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LT1956/LT1956-5

1956f

Output capacitor ripple current (RMS):

Lf V

RIPPLE RMS

OUT

()

.–

()(

)

()( )( )

029

Ceramic Capacitors

Ceramic capacitors are generally chosen for their good

high frequency operation, small size and very low ESR

(effective series resistance). Their low ESR reduces

output ripple voltage but also removes a useful zero in the

loop frequency response, common to tantalum capaci-

tors. To compensate for this, a resistor RC can be placed

in series with the VC compensation capacitor CC. Care

must be taken however, since this resistor sets the high

frequency gain of the error amplifier, including the gain

at 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. The LT1956 can be stabilized for VOUT = 5V at

1A using a 22

µF ceramic output capacitor and VC com-

ponent values of CC = 4700pF, RC = 4.7k and CF = 220pF.

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 LT1956 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 500kHz

switching frequency, the energy storage requirement of

the input capacitor suggests that values in the range of

2.2

µF to 10µF are suitable for most applications. If opera-

tion is required close to the minimum input required by the

output of the LT1956, a larger value may be required. This

is to prevent excessive ripple causing dips below the mini-

mum operating voltage resulting in erratic operation.

Depending on how the LT1956 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 LT1956 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 LT1956.

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 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

APPLICATIO S I FOR ATIO

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