"RTQ6363 is a industrial grade BUCK converter having a working voltage range of 4.5V to 60V. The minimum input voltage can be low to 4V, which is 3.5A, and the output voltage can be set between 0.8V ~ Vin. It uses The peak current mode control architecture can be set with an external resistor between 100 kHz to 2.5 MHz, and also supports synchronization operation with external clock signal control, and the frequency range of the external clock signal can be selected between 200 kHz to 2.2 MHz.
RTQ6363 can be supplied in two packages, one is PSOP-8, one is a 4mm x 4mm DFN-10L, the latter has adjustable soft start capability and PGOOD instruction signal due to more pins, and also Having higher heat dissipation, but the occupancy area is smaller, compared to the needs of applications with high timing control requirements.
In order to help users better use RTQ6363, one of my European colleagues recently wrote an application notes AN063, which cited two EXCEL design tools designed for RT (Q) 63xx series product design. The complete design and test verification of the case, and also gives the selection rules of the relevant components, comparison and verification of different results from the different options of key components, is a very reference value, And I have completed the translation of it, soon, it will be pushed to the reader who subscribes to the Rongu Electronic News by email.
When I completed the RTQ6363 specification after completing the AN063, I felt that the application instructions of its specifications were also written very well, so I would like to translate it to the use of our readers. The following is this translation. result. Since the writing method of the specification is not suitable to be presented in the form of a general article, I have some small modifications when I translate, I hope this can be understood by the reader. It is also considering that there are many content, it is not suitable for once, so it will be released in the form of serialization, please continue to pay attention.
Application note of RTQ6363
RTQ6363 model definition and pin layout
RTQ6363 internal circuit diagram
Typical application circuit of RTQ6363
The above-mentioned application circuits are given above. The choice of external components in each circuit is mainly determined by application requirements. The first step in the design is to determine their switch switching through the selection of external resistors RRT / SYNC. The frequency, then the inductance L, the input capacitor Cin, the output capacitor Cout, and the selection of the flow diode, and then the desired output voltage and crossover frequency are set by the selection of the feedback resistance, the compensation circuit parameter. The choice of self-lifting capacitance CBoot is relatively simple, enabling the peripheral components involved in the terminal EN, the soft start SS, PGOOD, and synchronous and other terminals can be selected according to the specific application needs, usually there will be no more problems.
Switch to switch change operation frequency setting
The switch switching operating frequency of the RTQ6363 is adjustable, and it can be set to the external resistance of the RRT / SYNC, and the set range is 100 kHz to 2.5 MHz. The difficulty of working frequency selection is to trade out in terms of conversion efficiency and component size, the higher operating frequency allows for smaller inductance and capacitance, while lower operating frequencies reduce the gate capacitance of internal switches. And the loss of the switching process, but also requires larger inductance and / or capacitance to maintain lower input voltage ripple.
The shortest conduction time and the shortest deadline of the switch will cause certain limits to the selection of the operating frequency. The shortest conduction time ton_min is the shorter time length of the upper bridge switch in the on state, and the typical value of this parameter of RTQ6363 is 100ns. In current continuous mode of operation, the highest operating frequency fsw_max is limited by the shortest conduction time, and the relationship between them is:
The VIN_MAX is the largest possible input voltage.
The shortest deadline TOFF_MIN is the shortest time required for the RTQ6363 internal current comparator to restore the normal state and enables the MOSFET on the bridge switch to the shortest time required, this parameter is 130ns. If the working frequency is constant, the actual deadline is longer than the shortest deadline. The following shortest deadline calculation formula is the result of considering various loss items:
The RDS (ON) _H is the on-resistance of the upper bridge MOSFET switch; the VD is the forward conduction voltage of the diode tube; RL is the DC impedance of the inductor.
The operating frequency FSW can be set by connecting the external resistance between the RT / SYNC and the ground. This terminal is designed according to the results of FMEA failure mode and its effect analysis, which ensures that it is encountered such as short-circuiting or The device does not allow the device to work in an abnormal frequency when the fault state is invalid. In this way, if the terminal is shorted, the operating frequency of the device will be 900kHz (typ); if the terminal is in an empty state, the actual operating frequency will be 240KHz (typical) If the terminal is normal connection, the relationship between the actual resistance value and the operating frequency is as follows:
The FSW (kHz) is the operating frequency we want to set. Setting the resistance of the operating frequency preferably uses 1% precision or higher precision resistance, and its temperature coefficient should be 100 ppm or less. The figure below shows the relationship between the operating frequency FSW and the value of the resistor RRT / SYNC:
Inductive selection
The choice of inductor needs to be trade-offs between dimensions, cost, efficiency, and transient response requirements. The key parameters contain three: inductance L, inductor saturation current ISAT and DC impedance DCR.
