AN1696 datasheet(5/11 Pages) STMICROELECTRONICS

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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 VOUT (VGATE>3.3V+VGS(TH)); moreover, increasing VGS voltage allows working

with a lower RDS(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:

(1)

(where VF is the forward drop of the diodes) supplying a current equal to:

(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

GAT E M AX

()

⋅

–

⋅

⋅⋅

PGND

SGND

+SOUT

OUT

VCC

PGND

+ IN

-IN

R14

R15

JP1

R13

SW 1

R11

R12

LED

C10

+ OUT

- OUT

R10

PUMP

To other

SECTIONS

To other

SECTIONS

Common

SHARE BUS

PGND

SGND

+SOUT

OUT

VCC

PGND

+ IN

-IN

R14

R15

JP1

R13

SW 1

R11

R12

LED

C10

+ OUT

- OUT

R10

PUMP

To other

SECTIONS

To other

SECTIONS

Common

SHARE BUS

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