LTC4267CDHC datasheet(19/32 Pages) LINEAR

19 / 32 page

LTC4267

4267fc

APPLICATIO S I FOR ATIO

Signature Disable Interface

To disable the 25kΩsignatureresistor,connectSIGDISApin

to the VPORTP pin. Alternately, SIGDISA pin can be driven

high with respect to VPORTN. An example of a signature

disable interface is shown in Figure 16, option 2. Note that

the SIGDISA input resistance is relatively large and the

threshold voltage is fairly low. Because of high voltages

present on the printed circuit board, leakage currents from

the VPORTP pin could inadvertently pull SIGDISA high. To

ensure trouble-free operation, use high voltage layout

techniques in the vicinity of SIGDISA. If unused, connect

SIGDISA to VPORTN.

Load Capacitor

The IEEE 802.3af speciﬁcation requires that the PD maintain

a minimum load capacitance of 5µF (provided by C1 in

Figure 11). It is permissible to have a much larger load

capacitor and the LTC4267 can charge very large load

capacitors before thermal issues become a problem. The

load capacitor must be large enough to provide sufﬁcient

energy for proper operation of the switching regulator.

However, the capacitor must not be too large or the PD

design may violate IEEE 802.3af requirements.

If the load capacitor is too large, there can be a problem

with inadvertent power shutdown by the PSE. Consider

the following scenario. If the PSE is running at – 57V

(maximum allowed) and the PD has detected and powered

up, the load capacitor will be charged to nearly – 57V. If

for some reason the PSE voltage is suddenly reduced to

–44V (minimum allowed), the input bridge will reverse bias

and the PD power will be supplied by the load capacitor.

Depending on the size of the load capacitor and the DC load

of the PD, the PD will not draw any power for a period of

time. If this period of time exceeds the IEEE 802.3af 300ms

disconnect delay, the PSE will remove power from the PD.

For this reason, it is necessary to ensure that inadvertent

shutdown cannot occur.

Very small output capacitors (≤10µF) will charge very

quickly in current limit. The rapidly changing voltage at

the output may reduce the current limit temporarily, caus-

ing the capacitor to charge at a somewhat reduced rate.

Conversely, charging a very large capacitor may cause the

current limit to increase slightly. In either case, once the

output voltage reaches its ﬁnal value, the input current

limit will be restored to its nominal value.

The load capacitor can store signiﬁcant energy when fully

charged. The design of a PD must ensure that this energy

is not inadvertently dissipated in the LTC4267. The polar-

ity-protection diode(s) prevent an accidental short on the

cable from causing damage. However, if the VPORTN pin

is shorted to VPORTP inside the PD while the capacitor

is charged, current will ﬂow through the parasitic body

diode of the internal MOSFET and may cause permanent

damage to the LTC4267.

Maintain Power Signature

In an IEEE 802.3af system, the PSE uses the maintain

power signature (MPS) to determine if a PD continues to

require power. The MPS requires the PD to periodically

draw at least 10mA and also have an AC impedance less

than 26.25kΩ in parallel with 0.05µF. If either the DC

current is less than 10mA or the AC impedance is above

26.25kΩ, the PSE may disconnect power. The DC current

must be less than 5mA and the AC impedance must be

above 2MΩ to guarantee power will be removed.

Selecting Feedback Resistor Values

The regulated output voltage of the switching regulator is

determined by the resistor divider across VOUT (R1 and

R2 in Figure 11) and the error ampliﬁer reference voltage

VREF. The ratio of R2 to R1 needed to produce the desired

voltage can be calculated as:

R2 = R1 • (VOUT – VREF)/VREF

In an isolated power supply application, VREF is determined

by the designer’s choice of an external error ampliﬁer.

Commercially available error ampliﬁers or programmable

shunt regulators may include an internal reference of

1.25V or 2.5V. Since the LTC4267 internal reference and

error ampliﬁer are not used in an isolated design, tie the

VFB pin to PGND.

In a nonisolated power supply application, the LTC4267

onboard internal reference and error ampliﬁer can be

used. The resistor divider output can be tied directly to

the VFB pin. The internal reference of the LTC4267 is 0.8V

nominal.

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