A balance point between the recommended size and the loss is to set the ripple peak of the inductor current to 30% of the IC rated current, plus the operating frequency, the input voltage, and the output voltage can calculate the value of the electric inductance together. :
The larger inductance will result in lower output voltage ripple and higher efficiency, but slightly reduce transient response ability, result in a hysteresis of loop phase, reduce crossover frequency, and compensate for ramp and detected current when slope compensate When the ratio between the ramp is increased, the control system of the current mode becomes more and more like the voltage mode. The lower inductance will result in a smaller size, but the growth of current ripple will result in a decrease in the accuracy of the current limit threshold, increase the communication loss of the inductor, and will cause insufficient compensation when the duty ratio is close or more than 50% , Severe loop instability problem. When the duty cycle is more than 50%, the following conditions need to be satisfied:
The inductor current ripple δ IL is set at 10% to 50% of the maximum rated output current (3.5a) to obtain a relatively good balance between dimensions, efficiency, and transient responses.
In order to obtain a good efficiency, low-loss inductance having the lowest DC impedance can be selected within the allowable spatial size. The inductance not only determines the size of the current ripple, but also determines the load current value at the time of DCM / CCM mode switching. The selected inductor should have a saturated current rated value greater than the IC peak current limit threshold, which should be large enough so that it does not have a saturation in the inductor current peak (IL_PEAK), where the inductor current peak The calculation formula is listed below:
That is, the current flowing through the inductor is composed of an inductive ripple current and load current. When the power generation, a fault or load occurs, the inductor current may occur more than the inductance current peak calculated above. Since there is a case where the inductive current top to the IC limit value may occur during transients, many conservative design will select a saturated current parameter rated value equal to or greater than the IC switch current limitation. In order to get good EMI performance, the selected inductor is preferably a shielded design.
Enter the choice of capacitance
The input capacitor CIN plays a function of filtering the upper bridge MOSFET switch drain pulse current, and the size should be able to avoid a large voltage change in the input. The input voltage ripple in the input capacitor can be evaluated by the following formula:
Including
Figure 5 below shows a ripple current flowing through the input capacitor CIN and a voltage ripple formed on a capacitor.
For ceramic capacitors, its equivalent series resistance ESR is small, and the ripple caused by it can be ignored, and we can use the following formula to estimate the actual input capacitance valid value:
The ΔVCIN_MAX is the maximum value of the input voltage ripple.
The input capacitor not only needs to have a very low ESR, but also has the ability to assume the effective value of the input current ripple in the worst case. The input current ripple valid value of the converter (IRMS) can be evaluated by the following formula according to the input voltage Vin, the output voltage Vout, and the rated load current IOUT:
The maximum value of the validity of the input current ripple occurs when the load is the largest, and this requirement must be incorporated into the input capacitor current bearing capacity. The maximum value of the input current ripple typically occurs when the duty cycle is 50%, there is VIN = 2 x Vout, so IRMS ≈ 0.5 x Iout_max when VIN = 2 X Vout is often used in the design. It should be noted that the ripple current carrying capacity provided by the capacitor manufacturer is usually based on the 2000 hour life, and thus a wise practice is to give capacitance more degradation to use space, or to choose more than actual needs. Higher capacitors use temperature level.
In reality, multiple capacitors are often used in parallel to meet the needs of applications, high and thermal performance. When you encounter a low application of input voltage, you need to use many large-capacity input capacitors to meet transient effects during load changes. The impact minimizes the needs.
The ceramic capacitor has become the optimal input capacitance selection of the switch mode converter due to small size, cossive, and ESR is very low, but it must be very careful when using them, because the ceramic input capacitor is formed after the line inductance in combination A resonant circuit with a high quality factor, if the RTQ6363 application circuit such as designed is directly inserted into the powered power supply, and the input is 2 times the ringing signal, which may exceed the maximum voltage of the IC. . To avoid this situation, the easiest way is to give a low ESR ceramic input capacitor in parallel with a large capacitance capacitance with a higher ESR, which can act as a suppression of the voltage ringing signal.
The input capacitor should be placed in a place where the IC's VIN and GND is as close as possible, and the inductance formed by the connection is as low as possible. The bypass capacitance required by the VIN end has at least 3μF, in an application of 400 kHz, can be connected between Vin and GND in an application of 400 kHz. If the operating frequency is lower, the required input capacitors must be larger. In order to eliminate high frequency noise, the additional 0.1μF small capacitance should be used with these devices, and the package specification should be 0402 or 0603. In order to achieve optimum performance in the wide temperature range and the input voltage, the selected ceramic capacitor is preferably X7R type. (Endlessly)
Reprinted from Richtektechnology
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