LTC1876 datasheet(27/36 Pages) LINER

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LTC1876

1876fa

Once the value of L is known, select an inductor that can

handle at least 1A without saturating. In addition, ensure

that the inductor has a low DCR (copper wire resistance)

to minimize I2R power losses.

Auxiliary Regulator’s Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multilayer ceramic capacitors are an excellent choice, as

they have extremely low ESR and are available in very

small packages. X5R dielectrics are preferred, followed by

X7R, as these materials retain the capacitance over wide

voltage and temperature ranges. A 4.7

µF to 10µF output

capacitor is sufficient for most applications, but systems

with very low output current may need only a 1

µF or 2.2µF

output capacitor. Solid tantalum or OS-CON capacitors

can be used, but they will occupy more board area than a

ceramic and will have a higher ESR. Always use a capacitor

with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, and should be placed as close as

possible to the AUXVIN pin. A 1µF to 4.7µF input capacitor

is sufficient for most applications. Table 2 shows a list of

several ceramic capacitor manufacturers. Consult the

manufacturers for detailed information on their entire

selection of ceramic parts.

Table 2. Ceramic Capacitor Manufacturers

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

AVX

(803) 448-9411

www.avxcorp.com

Murata

(714) 852-2001

www.murata.com

The decision to use either low ESR (ceramic) capacitors or

higher ESR (tantalum or OS-CON) capacitors can affect

the stability of the overall system. The ESR of any capaci-

tor, along with the capacitance itself, contributes a zero to

the system. For the tantalum and OS-CON capacitors, this

zero is located at a lower frequency due to the higher value

of the ESR, while the zero of a ceramic capacitor is a much

higher frequency and can generally be ignored.

A phase lead zero can be intentionally introduced by

placing a capacitor (C3) in parallel with the resistor (R8)

between VOUT3 and AUXVFB as shown in Figure 11. The

frequency of the zero is determined by the following

equation.

Z =

π ••

By choosing the appropriate values for the resistor and

capacitor, the zero frequency can be designed to slightly

improve the phase margin of the overall converter. The

typical target value for the zero frequency is between

50kHz to 150kHz.

Figure 11. Adding a Phase Lead Zero

Auxiliary Regulator’s Diode Selection

A Schottky diode is recommended for use with the auxil-

iary regulator. The ON Semiconductor MBR0520 is a very

good choice. Where the input to output voltage differential

exceeds 20V, use the MBR0530 (a 30V diode). These

diodes are rated to handle an average forward current of

0.5A. In applications where the average forward current of

the diode exceeds 0.5A, a Microsemi UPS5817 rated at 1A

is recommended.

Driving AUXSD Above 10V

The maximum voltage allowed on the AUXSD pin is 10V.

In some applications if the applied voltage on this pin is

going to exceed 10V, then a series resistor can be con-

nected to this pin. The value for this resistor is given by:

SERIES

AUXSD

(–

)

(•

)

–

60 10 6

By placing this series resistor, it ensures that the voltage

seen by the pin will not exceed 10V.

1876 F11

LTC1876

AUXVFB

VOUT3

APPLICATIO S I FOR ATIO

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