ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 26

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
I
classifi cation given in the electrical specifi cations. The
R
the overall accuracy of the classifi cation circuit. Resis-
tor power dissipation will be 100mW maximum and is
transient so heating is typically not a concern. In order
to maintain loop stability, the layout should minimize
capacitance at the R
can be disabled by fl oating the R
should not be shorted to V
LTC4268-1 classifi cation circuit to attempt to source very
large currents. In this case, the LTC4268-1 will quickly go
into thermal shutdown.
Power Good Interface
The LTC4268-1 provides complimentary power good
signals to simplify the DC/DC converter interface. Using
the power good signal to delay converter operation until
the load capacitor is fully charged is recommended as this
will help ensure trouble free start up.
The active high PWRGD pin is controlled by an open
collector transistor referenced to V
low PWRGD pin is controlled by a high voltage, open-
drain MOSFET referenced to V
designed to interface directly to the UVLO pin with the aid
of a pull-up resistor to Vcc. An example interface circuit
is shown in Figure 11.
Port Voltage Lockout
PoE applications require the PD interface to turn on below
42V and turn off above 30V. The LTC4268-1 includes an
internal port voltage lockout circuit to implement this basic
chip on/off control. Additionally, the LTC4268-1 includes
an enable/lockout function for the DC/DC converter that is
controlled by the UVLO pin and is intended to be driven by
PWRGD to ensure proper startup. (Refer to Power Good
Interface.) Users have the ability to implement higher turn
LTC4268-1
26
IN_CLASS
CLASS
resistor must be 1% or better to avoid degrading
is the LTC4268-1 IC supply current during
CLASS
node. The classifi cation circuit
PORTN
PORTN
CLASS
as this would force the
NEG
. The PWRGD pin is
pin. The R
while the active
CLASS
pin
on voltages if necessary by connecting the UVLO pin to
an external resistive divider between V
The UVLO pin also includes a bias current allowing imple-
mentation of hysteresis. When UVLO is below 1.24V, gate
drivers are disabled and the converter sits idle. When the
pin rises above the lockout threshold a small current is
sourced out of the UVLO pin, increasing the pin voltage and
thus creating hysteresis. As the pin voltage drops below
this threshold, the current is disabled, further dropping
the UVLO pin voltage. If not used, the UVLO pin can be
disabled by tying to V
Shutdown Interface
To disable the 25k signature resistor, connect SHDN to
the V
high with respect to V
that disable the signature and all LTC4268-1 functions are
shown in Figure 10, options 2 and 4. Note that the SHDN
input resistance is relatively large and the threshold volt-
age is fairly low. Because of high voltages present on the
printed circuit board, leakage currents from the V
could inadvertently pull SHDN high. To ensure trouble-free
operation, use high voltage layout techniques in the vicinity
of SHDN. If unused, connect SHDN directly to V
Load Capacitor
The IEEE 802.3af specifi cation requires that the PD maintain
a minimum load capacitance of 5μF . It is permissible to
have a much larger load capacitor and the LTC4268-1 can
charge very large load capacitors before thermal issues
become a problem. However, the load 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.
For example, if the PSE is running at –57V (IEEE 802.3af
maximum allowed) and the PD is detected and powered
up, the load capacitor will be charged to nearly –57V. If
for some reason the PSE voltage is suddenly reduced to
PORTP
pin. Alternately, the SHDN pin can be driven
PORTN
CC
.
. Examples of interface circuits
PORTP
and V
PORTP
PORTN
PORTN
42681fa
pin
.
.