ltc4269cdkd-2 Linear Technology Corporation, ltc4269cdkd-2 Datasheet - Page 23

• Manufacturer Part Number: ltc4269cdkd-2
• Description: Ieee 802.3at High Power Pd And Synchronous Forward Controller With Aux Support
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
loss of slope compensation during synchronization, the free
running oscillator frequency, f
to 80% of the external clock frequency (f
resistor chosen for nonsynchronized operation should be
increased by 1.25x (= f
Shutdown and Programming the Power Supply
Undervoltage Lockout
The LTC4269-2 has an accurate 1.32V shutdown threshold
at the SD_V
junction with a resistor divider to defi ne the power supply
undervoltage lockout threshold (UVLO) of the power supply
input voltage (V
before part turn-on, 0μA after part turn-on) allows power
supply UVLO hysteresis to be programmed. Calculation of
the on/off thresholds for the power supply input voltage
can be made as follows:
A simple open-drain transistor can be added to the resistor
divider network at the SD_V
of the LTC4269-2 (Figure 9).
The SD_V
be an external source current >11μA to lift the pin past its
1.32V threshold for part turn-on.
Micropower Start-Up: Selection of Start-Up Resistor
and Capacitor for V
The LTC4269-2 uses turn-on voltage hysteresis at the V
pin and low start-up current to allow micropower start-up
(Figure 10). The LTC4269-2 monitors V
allow the part to turn-on at 14.25V and the part to turn-
off at 8.75V. Low start-up current (460μA) allows a large
resistor to be connected between the power supply input
supply and V
increases to drive the IC (5.2mA) and the output drivers
(I
to prevent V
a bias winding in the converter takes over supply to V
This technique allows a simple resistor/capacitor for
start-up which draws low power from the system supply
to the converter.
DRIVE
V
V
S(OFF)
S(ON)
). A large enough capacitor is chosen at the V
Threshold = V
SEC
Threshold = 1.32[1 + (R1/R2)]
SEC
IN
IN
pin must not be left open since there must
. Once the part is turned on, input current
falling below its turn-off threshold before
S
) (Figure 9). A pin current hysteresis (11μA
pin. This threshold can be used in con-
IN
SYNC
S(OFF)
SEC
/f
OSC
OSC
+ (11μA • R1)
pin to control the turn-off
, should be programmed
).
SYNC
IN
pin voltage to
). The R
SLOPE
IN
pin
IN
IN
.
For system input voltages exceeding the absolute maximum
rating of the LTC4269-2 V
be connected from the V
condition where V
not turn on because SD_V
V
exceeding the rating for the V
should obey V
Programming Oscillator Frequency
The oscillator frequency (f
grammed using an external resistor, R
between the R
f
ning oscillator frequency is programmable in the range
of 100kHz to 500kHz.
OSC
IN
will continue to charge towards system V
ON
vs R
TRANSISTOR
SHUTDOWN
OPTIONAL
C
OFF
INPUT VOLTAGE (V
START
R
POWER SUPPLY
START
OSC
D1*
Figure 9. Programming Power Supply
Undervoltage Lockout (UVLO)
INPUT VOLTAGE (V
IN(ONMAX)
OSC
resistor values. The LTC4269-2 free-run-
Figure 10. Low Power Start-Up
POWER SUPPLY
IN
R1
R2
*FOR V
(V
pin and GND. Figure 11 shows typical
S
IN ON(MAX)
V
charges past V
)
IN
FROM AUXILIARY WINDING
S
> 25V, ZENER D1 RECOMMENDED
< V
SD_V
IN
IN
S
< V
OSC
SEC
)
1.32V
pin, an external zener should
Z
pin to GND. This covers the
Z
SEC
11μA
< 25V)
< 25V.
) of the LTC4269-2 is pro-
1.32V
< 1.32V. In this condition,
IN
pin. The zener voltage
IN(ON)
–
+
LTC4269-2
–
+
but the part does
LTC4269-2
OSC
LTC4269-2
, connected
42692 F10
IN
, possibly
42692 F09
23
42692f