Pin functional descriptions
VCC Supply (pin 4)
Start-up circuit The start-up circuit detects the VCC pin volt-
age, and makes the control IC start and stop operation. The power
supply of the control IC (VCC pin input) employs a circuit as
shown in figure 2. At start-up, C3 is charged through a start-up
resistor R2. The R2 value needs to be set for more than the hold
current of the latch circuit (140 μA max.) and to operate at the
minimum AC input.
If the value of R2 is too high, the C3 charge current will be
reduced. Consequently, it will take longer to reach the Operation-
Start voltage. The VCC pin voltage falls immediately after
the control circuit starts its operation. The voltage drop can be
reduced by increasing C3 capacitance. However, too large a
C3 capacitance will cause an improperly long time to reach the
Operation-Start voltage after the initial power turn on.
In general, SMPS performs its start-up operation properly
with a value of C3 between 4.7 and 47 μF, and R2 between
47 and 150 kΩ for 120 V narrow or universal AC input, and
82 to 330 kΩ for 200 V narrow AC input.
As shown in figure 3, the circuit current is limited to 100 μA max
(VCC = 15 V, and resistor R2 with appropriate high resistance
value for the circuit) until the control circuit starts its operation.
Once the VCC pin voltage reaches 18.2 V, the control circuit
starts its operation by the start-up circuit, and supply current is
increased. Once the VCC pin voltage drops down to lower than
the Operation-Stop voltage 9.7 V, the UVLO circuit operates to
stop the control circuit, and the IC returns to its initial state.
Bias/drive winding After the control circuit starts its operation,
the power supply is operated by rectifying and smoothing the
voltage of the bias winding. Figure 4 shows the start-up voltage
waveform of the VCC pin. The bias winding voltage does not
immediately increase up to the set voltage after the control circuit
starts its operation. That is why the VCC pin voltage starts drop-
ping. The Operation-Stop voltage is set as low as 10.6 V (max),
the bias winding voltage reaches a stabilized voltage before it
drops to the Operation-Stop voltage, and the control circuit conti-
ues its operation. The bias winding voltage, in normal power sup-
ply operation, is set for the voltage across C3 to be higher than
the Operation-Stop voltage, VCC(OFF) , 10.6 V (max.) and lower
than the OVP-operation voltage, VCC(OVP) , 25.5 V (min.).
100 μ A
15V 18. 2 V
Figure 3. VCC pin current versus voltage
Figure 2. External start-up circuit.
Bias Winding Voltage
Figure 4. VCC pin voltage after start-up, capacitor C3 installed
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.