LTM8028
11
8028fb
For more information www.linear.com/LTM8028
applicaTions inForMaTion
with digital ports. Pins that float may either actually float
or require logic that has Hi-Z output capability. This allows
the output voltage to be dynamically changed if necessary.
The output voltage is selectable from a minimum of 0.8V
to a maximum of 1.8V in increments of 50mV.
Table 2. V
O2
to V
O0
Setting vs Output Voltage
V
O2
V
O1
V
O0
V
OUT(NOM)
V
O2
V
O1
V
O0
V
OUT(NOM)
0 0 0 0.80V Z 0 1 1.35V
0 0 Z 0.85V Z Z 0 1.40V
0 0 1 0.90V Z Z Z 1.45V
0 Z 0 0.95V Z Z 1 1.50V
0 Z Z 1.00V Z 1 0 1.55V
0 Z 1 1.05V Z 1 Z 1.60V
0 1 0 1.10V Z 1 1 1.65V
0 1 Z 1.15V 1 X 0 1.70V
0 1 1 1.20V 1 X Z 1.75V
Z 0 0 1.25V 1 X 1 1.80V
Z 0 Z 1.30V
X = Don’t Care, 0 = Low, Z = Float, 1 = High
Capacitor Selection Considerations
The C
IN
, C
BKV
and C
OUT
capacitor values in Table 1 are the
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in T
able 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved dynamic response, if
it is necessar
y. Again, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable.
X5R and X7R types are stable over temperature and ap
-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir
-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
The output capacitance for BKV given in Table 1 specifies
an electrolytic capacitor
. Ceramic capacitors may also be
used in the application, but it may be necessary to use
more of them. Many high value ceramic capacitors have
a large voltage coefficient, so the actual capacitance of
the component at the desired operating voltage may be
only a fraction of the specified value. Also, the very low
ESR of ceramic capacitors may necessitate an additional
capacitor for acceptable stability margin.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8028. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8028 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possi
-
bly exceeding the device’s rating. This situation is easily
avoided; see the Hot-Plugging Safely section.
Why Do Multiple, Small Value Output Capacitors
Connected in Parallel W
ork Better?
The parasitic series inductance (ESL) and resistance
(ESR) of a capacitor can have a detrimental impact on the
transient and ripple/noise response of a linear regulator.
Employing a number of capacitors in parallel will reduce
this parasitic impedance and improve the performance of
the linear regulator. In addition, PCB vias can add significant
inductance, so the fundamental decoupling capacitors must
be mounted on the same copper plane as the LTM8028.
The most area efficient parallel capacitor combination is
a graduated 4/2/1 scale capacitances of the same case
size, such as the 37μF combination in Table 1, made up
of 22μF, 10μF and 4.7μF capacitors in parallel. Capacitors
with small case sizes have larger ESR, while those with
larger case sizes have larger ESL. As seen in Table 1, the
optimum case size is 0805, followed by a larger, fourth
bulk energy capacitor, case sized 1210. In general, the
large fourth capacitor is required only if very tight transient
response is required.
