Voltage Regulators

AN8038, AN8038S

19

s Application Notes (continued)

[2] Operation descriptions (continued)

5. Output block (continued)

• Operating power supply current characteristics

When the OVP circuit operates and the power supply current drops, this can induce a rise of the supply

Therefore, the circuit is provided with characteristics that cause the supply current to rise in constant

resistance mode when the OVP circuit operates, and thus the increases of the supply voltage.

Due to these characteristics, if the

operates is stabilized at a value (note that

this value depends on the value of the

startup resistor) that is larger than the OVP

release voltage (7.5 V typical), the OVP

circuit will not be reset as long as the AC

input is not cut. (See figure 11.) Note that

this does not apply to an external reset.

Figure 11. OVP operating circuit current

1.7 mA typ.

7.5 V 10 V

18 V

0.53 mA typ.

• Overcurrent protection circuit (pulse-by-pulse overcurrent protection)

This circuit uses the fact that overcurrents in the power supply output are proportional to the current flowing

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

sensitive to excessive current.

The current flowing in the main switch is detected by connecting a resistor between the power MOSFET

source 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

−205 mV with respect to ground at T

every cycle, once 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."

R6 and C6 in figure 12 form a

filter circuit that rejects noise gener-

ated due to the incidental equivalent

parasitic capacitance when the

power MOSFET is on.

For the grounding point, we rec-

ommend that the power MOSFET

source pin and the IC GND pin be

connected over as short a distance

as possible.

• Notes on the detection level precision

This overcurrent detection level is reflected on the operating current level of the power supply overcurrent

protection function. Therefore, if this detection level varies with sample-to-sample variations or with tem-

perature, 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.

Figure 12

CLM

GND

To AC

−

R6

R8 C6

R7

Use resistors with

no inductance.

Use a capacitor

with excellent frequency

characteristics.