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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 sug-

gested 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%. The formula for the resistor

(R1) from VOUT to FB is:

OUT

21 2

2 0 25

(– . )

.–

( .

)

–

1.2V

VSW

GND

1765 F02

10k

OUTPUT

ERROR

AMPLIFIER

LT1765 (ADJ)

Figure 2. Feedback Network

APPLICATIO S I FOR ATIO

rating and turn-on surge problems. Y5V or similar type

ceramics can be used since the absolute value of capaci-

tance is less important and has no significant effect on

loop stability. If operation is required close to the mini-

mum input required by the output or the LT1765, a larger

value may be required. This is to prevent excessive ripple

causing dips below the minimum operating voltage result-

ing in erratic operation.

If tantalum capacitors are used, values in the 22

µFto470µF

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 tantalum capacitors are

surge rated. AVX recommends derating capacitor operat-

ing 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 cur-

rent rating is not an issue. The current waveform is

triangular, with an RMS value given by:

Lf V

RIPPLE RMS

OUT

() =

() −

()

()( )( )

029

The LT1765 will operate with both ceramic and tantalum

output capacitors. Ceramic capacitors are generally cho-

sen for their small size, very low ESR (effective series

resistance), and good high frequency operation. Ceramic

output capacitors in the 1

µF to 10µF range, X7R or X5R

type are recommended.

Tantalum capacitors are usually chosen for their bulk

capacitance properties, useful in high transient load appli-

cations. ESR rather than absolute value defines output

ripple at 1.25MHz. Typical LT1765 applications require a

tantalum capacitor with less than 0.3

Ω ESR at 22µF to

500

µF, see Table 2. This ESR provides a useful zero in the

frequency response. Ceramic output capacitors with low

ESR usually require a larger VC capacitor or an additional

series R to compensate for this.

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 at the input of LT1765 and to force the switching

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

RMS ripple current can be calculated from:

RIPPLE RMS

OUT

() =−

()/ 2

Ceramic capacitors are ideal for input bypassing. At higher

switching frequency, the energy storage requirement of

the input capacitor is reduced so values in the range of 1

µF

to 4.7

µF are suitable for most applications. Their high

frequency capacitive nature removes most ripple current

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