MB3759-P Fujitsu Microelectronics, Inc., MB3759-P Datasheet - Page 16

• Manufacturer Part Number: MB3759-P
• Description: Pulse-width-modulation Control Circuit Push-pull/single-ended Output Mode
• Manufacturer: Fujitsu Microelectronics, Inc.

16
MB3759
• Initial limit current I
As the diode is reverse biased,
V
if for example the variation in I
in the offset voltage gives the following:
This indicates a setting of 100 mV or more is required.
• Polarity change point I
As this is the point where the diode becomes forward biased, it can be calculated by substituting [R4/(R3+R4)
V
• Final limit current I
The limit current for V
side of R5 are biased.
R3//R4 is the resistance formed by R3 and R4 in parallel (R3R4/(R3 + R4)). When R3//R4 << R5, equation (2)
becomes:
In addition to determining the limit current I
the power is turned on.
• Starter circuit
The figure below shows the case when the starter circuit formed by R3, R4, R5, and D is not present. The output
current I
When V
is positive, the output current is limited to being negative and therefore the output voltage does not rise.
Accordingly, if using a fallback system with a detection resistor inserted in the output, always include a starter
circuit, expect in the cases described later.
The condition for V
IO
REF
is the input offset voltage to the op-amp (-10 mV V
I
O
- V
=
D
R
O
O
1
] for V
S
= 0 such as when the power is turned on, the output current I
after the operation of the current limiting circuit is:
R1
I
R
I
L2
I
O
R
L3 C
S
R1
I
S
I
O
=
L3
L1
I
L3
R1 R2
I
R2
in equation (where V
R1 + R2
L1
=
=
=
=
R1
=
R1 + R2
R1
( V
R
R4R5 V
R
O
R1 + R2
L3
1
1
L1
R1
S
S
O
is:
O
R1
1 (R
= 0 when R2 >> R1 is the point where the voltages on either side of R
(
R4 / (R3 + R4)
L2
V
R
R3 + R4
V
R3R4 R3R5 R4R5
O
S
EE
REF
R4
V
R
V
V
O
3
S
)
O
O
L
// R
1
– V
is to be limited to 10 %, using equation (1) and only considering the variation
V
–
R
R3R5 V
V
V
R
IO
S
4
R3 R4
IO
R
REF
IO
S
) / R
V
R
S
IO
R4
S
( R2
D
·
– V
V
5
is the forward voltage of the diode).
REF
D
(
D
R3 R4
V
Eq. (1) (where R2 >> R1)
) –
R4R5 V
L3
– V
REF
R4
R1
for V
D
V
R
IO
)
S
–
V
O
D
REF
= 0, R3, R4, R5, and diode D also operate as a starter when
V
R
IO
S
V
IO
IO
V
D
+10 mV) and this causes the variation in I
)
V
R
IO
S
(2)
Eq.
O
= -V
I O
/ R
S
and, if the offset voltage V
S
L
and on either
. Accordingly,
IO