LTC1265CS datasheet(8/16 Pages) LINER

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LTC1265/LTC1265-3.3/LTC1265-5

APPLICATIONS INFORMATION

P-channel switch duty cycle. At high input voltages, the

diode conducts most of the time. As VIN approaches VOUT,

the diode conducts only a small fraction of the time. The

most stressful condition for the diode is when the output

is short circuited. Under this condition, the diode must

safely handle ISC(PK) at close to 100% duty cycle. Most

LTC1265 circuits will be well served by either a 1N5818 or

a MBRS130LT3 Schottky diode. An MBRS0520 is a good

choice for IOUT(MAX) ≤ 500mA.

CIN

In continuous mode, the input current of the converter is

a square wave of duty cycle VOUT/VIN. To prevent large

voltage transients, a low ESR input capacitor must be

used. In addition, the capacitor must handle a high RMS

current. The CIN RMS current is given by:

IRMS

≈

IOUT [VOUT (VIN – VOUT)]

1/2

VIN

(ARMS)

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

= IOUT/2. This simple worst case is commonly used for

design because even significant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours life-

time. This makes it advisable to further derate the capaci-

tor, or to choose a capacitor rated at a higher temperature

than required. Do not underspecify this component. An

additional 0.1

µF ceramic capacitor is also required on

PWR VIN for high frequency decoupling.

COUT

The selection of COUT is based upon the effective series

resistance (ESR) for proper operation of the LTC1265. The

required ESR of COUT is:

ESRCOUT < 50mV/IRIPPLE

where IRIPPLE is the ripple current of the inductor. For the

case where the IRIPPLE is 25mV/RSENSE, the required ESR

of COUT is:

ESRCOUT < 2(RSENSE)

To avoid overheating, the output capacitor must be sized

to handle the ripple current generated by the inductor. The

worst-case RMS ripple current in the output capacitor is

given by:

IRMS

≈ 150mV

2(RSENSE)

(ARMS)

Generally, once the ESR requirement for COUT has been

met, the RMS current rating far exceeds the IRIPPLE(P-P)

requirement.

ESR is a direct function of the volume of the capacitor.

Manufacturers such as Nichicon, AVX and Sprague should

be considered for high performance capacitors. The

OS-CON semiconductor dielectric capacitor available

from Sanyo has the lowest ESR for its size at a somewhat

higher price.

In surface mount applications, multiple capacitors may

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

current handling requirement of the application. Alumi-

num electrolyte and dry tantalum capacitors are both

available in surface mount configurations. In the case of

tantalum, it is critical that the capacitors are both available

in surface mount configuration and 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. Consult the

manufacturer for other specific recommendations.

When the capacitance of COUT is made too small, the

output ripple at low frequencies will be large enough to trip

the voltage comparator. This causes Burst Mode opera-

tion to be activated when the LTC1265 would normally be

in continuous operation. The effect will be most pro-

nounced with low value of RSENSE and can be improved at

higher frequencies with lower values of L.

Low-Battery Detection

The low-battery comparator senses the input voltage

through an external resistive divider. This divided voltage

connects to the (–) input of a voltage comparator (Pin 4)

which is compared with a 1.25V reference voltage. Ne-

glecting Pin 4 bias current, the following expression is

used for setting the trip limit:

VLB_TRIP = 1.25 1 +

)

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