LTC3589_12 Datasheet, PDF (23/50 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589/LTC3589-1/
LTC3589-2
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, V_IN, and V_OUT, as shown in this equation:

I_L
=
f
1
•L
•

V_OUT
1–

VV_OIUNT^
In an example application the LTC3589 step-down switching
regulator 3 has a maximum load of 1A, V_IN equals 3.8V,
and V_OUT 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 PV_IN pin. Refer to Table 12 for
recommended ceramic capacitor manufacturers.
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