LTC2921CGN データシートの表示（PDF） - Linear Technology

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LTC2921CGN

Power Supply Tracker with Input Monitors

Linear Technology 

LTC2921/LTC2922 Series

APPLICATIO S I FOR ATIO

Electronic Circuit Breaker

The LTC2921/LTC2922’s electronic circuit breaker pro-

tects against excessive current on VCC. The circuit breaker

trips when the SENSE pin falls more than 50mV below the

VCC 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. VCC 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 RSENSE to drop less than the mini-

mum ∆VSENSE at the desired trip current:

RSENSE

≤

∆VSENSE(MIN)

ILO(TRIP)

(8)

After selecting a resistor, use Equations 9a and 9b to

calculate the actual minimum and maximum trip current

threshold limits:

ITRIP(MIN)

=

∆VSENSE(MIN)

RSENSE(MAX)

(9a)

ITRIP(MAX)

=

∆VSENSE(MAX)

RSENSE(MIN)

(9b)

VSRC0

RSENSE

Q0

VLO

VCC SENSE

–+ 50mV

OVERCURRENT

COMPARATOR

LATCH CONTROL

LOGIC

VPUMP

GATE

ENABLE

SWITCH

ENABLE

4µA

GATE

REMOTE

VPUMP SENSE

SWITCH

GATE

VPUMP

RG0

10Ω

CGATE

4µA

PG

ENABLE

PG

V1 PULSE

WIDTH

GND

MEAS.

LTC2922

ILO

LOAD

2921/22 F07

Figure 7. Circuit Breaker Functional Schematic

Be mindful of thermal effects and power ratings when

choosing a resistor. Place RSENSE 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 VCC 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 VPUMP 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 VPG(OL):

RPG(MIN)

=

VLO(MAX) −

5mA

0.4V

(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 CPG.

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

29212fa

13

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