Datasheet

LTC4264
19
4264f
APPLICATIONS INFORMATION
R
CLASS
from Table 2. If a unique load current is required,
the value of R
CLASS
can be calculated as:
R
CLASS
= 1.237V/(I
LOAD
– I
IN_CLASS
)
I
IN_CLASS
is the LTC4264 IC supply current during clas-
sifi cation given in the electrical specifi cations. The R
CLASS
resistor must be 1% or better to avoid degrading the overall
accuracy of the classifi cation circuit. Resistor power dissi-
pation will be 100mW maximum and is transient so heating
is typically not a concern. In order to maintain loop stabil-
ity, the layout should minimize capacitance at the R
CLASS
node. The classifi cation circuit can be disabled by fl oating
the R
CLASS
pin. The R
CLASS
pin should not be shorted to
V
IN
as this would force the LTC4264 classifi cation circuit
to attempt to source very large currents. In this case, the
LTC4264 will quickly go into thermal shutdown.
GND
R
S
10k
R10
100k
PWRGD
D9
MMBD4148
Q1
FMMT2222
–54V
4264 F11
TO
PSE
LTC4264
ACTIVE-LOW ENABLE
V
IN
V
OUT
V
+
PD
LOAD
GND
R
S
10k
R9
100k
PWRGD
D9
5.1V
MMBZ5231B
–54V
TO
PSE
LTC4264
ACTIVE-LOW ENABLE
V
IN
V
OUT
PD
LOAD
–54V
TO
PSE
ACTIVE-HIGH ENABLE
PD
LOAD
RUN
SHDN
GND
PWRGD
LTC4264
V
IN
V
OUT
Power Good Interface
The LTC4264 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
OUT
while the active low PWRGD pin is controlled by a
high voltage, open-drain MOSFET referenced to V
IN
. The
designer has the option of using either of these signals to
enable the DC/DC converter and example interface circuits
are shown in Figure 11. When using PWRGD, diode D9
and resistor R
S
protects the converter shutdown pin from
excessive reverse voltage.
Figure 11. Power Good Interface Examples