Datasheet

SLUU155 – March 2003

Full-Featured –48-V Hot Swap Power Manager

4.5 Fault Timing With the TPS2392/93

Whenever the hot swap controller is limiting current to the load, an on-chip timer is monitoring this operation

against an established time limit. The timeout period is generated by the constant-current charging of a capacitor

at the FLTTIME pin. If current regulation ceases prior to expiration of the timer, the capacitor is discharged, and

normal steady-state operation of the load either starts or resumes. However, if the timer expires, then the pass

FET is turned off, disabling power to the load, and the FAULT

output is asserted.

On the EVM, several capacitor patterns are provided for adding or otherwise modifying the timeout period.

Capacitor C6 is hard-wired to the device FLTTIME pin, and provides a minimum fault timer for the default load.

C4 and C5 can be switched into the circuit via DIP switches SW1–4 and SW1–5, respectively. The EVM ships

from the factory with a 0.1-µF capacitor installed at C4; C5 is not populated for easier subsequent user

modification as required.

If the target application requires fault timing other than provided by the default EVM setup, a new value of timing

capacitor can be calculated from Equation 3. When selecting from the readily available capacitor values for the

Equation 3 result, default to a slightly larger, rather than smaller, capacitor.

FLT

55 t

FLT

3.75

where:

• C

FLT

is the calculated value in microfarads, and

• t

FLT

is the desired timeout period in seconds

4.6 Programming the UVLO and OVLO Thresholds and Hysteresis

The UVLO and OVLO pins can be used to set the circuit undervoltage and overvoltage thresholds (V

and V

respectively). When the input supply is below V

or above V

, the GATE pin is held low, disconnecting power

from the load, and the PG

output is deasserted. When input voltage is within the UV/OV window, the GATE drive

is enabled, assuming all other input conditions are valid for turn-on.

Threshold hysteresis is also externally programmable. Internal current sources are switched to the UVLO and

OVLO pins whenever the corresponding input voltage exceeds the nominal 1.4-V reference. Please refer to the

TPS2392/93 data sheet (TI Literature Number SLUS536) for additional details about the UVLO and OVLO

comparator operation.

On the –48-V hot swap EVM, the V

and V

thresholds are individually programmed via the three-resistor

divider R1, R2, and R3 (refer to Figure 2). The factory-installed resistor values result in the following nominal

voltage thresholds.

(3)