AN8038S Panasonic Semiconductor, AN8038S Datasheet - Page 21

• Manufacturer Part Number: AN8038S
• Description: AC-DC switching power supply control IC with RCC local resonance circuit for improved conformance with energy conservation laws
• Manufacturer: Panasonic Semiconductor

Voltage Regulators
[2] Operation descriptions (continued)
6. Notes on feedback control
Application Notes (continued)
fail or the output oscillation may become unstable.
applied to their pins. (Except for special applications.) This is because parasitic device operations may be induced
when negative voltages are applied due to the structure of ICs themselves.
(V
source voltage, V
high voltage. The voltage chattering that occurs at this
time is superposed on V
pacitance C
drain, and generates a negative voltage with respect
to the pin. No problems occur if the peak voltage, VEX,
of this negative voltage does not exceed the parasitic
device conduction voltage (about
for higher input voltages and for larger leakage in-
ductance in the transformer used. Also, the influence
of this phenomenon becomes more noticeable for the
larger C
peak value also increases. If the parasitic device con-
duction voltage is exceeded, then, in this IC, the parts
of the circuit around the feedback circuit (FB) (in par-
ticular, the FB discharge circuit) are influenced. This
can cause momentary drops in the IFB pin voltage
(the control voltage), and as a result increase the FB
current (I
on-period t
stabilization of the circuit. These are symptoms of the
case described here. (See figures 15 and 16.)
[Countermeasures]
described above, or similar symptoms, first insert a
Schottky diode between V
possible to completely remove the mechanism described above from a power supply system. It is also not possible
to prevent levels from being pulled down to negative voltages in the control IC itself. Therefore, the most impor-
tant point in designing countermeasures is to prevent such negative voltages from reaching the parasitic device
conduction voltage.
Note) If a Schottky diode is added to the circuit and the condition improves initially but the symptoms reappear when the
OUT
If the IC output pin (pin 7) falls to a negative voltage lower than that of the GND pin, the startup operation may
ICs in general, not just this IC, do not respond well when negative voltages lower than the ground level are
In the case mentioned above, when the IC output
However, the amplitude of the chattering is larger
If an application exhibits the symptoms of the case
input voltage or other parameter is increased, try replacing the Schottky diode with one that has a larger forward current
(both peak and average values). The current capability of the Schottky diode is sometime insufficient.
Reference: Panasonic Schottky Diode
) is turned off, the power MOSFET drain-to-
Note) : Former part number
MA2C700A (MA700A )
MA2C723 (MA723 )
MA2C719 (MA719 )
GD
IFB
of the power MOSFET used, and the V
GD
ON
) and thus does not allow the drive pulse
between the power MOSFET gate and
to be increased. It may also prevent
Part No.
DS
, jumps from a low voltage to a
OUT
OUT
through the parasitic ca-
and GND. It is not
Reverse voltage
0.7 V).
30 V
30 V
40 V
EX
Forward current (average)
V
V
I
OUT
FB
DS
200 mA
500 mA
30 mA
Rectified AC
Out
Figure 16
Forward current (peak)
Figure 15
AN8038, AN8038S
150 mA
300 mA
1 A
V
C
EX
GD
GND
Control
pin
V
21
DS