UCC3923, UCC3924, UCC3925, UCC3926, UCC3927, UCC3928

HIGHVOLTAGE HOT SWAP POWER MANAGER

SLUS471 – NOVEMBER 2000

5

POST OFFICE BOX 655303

detailed description (continued)

logic command to turn on power-to-load

Three logic inputs enable the load: INSA, INSB, and ON. To turn on the load, INSA and INSB must be lower

than a TTL threshold above –VIN and ON must be more than a TTL threshold above GND.

overvoltage/undervoltage lockout

In addition to the logic inputs INSA, INSB, and ON, two other inputs enable load current: OVLO and UVLO.

Specifically, OVLO must be less than 1.25 V with respect to –VIN and UVLO must be more than 1.25 V with

respect to –VIN to enable load current. These pins can be used as logic inputs or as precision overvoltage

shutdown and undervoltage lockout inputs by connecting these inputs to a voltage divider from GND to –VIN.

enable downstream power converter

The IC has five outputs to enable downstream power converters or loads. These outputs are open-drain

active-low drivers. During power–up, all five outputs are high–impedance. When load current is ramped up to

asserts low.

reference output

REF is a voltage reference output 5 V higher than –VIN. This output is enabled whenever –VIN to GND is

greater than 20 V. This reference can be used as a bias for cascode devices to buffer low-voltage logic outputs.

This reference should not be loaded with more than 50

µA.

programming for downstream load sequencing

These outputs turn on sequentially, with PG1 turning on first, PG2 second, etc. The delay from PG1 to PG2

is the time required to charge the capacitor from SEQTIME to –VIN by 1 V. SEQTIME is pulled high through

a 10-

µA current source, so the delay between output enables is C/10 µA.

negative supply input

–VIN is the negative supply input to the IC. All signals are measured with respect to –VIN except ON. In positive

supply systems, –VIN is the ground input to the IC.