KA7500B: Efficient and Stable PWM Switching Regulator Controller

05 March 2024 14


Ⅰ. What is KA7500B?

Ⅱ. Characteristics of KA7500B

Ⅲ. KA7500B working parts and principle

Ⅳ. Internal block diagram of KA7500B

Ⅴ. Applications of KA7500B

Ⅵ. Can KA7500B and KA7500BD be replaced?

Ⅶ. How to change the KA7500B chip to an adjustable power supply?

Ⅷ. How to judge the quality of KA7500B?



KA7500B and TL494 are actually the same chip, just with different names. It is a switching power supply pulse width modulation (PWM) control chip that is widely used in fields such as power management, lighting drive and drive control. This article will discuss in detail the characteristics, structure and applications of KA7500B. Let's start!



Ⅰ. What is KA7500B?


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The KA7500B serves as a crucial component within the control circuit of PWM switching regulators, operating efficiently across a broad spectrum of switching frequencies ranging from 1kHz to 300kHz. Classified under the category of switching controllers, this electronic device is conveniently packaged in DIP-16 and utilizes a through-hole installation method, ensuring ease of integration into various systems. To maintain optimal functionality, it is imperative for the device to operate within a temperature range of 0°C to 70°C. With a versatile supply voltage spanning from 7V to 42V, the KA7500B accommodates diverse power requirements. Notably, it features two outputs, enhancing its applicability and versatility across a range of applications.


Alternatives and equivalents:

LM78S40CN

LM78S40CN/NOPB

LM78S40N/NOPB

TL494CN



Ⅱ. Characteristics of KA7500B


• Complete PWM control circuit

• Variable duty cycle by dead time control (Pin 4)

• Internal circuit prohibits double pulse at either output

• On-chip oscillator with master or slave operation

• Output control cor push-pull or single ended operation

• Uncommitted output TR for 200mA sink or source current

• Internal regulator provides a stable 5V reference supply trimmed to five percent



Ⅲ. KA7500B working parts and principle


1. Pulse trigger

The pulse flip-flop is turned on when the falling edge of the sawtooth waveform and the comparator output is 1, causing one output terminal of the on-chip transistor to be turned on (in turn) and cut off when the comparator output drops to 0.


2. 5V reference source

KA7500B internally integrates a reference source based on the band gap principle. When the VCC voltage exceeds 7V, the reference source can provide a stable 5V output voltage with an error range of ±100mV. The output pin of the reference source is marked as pin 14 REF.


3. Comparator

The signal output from the operational amplifier (COMP pin) is introduced into the positive input of the comparator inside the chip and compared with the sawtooth wave at the negative input. When the sawtooth wave is higher than the signal at the COMP pin, the comparator outputs 0 and vice versa.


4. Sawtooth wave oscillator

KA7500B has a built-in linear sawtooth wave oscillator, which can generate a sawtooth wave signal from 0.3V to 3V. The frequency of this oscillator can be adjusted by an external resistor (Rt) and a capacitor (Ct). The specific oscillation frequency calculation formula is: f = 1 / (Rt × Ct), where the unit of Rt is ohm (Ω) and the unit of Ct is farad (F). The sawtooth signal can be measured at the Ct pin.


5. Quiet zone time comparator

The dead zone time is set via Dead Time Control pin 4. This pin intervenes in the pulse flip-flop via a comparator to limit the maximum duty cycle. The upper limit of the duty cycle at each end can be set up to 45 percent, but when the operating frequency is higher than 150KHz, the upper limit of the duty cycle is approximately 42 percent (when the DTC pin level is set to 0).


6. Operational amplifier

The KA7500B embeds two single-supply operational amplifiers. The transfer function of an op amp is ft(ni, inv) = A(ni - inv), but its output is limited by the output swing. In common power supply circuits, op amps are often configured for closed-loop operation. However, in a few special cases they may also operate in an open-loop manner, where the input signal is provided externally. A diode is connected to the output of each op amp, and these diodes are connected to the COMP pin and subsequent circuitry (i.e. comparators). This connection method ensures that higher output signals can be passed to the subsequent circuits.



Ⅳ. Internal block diagram of KA7500B


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Ⅴ. Applications of KA7500B


KA7500B is usually applied in the following fields:


• Motor driver: KA7500B can be used to design motor driver circuits, such as DC motor driver, stepping motor driver and so on. By controlling the switching tubes of the motor driver through PWM, the control of motor speed and steering can be realized.


