ISL6505 Datasheet, PDF (12/17 Pages) Intersil Corporation – Multiple Linear Power Controller with ACPI Control Interface

ISL6505
signals it is driving. Since the pin can nominally sink 1.2mA
with only a 0.1V drop, a 1kΩ resistor will match that
condition. The minimum input low logic level is typically
around 25-30% of the 1.2V supply (0.3V in this example),
and the 0.1V is well below it. So a resistor pull-up value as
low as 1kΩ is acceptable to get faster rise times.
Linear Regulator (V_OUT1) Compensation
V_OUT1 is a linear regulator, with an on-chip amplifier, and
external FET and feedback resistors. The output capacitors
should be selected to allow the output voltage to meet any
dynamic regulation requirements, paying attention to their
parasitic components ESR (Effective Series Resistance) and
ESL (Effective Series inductance).
V_OUT1 is internally compensated to cover a wide range of
load currents; however the output filter capacitor must be
chosen carefully. Ideally, the capacitor value and its ESR
combine to create a zero that cancels one of the amplifier
poles. However, this is only a first order approximation, since
that pole moves with load current, for example. In addition,
there are high frequency poles that may come into play
under certain conditions.
A lower capacitor ESR improves transient response. When
the output load changes quickly (faster than the amplifier
itself can respond), the differential load current is sourced or
sinked by the capacitor, until the regulator can respond and
catch up. In this case, the higher the ESR, the larger the
voltage drop across it, and thus the larger the voltage
transient on the output is.
However, lower output capacitor ESR pushes the zero
frequency higher, reducing the regulator phase margin.
Thus, it may be difficult to simultaneously satisfy both tight
dynamic regulation and a good stable loop with high phase
margin.
There are many factors that affect V_OUT1 stability, such that
a simple equation or formula is not practical. So the
recommendation is to choose a value from Figure 11, which
shows capacitance versus ESR. Values inside the polygon
will result in stable conditions over a full load range of 10mA
to 3A. Choosing a value outside the polygon is NOT
recommended; it may work in some cases, but the margin
may be much smaller. In addition, there are manufacturing
tolerances (of both the IC and the capacitor), load variations,
temperature, FET selection, and many other factors that can
create the potential for problems.
1000
100
10
100
1000
10000
CAPACITANCE (µF)
FIGURE 11. V_OUT1 OUTPUT CAPACITOR SELECTION
Other Considerations
See COMPONENT SELECTION section for more details on
choosing Q6. The minimum load assumed is 10mA. The
maximum load is based primarily on the ability of the FET to
dissipate the heat; for stability, the assumption was 3A.
The FET selection can affect the compensation. With light
(or no) load, the gm of the FET is very low, and looks like a
high series resistance to the load, thus reducing the loop
gain, and moving the pole formed by the output capacitor
down by as much as several decades. In addition, the FET
input capacitance can vary from hundreds to thousands of
pF; (higher gm FETs such as logic level FETs typically have
a higher gate capacitance). The FET capacitance, along with
the amplifier driver resistance is included in the stability
calculations. Finally, the slewing of the FET gate
(determined by its capacitance) affects the transient
response. So a lot of the parameters are inter-related.
Note that the latest low-ESR ceramic capacitors are NOT
well suited for this application; the ESR (typically only a few
mΩ) is too low to be inside the polygon, for any typical value
of capacitance.
1V2VID Regulator (V_OUT2) Compensation
1V2VID is an on-chip linear regulator, which is internally
compensated to cover loads up to its maximum rating of
180mA. However, the output capacitor choice can affect the
stability. The recommendation is to use a tantalum (or
similar) capacitor around 10µF (with high ESR in the 1-5mΩ
range), in parallel with a ceramic 1µF capacitor (with low
ESR in the 10mΩ range). The two capacitors (dominated by
the tantalum) will create a zero that will help cancel the pole
of the internal regulator; the ceramic capacitor will help the
frequency response. Note that a single bigger ceramic
capacitor is NOT recommended; the higher ESR is
necessary.
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