LTC3565_15 Datasheet, PDF(12/22 Page) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

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LTC3565_15 Datasheet, PDF (12/22 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

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LTC3565

APPLICATIONS INFORMATION

is adequate for filtering. The output ripple (ÎVOUT) is

determined by:

ÎVOUT

ÎIL

ï£«

ï£ï£¬ï£¬ESR +

8fO CO U T

ï£¶

ï£¸ï£·ï£·

where f = operating frequency, COUT = output capacitance

and ÎIL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since ÎIL increases

with input voltage. With ÎILâ¯=â¯0.4 â¢ IOUT(MAX), the output

ripple will be less than 100mV at maximum VIN, a minimum

COUT of 10ÂµF and fO = 1MHz with:

ESRCOUT < 150mÎ©

Once the ESR requirements for COUT have been met, the

RMS current rating generally far exceeds the IRIPPLE(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 tantalum

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 it has 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, available in case heights ranging from

2mm to 4mm. Aluminum electrolytic capacitors have a

significantly larger ESR, and is 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, 0.1ÂµF to 1ÂµF of ceramic capacitors should

also be placed close to the LTC3565 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. Their high ripple

current, high voltage rating and low ESR make them

ideal for switching regulator applications. Because the

LTC3565âs control loop does not depend on the output

capacitorâs ESR for stable operation, ceramic capacitors

can be used freely to achieve very low output ripple and

small circuit size.

However, care must be taken when ceramic capacitors are

used at the input. 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. Refer to Linear Technology Application Note 88 for

a detailed discussion of this potential issue.

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage charac-

teristics of all the ceramics for a given value and size.

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

to support the load. The time required for the feedback

loop to respond is dependent on the compensation com-

ponents and the output capacitor value. Typically, 3 to 4

cycles are required to respond to a load step, but only in

the first cycle does the output drop linearly. The output

droop, VDROOP, is usually about 2 to 3 times the linear

3565fc

For more information www.linear.com/LTC3565

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