an8027 Panasonic Corporation of North America, an8027 Datasheet - Page 25

• Manufacturer Part Number: an8027
• Description: Ac-dc Switching Power Supply Control Ic With Standby Mode
• Manufacturer: Panasonic Corporation of North America

Voltage Regulators
[1] Operation descriptions (continued)
15. Overcurrent protection circuit (pulse-by-pulse overcurrent protection)
16. Overcurrent protection circuit (input voltage correction function)
Application Notes (continued)
in the primary side main switch (power MOSFET). This circuit limits overcurrents in the power supply output by
constraining the upper limit of the pulse current flowing in the main switch, and thus protects components sensi-
tive to excessive current.
and ground and monitoring the voltage that appears across that resistor. When the power MOSFET is turned on
and the CLM (current limit) threshold voltage is detected, the output is turned off. This controls the circuit so that
a current in excess of that limit cannot flow by turning off the power MOSFET. The CLM threshold voltage is
about 0.2 V typical with respect to ground at T
an overcurrent is detected, the off state is held for the remainder of that cycle, and the circuit is not turned on until
the next period. This type of overcurrent detection
is called "pulse-by-pulse overcurrent detection."
rejects noise generated due to the incidental
equivalent parasitic capacitance when the power
MOSFET is on.
the power MOSFET source pin and the IC GND
pin be connected over as short a distance as pos-
sible.
protection function. Therefore, if this detection level varies with sample-to-sample variations or with temperature,
the operating current level of the overcurrent protection function of the power supply itself will vary. Since
variations in this level imply a need for increased ruggedness in parts used, or even the destruction of circuit
components, we have increased the precision of this IC as much as possible.
protection operating point is held fixed with respect to variations in the input voltage. This circuit uses the propor-
tional relationship between the input voltage and the inverted voltage of the bias winding, and superposes inverted
voltage of the bias winding on the overcurrent protection operating voltage. (Refer to figures 27 and 28.)
This circuit uses the fact that overcurrents in the power supply output are proportional to the current flowing
The current flowing in the main switch is detected by connecting a resistor between the power MOSFET source
R6 and C6 in figure 26 form a filter circuit that
For the grounding point, we recommend that
Notes on the detection level precision
This overcurrent detection level is reflected on the operating current level of the power supply overcurrent
As an extended application, this section presents a circuit design that applies a correction so that the overcurrent
E
E
IN
IN
V
CLM
CLM
AN8027
AN8037
GND
V
Out
TR
CC
Determines
the amount of correction
Figure 27
a
25 C. While this control operation is repeated every cycle, once
To AC
CLM
GND
R
rs
Input voltage
D
Z
correction
V
rs
rs
R6
B
R8
FB voltage applied; Shutdown
FF voltage applied; Correction operation
Determines the input voltage
at which correction starts.
Figure 26
I
DS
C6
R7
AN8027, AN8037
Use a capacitor with
excellent frequency
characteristics.
Use resistor with
no inductance.
25