AN1696 Datasheet, PDF (5/11 Pages) STMicroelectronics – Load sharing is a technique commonly

AN1696 APPLICATION NOTE
Besides the standard components necessary to perform DC-DC conversion, a charge pump (D3, D4, C12, C13,
R11) has been added to provide a voltage high enough to bring the gate of the OR-ing FET (on the motherboard)
at least one threshold above V_OUT (V_GATE>3.3V+V_GS(TH)); moreover, increasing V_GS voltage allows working
with a lower R_DS(ON) and reducing mosfet conduction losses.
This pump, running whenever PWM activity is present, pushes the gate of the OR-ing fet up to a voltage equal
to:
V_GATE(MAX) = 2 ⋅ V_IN – 2 ⋅ V_F
(1)
(where V_F is the forward drop of the diodes) supplying a current equal to:
I_CH = V_IN ⋅ C₁₂ ⋅ f_SW
(2)
From the daughter board, besides input/output voltages and ground, other signals exit toward the motherboard:
– SOFT-START: the soft-start voltage is brought out and connected to a switch (that can short soft-start
to ground) allowing enable/disable of the relevant section;
– +SOUT and SGND are the sense terminals for positive and negative load terminals; they are con-
nected to the relative power traces through two small resistors to avoid that any kind of open connec-
tion (or the sense pins left open) could cause a lost of control.
2.0 MOTHERBOARD
The motherboard accommodates all the auxiliary circuitry necessary to load sharing, to manage start-up and to
enable/disable each of the three daughter boards; a LED indicates the disabled section.
Terminals are available to connect input DC voltage and load and three series of connectors allow inserting the
daughter boards.
In fig. 4 the structure of one section of the motherboard is showed: replying this structure other two times the
entire board is obtained; table 2 reports the part list.
Figure 4. Motherboard schematic
+ IN
- IN
R3
LED
R5
R1
D1
D2
U2
C1
R4
R2
To other
SECTIONS
Q1
R7
R8
R10
SW 1
C2
R9
C3
R11
R12
C4
C8
C9
OUT
VCC
SS
PGND
PUMP
PGND
+SOUT
SGND
1
8
2
7
U1
3
6
4
5
D3
R13
C7
R14
C6
C5
R15
C10
J P1
+ OUT
- OUT
Common
SHARE BUS
To other
SECTIONS
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