LTC3630_15 Datasheet, PDF(14/26 Page) Linear Technology – High Efficiency, 65V 500mA Synchronous Step-Down Converter

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3630_15 Datasheet, PDF (14/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous Step-Down Converter

◁

LTC3630

Applications Information

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.

Input Voltage Steps

If the input voltage falls below the regulated output volt-

age, the body diode of the internal high side MOSFET will

conduct current from the output supply to the input sup-

ply. If the input voltage falls rapidly, the voltage across the

inductor will be significant and may saturate the inductor. A

large current will then flow through the high side MOSFET

body diode, resulting in excessive power dissipation that

may damage the part.

If rapid voltage steps are expected on the input supply, put

a small silicon or Schottky diode in series with the VIN pin

to prevent reverse current and inductor saturation, shown

below as D2 in Figure 5a. The diode should be sized for

a reverse voltage of greater than the input voltage, and to

withstand repetitive currents higher than the maximum

peak current of the LTC3630.

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 capaci-

tor with a series resistor may be required in parallel with

CIN to dampen the ringing of the input supply. Figure 5b

shows this circuit and the typical values required to

dampen the ringing.

LIN

CIN

4 â¢ CIN

LTC3630

VIN

CIN

3630 F05b

Figure 5b. Series RC to Reduce VIN Ringing

INPUT

SUPPLY

LTC3630

VIN

CIN

3637 F05a

VOUT

COUT

Figure 5a. Preventing Current Flow to the Input

Ceramic capacitors are also piezoelectric sensitive. The

LTC3630âs 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.

For more information www.linear.com/LTC3630

3630fd

▷