Datasheet

LTC2923
9
2923fa
Figure 6. Simplified Functional Diagram
The LTC2923 features an electronic circuit breaker func-
tion that protects the optional series FET against short
circuits. When the FET is fully enhanced (GATE > RAMP +
4.9V), the electronic circuit breaker is enabled. Then, if the
voltage across the FET (V
DS
) exceeds 200mV as measured
from V
CC
to the RAMP pin for more than about 10µs the
gate of the FET is pulled down with 20mA, turning it off.
Because the slaved supplies track the RAMP pin, they are
pulled low by the tracking circuit when a short-circuit fault
occurs. Following a short-circuit fault, the FET is latched
off until the fault is cleared by pulling the ON pin below
0.4V.
Ramp Buffer
The RAMPBUF pin provides a buffered version of the
RAMP pin voltage that drives the resistive dividers on the
TRACKx pins. When there is no external FET, it provides up
to 2mA to drive the resistors even though the GATE pin
only supplies 10µA. The RAMPBUF pin also proves useful
in systems with an external FET. Since the track cell in the
simplified functional diagram above drives 0.8V on the
TRACKx pins, if R
TBx
is connected directly to the FET’s
source, the TRACKx pin could potentially pull up the FET’s
source towards 0.8V when the FET is off. RAMPBUF
blocks this path.
APPLICATIO S I FOR ATIO
WUUU
–
+
–
+
–
+
+
–
V
CC
200mV
GATE
Q1
MASTER
SLAVE1
SLAVE2
C
GATE
1.2V
ON
RAMPBUF
10µA
V
CC
V
CC
V
CC
R
ONB
R
ONA
R
TA1
R
TB1
R
TA2
R
TB2
10µA
RAMP
FB1
TRACK1
0.8V
–
+
FB2
TRACK2
0.8V
R
FA1
R
FB1
DC/DC
R
FA2
2923 F06
R
FB2
DC/DC
1×