9

Voltage Regulators

AN8015SH

s Application Notes (continued)

[3] Function descriptions

1. Reference voltage block

This block is composed of the band gap circuit, and outputs the temperature compensated 2.46 V reference

and used as the operating power supply for the IC inside. It is possible to take out a load current of up to

−3 mA .

Also, an overcurrent protection circuit is built in for the load, thereby protecting the IC from destruction when

2. Triangular wave oscillation block

The triangular wave which swings from the highest wave of approximately 1.4 V to the lowest wave of

approximately 0.37 V will be generated by connecting a timing capacitor and a resistor to the CT terminal (pin 1) and

connected externally.

The triangular wave is connected with the inverted input of PWM comparator of the IC inside.

3. Error amplifier block

This block detects the output voltage of DC-DC converter by the PNP transistor input type error amplifier, and

inputs the amplified signal to the PWM comparator.

The common-mode input voltage range is

− 0.1 V to 0.8 V, and is a voltage obtained by dividing the reference

voltage with resistors.

Also, it is possible to perform the gain setting and the phase compensation arbitrarily by inserting the feedback

resistor and capacitor between the error amplifier output terminal (pin 10) and the inverted input terminal (pin 9).

connecting to the inverted input terminal as shown in figure 3. 2). The output voltages in each of these cases are

given in the following equations.

R1

R2

7

PWM comparator

input

Error amp.

IN

+ 8

IN

− 9

Figure 3. Connection method of error amplifier

1) If output is positive

R3

R4

2) If output is negative

R3

+ R4

R3

×

R1

+ R2

R2

×

R4

R3

+ R4

×

R2

R1

+ R2

R1

R2

7

PWM comparator

input

Error amp.

IN

+ 8

IN

− 9

R3

R4

4. Timer latch short-circuit protection circuit

This circuit protects the external main switching devices, flywheel diodes, and choke coils, etc. from

destruction or deterioration if overload or short-circuit of power supply output lasts for a certain time.

The timer latch short-circuit protection circuit detects the output level of the error amplifier. When the output

voltage of DC-DC converter drops and the output level of error amplifiers exceeds 1.85 V, the low-level output is

given and the timer circuit is actuated to start the charge of the external protection enable capacitor.

If the output of the error amplifier does not return to a normal voltage range by the time when the voltage of

this capacitor reaches 1.83 V, it sets the latch circuit, cuts off the output drive transistor, and sets the dead-time at

100%.