LTC3622 datasheet(13/24 Pages) AD

13 / 24 page

LTC3622/

LTC3622-2/LTC3622-23/5

Rev D

For more information www.analog.com

Output Voltage Programming

For non-fixed output voltage parts, the output voltage is

set by external resistive dividers according to the follow-

ing equation:

VOUT = 0.6V • 1+

⎛

⎝

⎜

⎞

⎠

⎟

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 2.

For fixed VOUT parts, tie FB directly to VOUT, as R2 and R1

are matched internal resistors.

APPLICATIONS INFORMATION

Figure 2. Setting the Output Voltage

Note that ripple current ratings from capacitor manufac-

turers are often based on only 2000 hours of life which

makesitadvisabletofurtherderatethecapacitor,orchoose

a capacitor rated at a higher temperature than required.

Several capacitors may also be paralleled to meet size or

height requirements in the design. For low input voltage

applications, sufficient bulk input capacitance may be

needed to minimize transient effects during output load

changes.

Output Capacitor (COUT) Selection

The selection of COUT is determined by the effective series

resistance(ESR)thatisrequiredtominimizevoltageripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response. The output ripple, ΔVOUT, is

determined by:

ΔVOUT < ΔIL

8 • ƒ • COUT

+ ESR

⎛

⎝

⎜

⎞

⎠

⎟

Theoutputrippleishighestatmaximuminputvoltagesince

ILincreaseswithinputvoltage.Multiplecapacitorsplaced

inparallelmaybeneededtomeettheESRandRMScurrent

handlingrequirements.Drytantalum,specialpolymerand

hybridconductivepolymercapacitorsareverylowESRbut

have lower capacitance density than other types. Tantalum

capacitors have the highest capacitance density but it is

importance to only use types that have been surge tested

foruseinswitchingpowersupplies.Aluminumelectrolytic

capacitors have significantly higher ESR, but can be used

in cost-sensitive applications provided that consideration

is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteristics

and small footprints.

Using Ceramic Input and Output Capacitors

Higher capacitance value, lower cost ceramic capacitors

are now becoming available in smaller case sizes. Their

high ripple current, high voltage rating and low ESR make

them ideal for switching regulator applications. However,

care must be taken when these capacitors are used at

the input and output. When a ceramic capacitor is used

VOUT

CFF

3622 F02

GND

LTC3622

Input Capacitor (CIN) Selection

The input capacitance, CIN, is needed to filter the square

wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring, a low

ESR input capacitor sized for the maximum RMS current

should be used. The RMS current calculation is different

if the part is used in in-phase or out-of-phase.

For "in phase", when VOUT1 = VOUT2

VOUT(VIN – VOUT)

VIN

This formula has a maximum at VIN = 2VOUT. This simple

worstcaseiscommonlyusedtodeterminethehighestIRMS.

For out-of-phase case, the ripple current can be lower than

the "in phase" current. The maximum current typically oc-

curs when VOUT1 – VIN/2 = VOUT2 or when VOUT2 – VIN/2

= VOUT1. As a good rule of thumb, the amount of worst

case ripple is about 75% of the worst case ripple in the

in-phase mode. Also note that when VOUT1 = VOUT2 = VIN/2

and I1 = I2, the input current ripple is at its minimum.

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