
12
signals it is driving. Since the pin can nominally sink 1.2mA 
with only a 0.1V drop, a 1k
 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 1k
 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.
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 
m
) 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 1-5m
 
range), in parallel with a ceramic 1μF capacitor (with low 
ESR in the 10m
 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. 
FIGURE 11. V
OUT1
 OUTPUT CAPACITOR SELECTION
100
1000
10000
CAPACITANCE (μF)
1000
100
10
E
)
ISL6505