TL3845P PWM Controller: Symbol, Features and Layout Guidelines
09 November 2023
Ⅱ. Overview of TL3845P PWM controller
Ⅲ. Symbol, footprint and pin configuration of TL3845P
Ⅳ. Features of TL3845P PWM controller
Ⅴ. Technical parameters of TL3845P PWM controller
Ⅵ. Working principle of TL3845P PWM controller
Ⅶ. Applications of TL3845P PWM controller
Ⅷ. Layout guidelines for TL3845P PWM controller
Pulse width modulation (PWM) refers to the use of the digital output of a microprocessor to control analog circuits. It is a method of digitally encoding analog signal levels that is used in many areas from measurement and communications to power control and conversion. Controlling analog circuits digitally can significantly reduce system cost and power consumption. Many microcontrollers contain built-in PWM controllers.
One advantage of PWM is that the signals from the processor to the controlled system are all in digital form, without the need for digital-to-analog conversion. Keeping the signal in digital form minimizes the effects of noise. Noise can have an impact on a digital signal only if it is strong enough to change a logic 1 to a logic 0 or a logic 0 to a logic 1. Another advantage of PWM is its enhanced immunity to noise, which is the main reason why PWM is used for communication in some cases. Converting analog signals to PWM can greatly extend the communication distance. At the receiving end, through an appropriate RC or LC network, the modulated high-frequency square wave can be filtered out and the signal restored to its analog form.
TL3845P is a PWM controller. In a switching power supply, when the input voltage at the COMP terminal changes, it will adjust the frequency of the switching signal accordingly. Through this function, it can keep the output voltage of the switching power supply stable.
Some of the circuitry implemented internally is low voltage lockout (UVLO), which picks up less than 1mA of current and has a precision reference voltage to ensure accuracy at the error amplifier input. Other internal circuitry includes logic to enable latching operation, a pulse-width modulation comparator that also provides current limit control, and a totem-pole output stage designed to source or sink high peak currents. Its single PWM output can produce 200 mA. Its switching frequency is 500 kHz. The device has a maximum voltage of 30 V. This current mode controller has a minimum operating temperature of 0°C and a maximum operating temperature of 70°C.
Replacements and equivalents:
• TL3843P
• TL3844P
• Error amplifier with low output resistance
• With hysteretic undervoltage lockout function
• With double pulse suppression function
• With low startup current (less than 1mA) and automatic feed-forward compensation function
• Features high current totem pole output
• Features pulse-by-pulse current limiting and enhanced load response characteristics
• Optimized for offline and DC-DC converters
When the input voltage is applied to the VCC pin of TL3845P, the chip starts working. First, the error amplifier compares the feedback voltage with the reference voltage to generate an error signal. This error signal is then amplified and fed into the PWM comparator. The PWM comparator compares the error signal with the triangle wave to generate a PWM signal. The duty cycle of this PWM signal is proportional to the input voltage. The PWM control logic will control the switching time and off time of the switch tube based on this PWM signal, thereby controlling the output voltage.
During the working process, TL3845P will also limit the current according to the output current. When the output current is too large, the current limiting logic will limit the conduction time of the switch tube to protect the switch tube from being damaged by overload.
• Power supply systems in industrial equipment, communication equipment and other fields
• Power supply systems for consumer electronics such as computers, mobile phones, and televisions
• Isolated flyback power controller: used to provide constant voltage (CV) and constant current (CC) output regulation to improve transient response to larger load steps
We always try to use a low EMl inductor with a ferrite type closed core. Some examples would be toroid andencased E core inductors. Open core can be used if they have low EMI characteristics and are located a bit more away from the low power traces and components . Make the poles perpendicular to the PCB as well if using an open core. Stick cores usually emit the most unwanted noise.
1. Compensation components
External compensation components for stability should also be placed dose to the lC. Surface mount components are recommended here as well for the same reasons discussed for the filter capacitors. These should not be located very close to the inductor either.
2. Feedback traces
Try to run the feedback trace as far from the inductor and noisy power races as possible. Also, keep the feedback trace to be as direct as possible and somewhat thick. These two sometimes involve a trade-off, but keeping it away from inductor EMI and other noise sources is the more critical of the two. Run the feedback trace on the side of the PCB opposite of the inductor with a ground plane separating the two.
3. Input/output capacitors
When using a low value ceramic input flter capactor, it should be located as dose to the Vcc pin of the IC as possible. This will eliminate as much trace inductance effecs as possible and give the intermal lC rail a cleaner voltage supply. Some designs require the use of a feed-forward capacitor connected from the output to the feedback pin as well, usually for stability reasons. In this case it should also be positioned as close to the IC as possible. Using surface mount capacitors also reduces lead length and lessens the chance of noise coupling into the effective antenna created by through-hole components.
4. Traces and ground planes
Make all of the power (high current) traces as short, direct, and thick as possible. lt is good practice on a standard PCB board to make the traces an absolute minimum of 15 mils (0.381 mm) per Ampere. The inductor, output capacitors, and output diode should be as close to each other possible. This helps reduce the EMl radiated by the power traces due to the high switching currents through them. This will also reduce lead inductance and resistance as well, which in turn reduces noise spikes , ringing, and resistive losses that produce voltage errors.
The grounds of the lC, input capacitors, output capacitors, and output diode (if applicable) should be connected close together directly to a ground plane. it would also be a good idea to have a ground plane on both sides of the PCB. This will reduce noise as well by reducing ground loop ermors as well as by absorbing more of the EMI radiated by the inductor. For mult-layer boards with more than two layers, a ground plane can be used to separate the power plane (where the power traces and components are) and the signal plane (where the feedback and compensation and components are) for improved performance. On multi-layer boards the use of vias will be required to connect traces and different planes. lt is good practice to use one standard via per 20omA of current if the trace will need to conduct a significant amount of current from one plane to the other.
Arrange the components so that the switching current loops curl in the same direction. Due to the way switching regulators operate, there are two power states. One state when the switch is on and one when the swtch is off. During each state there will be a current loop made by the power components that are currently conducting. Place the power components so that during each of the two states the current loop is conducting in the same direction. This prevents magnetic field reversal caused by the traces between the two half-cycles and reduces radiated EMI.
Frequently Asked Questions
1. What are the typical applications of the TL3845P IC?
The TL3845P is commonly used in applications such as DC-DC converters, voltage regulators, and power supplies for various electronic devices and systems.
2. What is the replacement and equivalent of TL3845P?
You can replace the TL3845P with the TL3843P, TL3844P or TL3845PE4.
3. What is the TL3845P IC used for?
The TL3845P is used as a voltage mode PWM (Pulse Width Modulation) controller IC. It is commonly used in power supply and voltage regulation applications.
4. What protections or features does the TL3845P provide?
The TL3845P provides features like overcurrent protection, overvoltage protection, undervoltage lockout, and soft start to ensure safe and stable operation of power supply circuits.
