MB3759 Datasheet, PDF (16/27 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Switchable between push-pull and single-end functions)

MB3759
• Initial limit current IL1
VO >
R4
R3 + R4
VREF
The condition for VO is:
As the diode is reverse biased
RS
IL1
=
R1
R1 + R2
VO
–
VIO
∴IL1 = ^{R1 VO} – ^VIO
R1 + R2 RS RS
Eq. (1) (where R2 >> R1)
VIO is the input offset voltage to the op-amp (-10 mV ≤ VIO ≤ +10 mV) and this causes the variation in IL. Accordingly,
if for example the variation in IL is to be limited to ±10 %, using equation (1) and only considering the variation
in the offset voltage gives the following:
IO
=
1
RS
R1
R1 + R2
(
VO
+
VEE
)
−
VIO
RS
( R2 >> R1 )
This indicates a setting of 100 mV or more is required.
• Polarity change point IL2
As this is the point where the diode becomes forward biased, it can be calculated by substituting [R4/(R3+R4)
VREF - VD] for VO in equation (where VD is the forward voltage of the diode).
IL2 =
R1 R4 / (R3 + R4) · VREF – VD
R1 + R2
RS
–
VIO
RS
• Final limit current IL3
The limit current for VO = 0 when R2 >> R1 is the point where the voltages on either side of RS and on either
side of R5 are biased.
RS
IL3
=
R4R5 VREF − R3R5 VD − R4R5 VD
R3R4 + R3R5 + R4R5
−
VIO
∴IL3 =
1
RS
1
( ^R4 VREF − VD ) −
1 + (R 3 // R 4) / R5 R3 + R4
VIO
RS
(2) Eq.
R3//R4 is the resistance formed by R3 and R4 in parallel (R3R4/(R3 + R4)). When R3//R4 << R5, equation (2)
becomes:
IL3 C =
1 ₍ R4 _{VREF – VD) –}
RS R3 + R4
VIO
RS
In addition to determining the limit current IL3 for VO = 0, R3, R4, R5, and diode D also operate as a starter when
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 IO after the operation of the current limiting circuit is:
IO
=
R1
R1 + R2
VO
RS
−
VIO
RS
When VO = 0 such as when the power is turned on, the output current IO = -VI O / RS and, if the offset voltage VIO
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.
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