APPLICATION INFORMATION
OVERVIEW
Gate
ON
Gate
OFF
Programmable
FastTurn-off
Threshold
Gnd
Programmable
FastTurn-off
Threshold
Active
Regulation
SlowTurn-off
Range
TPS2413
(SeeText)
TPS2412
(SeeText)
V
+10V
(A)
V
+V
(A)
(T)
V(AC)
V
(G
A
TE)
10mV
V
(G
A
TE)
3mV
10mV
3mV
SLVS728B – JANUARY 2007 – REVISED SEPTEMBER 2008.......................................................................................................................................... www.ti.com
The TPS2412/13 is designed to allow an output ORing in N+1 power supply applications (see
Figure 12), and an
input-power bus ORing in redundant source applications (see
Figure 13). The TPS2412/13 and external
MOSFET emulate a discrete diode to perform this unidirectional power combining function. The advantage to this
emulation is lower forward voltage drop and the ability to tune the operation.
The TPS2412 turns the MOSFET on with a linear control loop that regulates V(AC) to 10 mV as shown in
Figure 11. With the gate low, and V(AC) increasing to 10 mV, the amplifier drives GATE high with all available output current until regulation is reached. The regulator controls V(GATE) to maintain V(AC) at 10 mV as long as the
MOSFET rDS(on) × I(DRAIN) is less than this the regulated voltage. The regulator drives GATE high, turning the
MOSFET fully ON when the rDS(on) × I(DRAIN) exceeds 10 mV; otherwise, V(GATE) will be near V(A) plus the
MOSFET gate threshold voltage. If the external circuits force V(AC) below 10 mV and above the programmed fast
turnoff, GATE is slowly turned off. GATE is rapidly pulled to ground if V(AC) falls to the RSET programmed fast
turn-off threshold.
The TPS2413 turns the MOSFET on and off like a comparator with hysteresis as shown in
Figure 11. GATE is
driven high when V(AC) exceeds 10 mV, and rapidly turned off if V(AC) falls to the RSET programmed fast turn-off
threshold.
System designs should account for the inherent delay between a TPS2412/13 circuit becoming forward biased,
and the MOSFET actually turning ON. The delay is the result of the MOSFET gate capacitance charge from
ground to its threshold voltage by the 290
A gate current. If there are no additional sources holding the ORed
rail voltage up, the MOSFET internal diode will conduct and maintain voltage on the ORed output, but there will
be some voltage droop. This condition is analogous to the power source being ORed in this case. The DC/DC
converter output voltage droops when its load increases from zero to a high value. Load sharing techniques that
keep all ORed sources active solve this condition.
Figure 11. TPS2412/13 Operation
The operation of the two parts is summarized in
Table 2.
Table 2. Operation as a Function of VAC
Turnoff Threshold(1)
≤ VAC ≤ 10 mV
V(AC) ≤ Turnoff Threshold
(1)
V(AC) > 10 mV
(MOSFET
V(AC) Forced < 10 mV
rDS(on) × ILOAD) ≤ 10 mV
Weak GATE pull-down
TPS2412 Strong GATE pull-down (OFF)
V(AC) regulated to 10 mV
GATE pulled high (ON)
(OFF)
TPS2413 Strong GATE pull-down (OFF)
Depends on previous state (Hysteresis region)
GATE pulled high (ON)
(1)
Turnoff threshold is established by the value of RSET.
10
Copyright 2007–2008, Texas Instruments Incorporated