LTC2921/LTC2922 Series
13
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APPLICATIO S I FOR ATIO
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Electronic Circuit Breaker
The LTC2921/LTC2922’s electronic circuit breaker pro-
tects against excessive current on V
CC
. The circuit breaker
trips when the SENSE pin falls more than 50mV below the
V
CC
pin for more than 1µs. When the breaker trips, the
remote sense switches are opened and the PG and GATE
pins are pulled to ground, disconnecting the supplies. An
internal latch guarantees that the monitoring sequence
cannot start until the breaker is reset. To reset the circuit
breaker, cycle the V1 input below 0.5V(nom) for more than
150µs. V
CC
falling below the undervoltage threshold also
resets the breaker. After reset, the sequence returns to
Step 1, awaiting valid monitor levels.
Figure 7 shows an equivalent schematic for the electronic
circuit breaker function. Using Equation 8, set the circuit
breaker by selecting R
SENSE
to drop less than the mini-
mum ∆V
SENSE
at the desired trip current:
R
V
I
SENSE
SENSE MIN
LO TRIP
≤
∆
()
()
(8)
After selecting a resistor, use Equations 9a and 9b to
calculate the actual minimum and maximum trip current
threshold limits:
I
V
R
TRIP MIN
SENSE MIN
SENSE MAX
()
()
()
=
∆
(9a)
I
V
R
TRIP MAX
SENSE MAX
SENSE MIN
()
()
()
=
∆
(9b)
Be mindful of thermal effects and power ratings when
choosing a resistor. Place R
SENSE
as close as possible to
the LTC2921/LTC2922 pins to reduce noise pickup, and
use Kelvin sensing to ensure accurate measurement of the
voltage drop. In applications not requiring the current
sensing circuit breaker, tie the SENSE pin to the V
CC
pin.
Configuring the PG Pin Output
The LTC2921 and LTC2922 each include a power good
indicator, the PG pin. During the turn-on sequence, and
upon detection of errors, a strong FET pulls PG to ground
with >10mA of current. When all supplies have satisfied
their monitor and overvoltage thresholds, the circuit breaker
has not tripped, the GATE pin has reached its peak, and the
remote sense switches have turned on, a 4µA current
source from V
PUMP
pulls up PG.
Configure PG as a logic signal by adding an external pull-
up resistor to a voltage source. For example, create a
negative-logic system reset signal by adding an external
pull-up resistor to the load side of a supply voltage, as in
Figure 8. Calculate the minimum pull-up resistor value that
meets the output low voltage specification for V
PG(OL)
:
R
VV
mA
PG MIN
LO MAX
()
()
.
=
−04
5
(10)
Do not pull PG above the GATE pin’s fully ramped voltage.
An internal clamp limits the PG voltage to ≤12.2V relative
to ground. In applications that do not require the PG
output, leave the pin unconnected.
The PG output can also be used as the gate drive for
external N-channel MOSFETs, as in Figure 9. The delay
between the GATE ramp and the PG activation makes a
supply sequencer, useful when two supplies (or two
groups of supplies) need to be ramped one after another.
Choose the FETs and design the ramp rate in the same way
as for the GATE pin. Refer to Equations 5 and 6, substitut-
ing 4µA for 10µA, to choose capacitor C
PG
.
Integrated Switches for Remote Sensing
A significant feature of the LTC2921/LTC2922 series
is a set of remote sense switches that allow for
compensation of voltage drops in the load path. Switch
activation occurs in the turn-on sequence after the GATE
Figure 7. Circuit Breaker Functional Schematic
GATE
ENABLE
CONTROL
LOGIC
V
PUMP
V
PUMP
V
PUMP
SWITCH
ENABLE
PG
ENABLE
4µA
4µA
PG
GATE
REMOTE
SENSE
SWITCH
GATE
LATCH
V
PULSE
WIDTH
MEAS.
+
+
–
50mV
V
CC
SENSE
R
SENSE
OVERCURRENT
COMPARATOR
V1
V
LO
I
LO
V
SRC0
GND
C
GATE
R
G0
10Ω
2921/22 F07
Q0
LOAD
LTC2922
