LTC3632 datasheet(12/22 Pages) LINER

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LTC3632

3632fb

APPLICATIONS INFORMATION

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even signiﬁcant deviations

do not offer much relief. Note that ripple current ratings

from capacitor manufacturers are often based only on

2000 hours of life which makes it advisable to further

derate the capacitor, or choose a capacitor rated at a

higher temperature than required. Several capacitors may

also be paralleled to meet size or height requirements in

the design.

The output capacitor, COUT, ﬁlters the inductor’s ripple

current and stores energy to satisfy the load current

when the LTC3632 is in sleep. The output voltage ripple

during a burst cycle is dominated by the output capacitor

equivalent series resistance (ESR) and can be estimated

by the following equation:

VOUT

160

< ΔVOUT ≤IPEAK •ESR

where the lower limit of VOUT/160 is due to the 5mV

feedback comparator hysteresis.

The value of the output capacitor must be large enough

to accept the energy stored in the inductor without a large

change in output voltage. Setting this voltage step equal to

1% of the output voltage, the output capacitor must be:

COUT > 50 • L •

IPEAK

VOUT

Typically, a capacitor that satisﬁes the ESR requirement is

adequate to ﬁlter the inductor ripple. To avoid overheating,

the output capacitor must also be sized to handle the ripple

current generated by the inductor. The worst-case ripple

current in the output capacitor is given 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 ringing.

Using Ceramic Input and Output Capacitors

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 applications with inductive source impedance, such

as a long wire, a series RC network 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.

LTC3632

VIN

CIN

LIN

3632 F05

4 • CIN

R =

LIN

CIN

Figure 5. Series RC to Reduce VIN Ringing

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation:

VOUT = 0.8V • 1+

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