Datasheet

25 V
Gnd
10 mV
- 3 mV
V
GATE
12xHP = 0
V
OR
25 V
Gnd
10 mV
3 mV
V
GATE
12xHP = 1
V
OR
10
LOAD
SENSE DSonPASS DSonBLK
mV
I
R R R
>
+ +
TPS2359
SLUS792H –FEBRUARY 2008–REVISED MAY 2013 www.ti.com
12-V ORing Operation for Redundant Systems
The 12-V channels use external pass FETs to provide reverse current blocking. The TPS2359 pulls the BLKx pin
high when the input-to-output differential voltage IN12x-OUT12x exceeds a nominal value of 10 mV, and it pulls
the pin low when this differential falls below a nominal value of -3 mV. These thresholds provide a nominal 13
mV of hysteresis to help prevent false triggering.
The source of the blocking FET connects to the source of the pass FET, and the drain of the blocking FET
connects to the load. This orients the body diode of the blocking FET such that it conducts forward current and
blocks reverse current. The body diode of the blocking FET does not normally conduct current because the FET
turns on when the voltage differential across it exceeds 10 mV.
Applications that do not use the blocking FET should clear the associated 12xOR bit to turn off the internal
circuitry that drives the BLKx pin.
12-V ORing for High-Power Loads
The 12AHP and 12BHP bits adjust the ORing turn-off thresholds of the 12A and 12B channels, respectively.
Clearing these bits sets the ORing turn-off thresholds to the default nominal value of -3 mV. Setting these bits
shifts the thresholds up by 6 mV to a nominal value of +3 mV (Figure 29). Shifting the turn-off threshold to a
postive value ensures that the blocking FET shuts off before any reverse current flows.
A light load may not draw sufficient current to keep the input-to-output differential VIN12x–OUT12x above 3 mV.
When this happens, the blocking FET shuts off and then the differential voltage increases until it turns back on.
This process endlessly repeats, wasting power and generating noise. Therefore 12AHP or 12BHP should only be
set for high-power loads that satisfy the relationship
(8)
where I
LOAD
equals the current drawn by the load, R
SENSE
equals the value of the sense resistor, R
DS(on)PASS
equals the maximum on-resistance of the pass FET, and R
DSo(n)BLK
equals the maximum on-resistance of the
blocking FET.
Example: If R
SENSE
= R
HSFET
= R
ORFET
= 5 mΩ then a high-power load must always draw at least 667 mA. Most,
although not all, AdvancedMC™ loads can benefit from using the high-power bits 12AHP and 12BHP.
Figure 29. 12-V ORing Thresholds - High Power vs. Low Power
38 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: TPS2359