AN8037 Panasonic Semiconductor, AN8037 Datasheet - Page 19

• Manufacturer Part Number: AN8037
• Description: AC-DC switching power supply control IC with standby mode
• Manufacturer: Panasonic Semiconductor

Voltage Regulators
[1] Operation descriptions (continued)
10. Output block
11. Timer latch
Application Notes (continued)
15 sinks and sources current to rapidly drive the power MOSFET which is a capacitive load.
Furthermore, this circuit has a sink capability of 1 mA (typical) even when the supply voltage has fallen under the
stop voltage, and thus can turn off the power MOSFET reliably.
That is to say, a high average current is not required in steady state operation. This is because the power MOSFET
is a capacitive load, and while a large peak current is required to drive such a load rapidly, once the load has been
charged or discharged a much smaller current suffices to retain that state.
capacitance of the power MOSFETs that will be used.
MOSFET can cause ringing, and pull down the output
pin below the ground level. If the output pin goes to a
negative voltage that is larger than the voltage drop of the
diode, this state can turn on the parasitic diode formed by
the collector of the output NPN transistor and the sub-
strate. Insert a Schottky barrier diode between the output
and ground if this is a problem. (Refer to figure 15.)
diode or Schottky diode on the secondary side, or the power MOSFET, if the power output is overloaded or short-
circuited for a certain period. If the overcurrent state continues for a certain period or longer time, the timer latch
function makes it possible to interrupt the IC in a certain period by charging the capacitor connected to the SD pin.
output, at which time the pulse-by-pulse overcurrent protection is in operation. The voltage drop in power output is
detected as a decrease in current through the FB current feedback pin (pin 9). If this current is less than a certain value,
the on-chip comparator of the IC is in inverted operation to provide constant current to the SD pin. Refer to figure 16.
operating threshold voltage (i.e., 3.9 V typical), overvoltage protection (OVP) is triggered, and the operation of
the IC is suspended. Refer to figure 17.
This IC adopts a totem pole (push-pull) structure output circuit in which NPN transistors as shown in figure
This circuit provides maximum sink and source currents of 0.1 A (DC), and peak currents of 1.0 A.
The main requirement on the control IC in this type of power supply is the ability to provide a large peak current.
This IC has a guaranteed peak current capability of 1 A, values which were determined by considering the
The parasitic inductance and capacitance of the power
The pulse-by-pulse overcurrent protection function by itself cannot fully protect the transformer, the first recovery
The overload of the power output or short-circuiting of the power output is monitored as a voltage drop in power
Then external capacitor connected to the SD pin is charged and the voltage on the SD pin rises to the SD
V
V
CC
CC
discharging conditions
Latch
Shut-
down
Timer capacitor
U.V.L.O. stop voltage
OVP reset voltage
3.9 V typ.
AN8027, AN8037
V
OVP
OVP
Timer capacitor
discharging transistor
Overcurrent
V
I
detection
SD
OCP
On
V
On
FB
Figure 16
SD
FB
GND
V
CC
The timer period is set according to the capacitance.
PC
Discharging
Input voltage correction: Voltage input is detected
from the flow of current from the TR pin.
I
SD
I
FB
The pin voltage rises by
charging the timer to trigger OVP.
n
I
TR
C
Charging
I
TIM
TIM
70 A, n 8.4
Figure 15
AN8027, AN8037
Schottky barrier diode
D
B
NB
19