LTC3589-2_15 datasheet(23/50 Pages) LINER

部件名	LTC3589-2

功能描述	8-Output Regulator with Sequencing and I2C

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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LTC3589/LTC3589-1/

LTC3589-2

23

3589fg

For more information www.linear.com/LTC3589

operaTion

frequency is determined by desired efficiency, component

size and converter duty cycle.

Operation at lower frequency improves efficiency by

reducing internal gate charge and switching losses but

requires larger inductance and capacitance values for

comparable output ripple voltage. The lowest duty cycle

of the step-down switching regulator is determined by

the converters minimum on-time. Minimum on-time is

the shortest time duration that the converter is capable of

turning its top PMOS on and off again. The time consists

of the gate charge time plus internal delays associated

with peak current sensing. The minimum on-time of the

LTC3589 is approximately 90ns. If the duty cycle falls

below what can be accommodated by the minimum on-

time, the converter will begin to skip cycles. The output

voltage will continue to be regulated but the ripple voltage

and current will increase. With the switching frequency

set to 2.25MHz, the minimum supported duty cycle is

20%. Switching at 1.125MHz the converter can support

a 10% duty cycle.

Phase Selection

To reduce the cycle by cycle peak current drawn by the

switching regulators, the clock phase of each of the

LTC3589 step-down switching regulators can be set using

I2C command register bits B1DTV2[6], B2DTV2[6] and

B3DTV2[6]. The internal full-rate clock has a nominal

duty cycle of 20% while the half-rate clocks have a 50%

duty cycle. Setting the command register bits high will

delay the start of each converter switching cycle by 20%

or 50% depending on the selected operating frequency.

Inductor Selection

The choice of step-down switching regulator inductor

influencestheefficiencyoftheconverterandthemagnitude

of the output voltage ripple. Larger inductance values

reduce inductor current ripple and therefore lower output

voltage ripple. A larger value inductor improves efficiency

by lowering the peak current to be closer to the average

output current. Larger inductors, however, generally

have higher series resistance that counters the efficiency

advantage of reduced peak current.

Inductorripplecurrentisafunctionofswitchingfrequency,

inductance, VIN, and VOUT, as shown in this equation:

ΔIL =

1

f •L

• VOUT 1–

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

In an example application the LTC3589 step-down

switching regulator 3 has a maximum load of 1A, VIN

equals 3.8V, and VOUT is set for 1.2V. A good starting

design point for inductor ripple is 30% of output current

or 300mA. Using the equation for ripple current, a 1.2µH

inductor should be selected.

An inductor with low DC resistance will improve converter

efficiency.SelectaninductorwithaDCcurrentratingatleast

1.5 times larger than the maximum load current to ensure

the inductor does not saturate during normal operations.

If short-circuit is a possible condition, the inductor should

be rated to handle the maximum peak current specified

for the step-down converter. Table 8 shows inductors

that work well with the step-down switching regulators.

Input/Output Capacitor Selection

LowESR(equivalentseriesresistance)ceramiccapacitors

should be used at both the output and input supply of the

switching regulators. Only X5R or X7R ceramic capacitors

should be used because they retain their capacitance over

wider voltage and temperature ranges than other ceramic

types. A 22µF capacitor is sufficient for the step-down

switching regulator outputs. For good transient response

and stability the output capacitor should retain at least

10µF of capacitance over operating temperature and bias

voltage. Place at least 4.7µF decoupling capacitance as

close as possible to each PVIN pin. Refer to Table 12 for

recommended ceramic capacitor manufacturers.

类似零件编号 - LTC3589-2_15

制造商	部件名	数据表	功能描述
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