7

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

sn1767 1767fas

FB RESISTOR NETWORK

If an output voltage of 1.8V, 2.5V, 3.3V or 5V is required, the

respective fixed option part, -1.8, -2.5, -3.3 or -5, should be

used. The FB pin is tied directly to the output; the necessary

resistive divider is already included on the part. For other

voltage outputs, the adjustable part should be used and an

external resistor divider added. The suggested resistor (R2)

from FB to ground is 10k. This reduces the contribution of

FB input bias current to output voltage to less than 0.25%.

R

RV

RA

OUT

1

21 2

12

2 0 25

=

−

()

−µ

.

.( .

)

of capacitance is less important and has no significant

effect on loop stability. If operation is required close to the

minimum input required by the output of the LT1767, a

larger value may be required. This is to prevent excessive

ripple causing dips below the minimum operating voltage,

resulting in erratic operation.

If tantalum capacitors are used, values in the 22

µF to 470µF

range are generally needed to minimize ESR and meet ripple

current and surge ratings. Care should be taken to ensure

the ripple and surge ratings are not exceeded. The AVX TPS

and Kemet T495 series are surge rated. AVX recommends

derating capacitor operating voltage by 2:1 for high surge

applications.

OUTPUT CAPACITOR

Unlike the input capacitor, RMS ripple current in the output

capacitor is normally low enough that ripple current rating

is not an issue. The current waveform is triangular, with an

RMS value given by:

I

VV

V

Lf V

RIPPLE RMS

OUT

IN

OUT

IN

()

()( )( )

029

.

The LT1767 will operate with both ceramic and tantalum

output capacitors. Ceramic capacitors are generally chosen

for their small size, very low ESR (effective series resis-

tance), and good high frequency operation, reducing out-

put ripple voltage. Their low ESR removes a useful zero in

the loop frequency response, common to tantalum capaci-

pole frequency must typically be reduced by a factor of 10.

Typical ceramic output capacitors are in the 1

µF to 10µF

range. Since the absolute value of capacitance defines the

pole frequency of the output stage, an X7R or X5R type

ceramic, which have good temperature stability, is recom-

mended.

Tantalum capacitors are usually chosen for their bulk

capacitance properties, useful in high transient load appli-

cations. ESR rather than capacitive value defines output

ripple at 1.25MHz. Typical LT1767 applications require a

tantalum capacitor with less than 0.3

Ω ESR at 22µF to

500

µF, see Table 2.

Figure 2. Feedback Network

–

+

1.2V

GND

1767 F02

R1

R2

10k

OUTPUT

ERROR

AMPLIFIER

FB

LT1767

+

INPUT CAPACITOR

Step-down regulators draw current from the input supply in

pulses. The rise and fall times of these pulses are very fast.

The input capacitor is required to reduce the voltage ripple

this causes at the input of LT1767 and force the switching

current into a tight local loop, thereby minimizing EMI. The

RMS ripple current can be calculated from:

II

V

V

V

V

RIPPLE RMS

OUT

OUT

IN

OUT

IN

Higher value, lower cost ceramic capacitors are now avail-

able in smaller case sizes. These are ideal for input bypass-

ing since their high frequency capacitive nature removes

most ripple current rating and turn-on surge problems. At

higher switching frequency, the energy storage require-

ment of the input capacitor is reduced so values in the range

of 1

µF to 4.7µF are suitable for most applications. Y5V or

similar type ceramics can be used since the absolute value