LTC3589_1 Datasheet, PDF (22/46 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589
OPERATION
Operating Frequency
The switching frequency of each of the LTC3589 step-
down switching regulators may be independently set using
I²C command register bits B1DTV2[5], B2DTV2[5] and
B3DTV2[5]. The power-on default frequency is 2.25MHz.
Writing bit BxDTV2[5] HIGH will reduce the switching fre-
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
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 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:
ΔI_L
=
1
f •L
•

V_OUT 1–

V_OUT
V_IN



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 7 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
3589fb
22