AN1696 APPLICATION NOTE

8/11

Figure 5. Adjustment network

2.3 Gate driver of OR-ing FET

The OR-ing FET gate is driven by the output of a comparator (one section of an LM293, standard double com-

parator) sensing the voltage upstream (source) and downstream (drain) the OR-ing FET (see fig. 6). This turns-

off the transistor whenever a current tries flowing from the load towards the output stage of a daughter board.

Whenever one section is enabled with the two others already running (hot plug), its output capacitor is com-

the FET is turned on.

Moreover the load must be isolated from a failed section; for example in case of a short circuit on the low side

power MOS of one section, then a current will tend to flow in reverse direction through the corresponding OR-

ing FET.

Figure 6. OR-ing FET management

V

R

A

ADJ

R

13

+

-

I

ADJ

L6615

MOTHERBOARD

DAUGTHERBOARD

L6910

V

FB

R

8

R

6

R

7

C

10

V

R

A

ADJ

R

13

+

-

I

ADJ

L6615

MOTHERBOARD

DAUGTHERBOARD

L6910

V

FB

R

8

R

6

R

7

C

10

V

CO UT

() ON

–

V

REF

1

R5

R7

--------

+





⋅

V

LO AD

R8

R8

R9

+

----------------------





1

R5

R7

--------

⋅





⋅⋅

==

L

O

A

D

-

+

R8

R9

C2

R7

DAUGHTERBOARD

MOTHERBOARD

R5

V

C(OUT)

V

LOAD

V

REF

V

PS+

V

GATE

V

REF

V

C(OUT)

V

PS+

V

GATE

∆V

HYST

V

PUMP

V

C(OUT)-ON

V

C(OUT)-OFF

V

LOAD

SECTIONS

ALREADY

RUNNING

R11

V

PUMP

PUMP

CHARGE

L

O

A

D

L

O

A

D

-

+

R8

R9

C2

R7

DAUGHTERBOARD

MOTHERBOARD

R5

V

C(OUT)

V

LOAD

V

REF

V

PS+

V

GATE

V

REF

V

C(OUT)

V

PS+

V

GATE

∆V

HYST

V

PUMP

V

C(OUT)-ON

V

C(OUT)-OFF

V

LOAD

SECTIONS

ALREADY

RUNNING

R11

V

PUMP

PUMP

CHARGE