

compensation.

INPUT CAPACITOR

Step-down regulators draw current from the input supply

in pulses. The rise and fall times of these pulses are very

fast. The input capacitor is required to reduce the voltage

ripple this causes at the input of LT3430 and force the

switching current into a tight local loop, thereby minimiz-

ing EMI. The RMS ripple current can be calculated from:

RIPPLE RMS

OUT

() =

()

–/

Ceramic capacitors are ideal for input bypassing. At 200kHz

switching frequency, the energy storage requirement of

the input capacitor suggests that values in the range of

4.7

µF to 20µF are suitable for most applications. If opera-

tion is required close to the minimum input required by the

output of the LT3430, a larger value may be required. This

is to prevent excessive ripple causing dips below the

minimum operating voltage resulting in erratic operation.

Depending on how the LT3430 circuit is powered up you

may need to check for input voltage transients.

The input voltage transients may be caused by input

voltage steps or by connecting the LT3430 converter to an

already powered up source such as a wall adapter. The

sudden application of input voltage will cause a large

surge of current in the input leads that will store energy in

the parasitic inductance of the leads. This energy will

cause the input voltage to swing above the DC level of input

power source and it may exceed the maximum voltage

rating of input capacitor and LT3430.

The easiest way to suppress input voltage transients is to

add a small aluminum electrolytic capacitor in parallel with

the low ESR input capacitor. The selected capacitor needs

to have the right amount of ESR in order to critically

dampen the resonant circuit formed by the input lead

inductance and the input capacitor. The typical values of

ESR will fall in the range of 0.5

Ωto2Ωandcapacitancewill

fall in the range of 5

µF to 50µF.

If tantalum capacitors are used, values in the 22

µF to

470

µF range are generally needed to minimize ESR and

meet ripple current and surge ratings. Care should be

taken to ensure the ripple and surge ratings are not

exceeded. The AVX TPS and Kemet T495 series are surge

rated. AVX recommends derating capacitor operating

voltage by 2:1 for high surge applications.

CATCH DIODE

Highest efficiency operation requires the use of a Schottky

type diode. DC switching losses are minimized due to its

low forward voltage drop, and AC behavior is benign due

to its lack of a significant reverse recovery time. Schottky

diodes are generally available with reverse voltage ratings

of up to 60V and even 100V, and are price competitive with

other types.

The use of so-called “ultrafast” recovery diodes is gener-

ally not recommended. When operating in continuous

mode, the reverse recovery time exhibited by “ultrafast”

diodes will result in a slingshot type effect. The power

internal switch will ramp up VIN current into the diode in an

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