• Power management: The design flexibility and high configurability of the KA7500B enable it to be used in a wide range of power management circuits, including battery charge or discharge management and output voltage regulation of power modules.


• Lighting control: The KA7500B can also be used to design LED dimming control circuits or other types of lighting control circuits. By adjusting the duty cycle of the PWM signal, it can realize the brightness adjustment of LED lights or other types of lighting devices.


• Inverter design: The KA7500B can also be used to design inverter circuits to convert DC power to AC power. By controlling the switching tubes of the inverter through PWM, it can realize the frequency and amplitude control of the output AC power, which is commonly used in circuits such as solar inverters and UPS (uninterruptible power supply).


• Switching power supply design: The KA7500B is commonly used to design control circuits for switching power supplies. By adjusting the duty cycle of the PWM signal, it can effectively control the conduction time of the switching tube, thus realizing the precise regulation and stabilization of the output voltage. According to specific application requirements, it can design different types of switching power supplies, including off-line switching power supplies and DC-DC converters.



Ⅵ. Can KA7500B and KA7500BD be replaced?


No. KA7500B is an IC designed for switching power supply control, mainly used to accurately control the output voltage and current of switching power supply. The A7500BD is also an IC that is usually used in the control circuit of a switching power supply and integrates functions such as an oscillator and an error amplifier circuit. It should be noted that the KA7500B and A7500BD are not interchangeable because of the differences in their functions and characteristics. In practical applications, we should choose the appropriate IC according to the specific needs and circuit design requirements.



Ⅶ. How to change the KA7500B chip to an adjustable power supply?


The adjustable methods are as follows:


1. We short the only green wire on the main board interface of the power supply to any black wire, and then energize and turn on the switch to start the power supply. At this time, the power supply can output +12V (yellow wire), +5V (red wire) and +3.3V (orange wire) voltage. It is worth noting that all black wires are used as ground wires. In addition, by matching the yellow and red wires, it can also output +7V (other matching methods can be selected according to demand).


2. The number of output wires will vary for differently positioned PC power supplies, but what they all have in common is the use of these nine distinctive colors of wire: yellow, red, orange, purple, blue, white, gray, green, and black. Standard PC power supplies usually include these nine colors of wires (although current mainstream power supplies have omitted the white wire).



Ⅷ. How to judge the quality of KA7500B?


1. We use the 1K setting of the multimeter, connect the red pen to pin 4 of KA7500B, and connect the black pen to pin 13. Under normal circumstances, the measured DC resistance value should be around 50K. Then, we used the 1K setting of the multimeter again. This time, we connected the red pen to pin 13 of KA7500B and the black pen to pin 4. At this time, the measured DC resistance value should be above 5M, or it should be displayed as infinity.


2. We use the 1K setting of the multimeter, connect the red pen to pin 12 of KA7500B, and connect the black pen to pin 7. At this time, the measured DC resistance value should be between 8K and 9K. Then, we use the 1K setting of the multimeter again, connect the red pen to pin 7 of KA7500B, and connect the black pen to pin 12. The measured DC resistance value at this time should be around 30K. These measurements show that the resistance values between the corresponding pins of the KA7500B are within the normal range.


3. The simplest way to test KA7500B is to use the 1K range of a multimeter. We connect the black pen to pin 7 of KA7500B and the red pen to pins 11 and 12 respectively. The measured DC resistance value at this time should be between 8K and 9K. We then connect the red pen to pin 14. The DC resistance value should be between 5K and 6K. If the measured results are within the above range, we can judge that the KA7500B integrated circuit is basically normal. If the resistor value is less than 2K, we can be sure it is damaged. These DC resistance values are derived from multiple test comparisons of multiple normal and damaged KA7500B integrated circuits.




Frequently Asked Questions


1. What is the KA7500B used for?


The KA7500B is used for the control circuit of the PWM switching regulator.


2. What are the key features of the KA7500B?


The KA7500B features an internal regulator that provides a stable 5V reference voltage, precise timing and duty cycle control through its oscillator, and built-in error amplifier for voltage regulation.


3. What is the voltage of KA7500B?


The KA7500B consists of 5V reference voltage circuit, two error amplifiers, a flip flop, an output control circuit, a PWM comparator, a dead time comparator and an oscillator. This device can be operated in the switching frequency of 1kHz to 300kHz.




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