LTC3104 datasheet(14/26 Pages) LINER

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LTC3630

3630fb

APPLICATIONS INFORMATION

by IRMS = IPEAK/2. Multiple capacitors placed in parallel

may be needed to meet the ESR and RMS current handling

requirements.

Dry tantalum, special polymer, aluminum electrolytic,

and ceramic capacitors are all available in surface mount

packages. Special polymer capacitors offer very low ESR

but have lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density

but it is important only to use types that have been surge

tested for use in switching power supplies. Aluminum

electrolytic capacitors have significantly higher ESR but

can be used in cost-sensitive applications provided that

consideration is given to ripple current ratings and long-

term reliability. Ceramic capacitors have excellent low ESR

characteristics but can have high voltage coefficient and

audible piezoelectric effects. The high quality factor (Q)

of ceramic capacitors in series with trace inductance can

also lead to significant input voltage ringing.

Ceramic Capacitors and Audible Noise

Higher value, lower cost ceramic capacitors are now be-

coming available in smaller case sizes. Their high ripple

current, high voltage rating, and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input and

the power is supplied by a wall adapter through long wires,

a load step at the output can induce ringing at the input,

VIN. At best, this ringing can couple to the output and be

mistaken as loop instability. At worst, a sudden inrush

of current through the long wires can potentially cause

a voltage spike at VIN large enough to damage the part.

For application with inductive source impedance, such as

a long wire, an electrolytic capacitor or a ceramic capacitor

with a series resistor may be required in parallel with CIN

to dampen the ringing of the input supply. Figure 5 shows

this circuit and the typical values required to dampen the

ringing.

Ceramic capacitors are also piezoelectric sensitive. The

LTC3630’s burst frequency depends on the load current,

and in some applications at light load the LTC3630 can

excite the ceramic capacitor at audio frequencies, gen-

erating audible noise. If the noise is unacceptable, use

a high performance tantalum or electrolytic capacitor at

the output.

Output Voltage Programming

The LTC3630 has three fixed output voltage modes that

can be selected with the VPRG1 and VPRG2 pins and an

adjustable mode. The fixed output modes use an internal

feedback divider which enables higher efficiency, higher

noise immunity, and lower output voltage ripple for 5V,

3.3V and 1.8V applications. To select the fixed 5V output

voltage, connect VPRG1 to SS and VPRG2 to GND. For 3.3V,

connect VPRG1 to GND and VPRG2 to SS. For 1.8V, connect

both VPRG1 and VPRG2 to SS. For any of the fixed output

voltage options, directly connect the VFB pin to VOUT.

For the adjustable output mode (VPRG1 = 0V, VPRG2 = 0V),

the output voltage is set by an external resistive divider

according to the following equation:

VOUT = 0.8V • 1+

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LIN

3630 F05

VIN

Figure 5. Series RC to Reduce VIN Ringing

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