LTC3589 datasheet(23/50 Pages) LINER

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LTC3589/LTC3589-1/

LTC3589-2

3589fe

OPERATION

quency to 1.125MHz. Selection of the operating frequency

is determined by desired efﬁciency, component size and

converter duty cycle.

Operation at lower frequency improves efﬁciency by reduc-

ing 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 com-

mand 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 inﬂu-

ences the efﬁciency of the converter and the magnitude 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 low-

ering 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.

Inductor ripple current is a function of switching frequency,

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

IL =

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. Select an inductor with a DC current rating at least

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

Low ESR (equivalent series resistance) ceramic capacitors

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.

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