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LTM8001

8001f

For more information www.linear.com/8001

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 VOUT0 output voltage.

2. Apply the recommended CIN0, COUT0, RFB0 and RT 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 VOUT1, VOUT2, VOUT3,

VOUT4 and VOUT5 regulators. To set the voltage of each

linear regulator, use the equation

RSETX =

VOUTX

10µA

where the value of RSET 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:

RSET =

VOUT

10µA •n

where n is the number of regulators paralleled.

4. ApplytheoutputcapacitorsfortheVOUT1,VOUT2,VOUT3,

VOUT4andVOUT5regulators.Aminimumoutputcapaci-

torof2.2μFwithanESRof0.5Ωorlessisrecommended

to prevent oscillations.

Whilethesecomponentcombinationshavebeentestedfor

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

Performance Characteristics section for guidance.

The maximum frequency (and attendant RT value) at

which the LTM8001 should be allowed to switch is given

in Table 1 in the fMAX column, while the recommended

frequency (and RT value) for optimal efficiency over the

given input condition is given in the fOPTIMAL 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 CIN and COUT capacitor values in Table 1 are the

minimum recommended values for the associated oper-

ating conditions. Applying capacitor values below those

indicated in Table 1 is not recommended, and may result

in undesirable operation. Using larger values is generally

acceptable, and can yield improved dynamic response, if

necessary. Again, it is incumbent upon the user to verify

proper operation over the intended system’s line, load and

environmental conditions.

Ceramiccapacitorsaresmall,robustandhaveverylowESR.

However, not all ceramic capacitors are suitable. X5R and

X7Rtypesarestableovertemperature,appliedvoltageand

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 Table 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.

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