LTC3446IDE#PBF【PDF 13/20 页】IC REG TRPL BCK/LINEAR 14-DFN

LTC3446

13

3446ff

applicaTions inForMaTion

Output Capacitor (C

OUTB

) Selection

The selection of C

OUTB

is driven by the required ESR to

minimize voltage ripple and load step transients. Typically,

once the ESR requirement is satisfied, the capacitance

is adequate for filtering. The output ripple (V

OUTB

) is

determined by:

V

OUTB

HI

L

ESR+

1

8f

O

C

OUTB

?/DIV>

where f = 2.25MHz, C

OUTB

= output capacitance and

I

L

= ripple current in the inductor. The output ripple is

highest at maximum input voltage since I

L

increases

with input voltage.

Once the ESR requirements for C

OUTB

have been met, the

RMS current rating generally far exceeds the I

RIPPLE(P-P)

requirement, except for an all ceramic solution.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Aluminum

electrolytic, special polymer, ceramic and dry tantulum

capacitors are all available in surface mount packages. The

OS-CON semiconductor dielectric capacitor available from

Sanyo has the lowest ESR(size) product of any aluminum

electrolytic at a somewhat higher price. Special polymer

capacitors, such as Sanyo POSCAP, offer very low ESR,

but have a lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density,

but have a larger ESR and it is critical that the capacitors

are surge tested for use in switching power supplies.

An excellent choice is the AVX TPS series of surface

mount tantalums, avalable in case heights ranging from

2mm to 4mm. Aluminum electrolytic capacitors have a

significantly larger ESR, and are often used in extremely

cost-sensitive applications provided that consideration is

given to ripple current ratings and long-term reliability.

Ceramic capacitors have the lowest ESR and cost but also

have the lowest capacitance density, a high voltage and

temperature coefficient and exhibit audible piezoelectric

effects. In addition, the high Q of ceramic capacitors along

with trace inductance can lead to significant ringing. Other

capacitor types include the Panasonic specialty polymer

(SP) capacitors.

In most cases, 0.1礔 to 1礔 of ceramic capacitors should

also be placed close to the LTC3446 in parallel with the

main capacitors for high frequency decoupling.

Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now be-

coming available in smaller case sizes. These are tempting

for switching regulator use because of their very low ESR.

Unfortunately, the ESR is so low that it can cause loop

stability problems. Solid tantalum capacitor ESR gener-

ates a loop zero at 5kHz to 50kHz that is instrumental in

giving acceptable loop phase margin. Ceramic capacitors

remain capacitive to beyond 300kHz and usually resonate

with their ESL before ESR becomes effective. Also, ceramic

caps are prone to temperature effects which requires the

designer to check loop stability over the operating tem-

perature range. To minimize their large temperature and

voltage coefficients, only X5R or X7R ceramic capacitors

should be used. A good selection of ceramic capacitors

is available from Taiyo Yuden, TDK and Murata.

Great care must be taken when using only ceramic input

and output capacitors. When a ceramic capacitor is used

at the input and the power is being supplied through long

wires, such as from a wall adapter, a load step at the output

can induce ringing at the V

IN

pin. At best, this ringing can

couple to the output and be mistaken as loop instability.

At worst, the ringing at the input can be large enough to

damage the part.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement. During a load step, the output capacitor must

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

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