MD3221N 查看數據表（PDF） - Shindengen
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MD3221N Datasheet PDF : 26 Pages
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◆Short-circuit Protection
◆The Overheat Protection Function
The short-circuit protection function described above
operates when the output pin is completely short-
circuited, or if power is switched on in the short-
circuited condition, however as an extremely high
current flows in the converter the latch circuit may not
operate due to noise resulting from this short-circuit
current. A protection circuit independent of the
MD3221N should therefore be provided to
accommodate such cases (refer to '20. External
Protection Circuits').
The MD3221N incorporates an overheat protection
function. The oscillator is halted when the junction
temperature reaches 140℃ due to operation under
adverse conditions. The oscillator begins operation
again when the temperature drops to 110℃. A reset
signal is not required.
◆Error Amp Gain Adjustment
◆The Current Limiting Function
(external resistance detection)
In addition to over-current protection using the timer
latch, the MD3221N incorporates a function to limit
output current. Connection of an external resistor
facilitates incorporation of droop characteristics for the
output at any desired current value. Limiting the
maximum output power simplifies start-up when the
current supply capacity of the input power supply is low.
As shown in the diagram, the voltage drop across the
inserted resistor (R2) is detected at 100mV.
The timer latch is disabled when droop characteristics
are incorporated in the output using this function. When
the impedance at the load side approaches zero,
current increases and the timer latch operates to
protect the circuit. The OCL+ and OCL- should be
shorted if this function is not used.
Error amp gain adjustment is effective in ensuring
stable operation of the power supply circuit and good
transient response. As the appropriate constant
varies with the components used (eg output
condenser), the MD3221N has an external error amp
input / output pin for adjustment following selection
of the primary components.
● Error Amp Gain Adjustment Circuit
28
ampOUT
C6
R3
MD3221N
29
amp-
● Circuit for Current Limiting Function (external resistance detection)
VOUT
L1
11~14
OCL+
MD3221N 25
−
＋
27
OCL-
R2
+V0
◆Bootstrap
The MD3221N employs an N-ch MOSFET in the
high-side switch. The load supply circuit is
bootstrapped to the gate of this MOSFET. A
condenser (C3) is connected across the VB and VOUT
pins as part of the power supply.
The capacitance of C3 must be sufficient in relation
to the capacitance of the MOSFET gate. Use a
ceramic condenser of approximately 0.1μF.
13
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