LTC2921/LTC2922 Series

10

29212fa

The timing of a circuit breaker trip and reset, and a

subsequent regular turn-on are shown in Figure 4. Prior to

time 1, a successful turn-on sequence had completed. At

time 1, excessive current pulls SENSE more than 50mV

switches fall at rates determined by the pull-down currents

and loading conditions of each (times 2, 3, 4). Note that the

excessive current condition ceases at time 4. A circuit

breaker reset pulse is initiated at time 5. The latch resets

at time 6 since the V1 pulse is wide enough. A normal turn-

on begins when V1 rises above the monitor threshold

(time 7 onward).

Multiple supply systems have become common to accom-

modate circuits on the same board with different voltage

requirements. Desktop PC motherboards, instrumenta-

tion circuits and plug-in boards of all kinds often require

tracking and control of several supply voltages.

The LTC2921 and LTC2922 ramp and monitor up to five

supply voltages in such systems. External resistive volt-

age dividers independently program four monitor levels,

level. Time delays in the monitoring sequence are set by an

external capacitor at the TIMER pin.

The GATE pin provides a high side drive voltage appropri-

ate to logic-level and sublogic-level N-channel power

TI I G DIAGRA S

V1

V2

V3

V4

0.7V

0.5V

0.7V

0.5V

0.7V

0.5V

0.7V

0.5V

1.2V

1.2V

12 3 4

6 7

5

SENSE

TIMER

GATE

PG

REMOTE

SENSE

SWITCH

GATE

INTERNAL

CIRCUIT

BREAKER

LATCH

UNDERVOLTAGE

LOCKOUT LEVEL

APPLICATIO S I FOR ATIO

Figure 4. Circuit Breaker Trip, Reset and Start-Up Sequence Timing

MOSFETs. The external RC network on GATE programs

the supply ramp rate and eliminates possible high fre-

quency oscillations in the power path. Featured in the

LTC2921/LTC2922 series are sub-10

Ω internal remote

sense switches to compensate for voltage drops between

the supplies and the loads.

At the end of a successful power-on sequence, the LTC2921/

LTC2922 asserts the PG output. A typical application uses

an external pull-up resistor between PG and the load side

of a supply. In applications where supply power-on se-

quencing is required, the PG pin can function as a second,

separate high side driver.