LTC2921/LTC2922 Series

13

29212fa

APPLICATIO S I FOR ATIO

Electronic Circuit Breaker

The LTC2921/LTC2922’s electronic circuit breaker pro-

trips when the SENSE pin falls more than 50mV below 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

150

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

mum

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

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

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 4

µA current

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

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 4

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

SWITCH

ENABLE

PG

ENABLE

4

µA

4

µA

PG

GATE

REMOTE

SENSE

SWITCH

GATE

LATCH

PULSE

WIDTH

MEAS.

+

+

SENSE

OVERCURRENT

COMPARATOR

V1

GND

10

Ω

2921/22 F07

Q0

LOAD

LTC2922