Datasheet

LTM8001
12
8001fb
For more information www.linear.com/LTM8001
APPLICATIONS INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 1 and find the row that has the desired
input range and V
OUT0
output voltage.
2. Apply the recommended C
IN0
, C
OUT0
, R
FB0
and R
T
val-
ues. Note that ceramic and electrolytic capacitors are
recommended. These are intended
to work in concert
to optimize performance and solution size; apply both
capacitors.
3. Apply the set resistors for the V
OUT1
, V
OUT2
, V
OUT3
,
V
OUT4
and V
OUT5
regulators. To set the voltage of each
linear regulator, use the equation
R
SETX
=
V
OUTX
10µA
where the value of R
SET
is in Ohms. Note that there is
no minimum positive output voltage for the regulator,
but a minimum load current is required to maintain
regulation regardless of output voltage, (please see
Electrical Characteristics table). For true zero voltage
output operation, this minimum load current must be
returned to a negative supply voltage. If paralleling the
linear regulators, set the output of each regulator to
the same voltage by tying the SETx pins together and
applying a single resistor. The value of the single set
resistor is given by the equation:
R
SET
=
V
OUT
10µA •n
where n is the number of regulators paralleled.
4. Apply the output capacitors for the V
OUT1
, V
OUT2
, V
OUT3
,
V
OUT4
and V
OUT5
regulators. A minimum output capaci-
tor of 2.2μF with an ESR of 0.5Ω or less is recommended
to prevent oscillations.
While these component combinations have been tested for
proper operation, it is incumbent upon the user to verify
proper operation over the intended system’
s line, load and
environmental conditions. Bear in mind that the maximum
output current is limited by junction temperature, the rela
-
tionship between the input and output voltage magnitude
and other factors. Please refer to the graphs in the Typical
Per
formance Characteristics section for guidance.
The maximum frequency (and attendant R
T
value) at
which the LTM8001 should be allowed to switch is given
in Table 1 in the f
MAX
column, while the recommended
frequency (and R
T
value) for optimal efficiency over the
given input condition is given in the f
OPTIMAL
column.
There are additional conditions that must be satisfied if
the synchronization function is used. Please refer to the
Switching Frequency Synchronization section for details.
Capacitor Selection Considerations
The C
IN
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
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, applied voltage and
give dependable service. Other types, including Y5V and
Z5U have very large temperature and voltage coefficients
of capacitance. In an application circuit they may have
only a small fraction of their nominal capacitance result
-
ing in much higher output voltage ripple than expected.
Many of the output capacitances given in T
able 1 specify
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 additional capacitors
for acceptable stability margin.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8001. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8001 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.