What should be paid attention to in the PCB design of DC- DC
Time:2023-01-07
Views:1391
The circuit of DC-DC is much more complex than that of LDO, with greater noise and higher requirements for layout and layout. The quality of layout directly affects the performance of DC-DC, so it is important to understand the layout of DC-DC.
01 Bad layout
There will be higher dv/dt on the SW pin of EMI and DC-DC, and higher dv/dt will cause greater EMI interference;
Ground wire noise, poor ground wire routing, will produce relatively large switching noise on the ground wire, and these noises will affect other parts of the circuit;
Voltage drop is generated on the wiring. If the wiring is too long, voltage drop will be generated on the wiring, which will reduce the efficiency of the entire DC-DC;
02 General principles
The high current circuit of the switch shall be as short as possible;
The signal ground and high current ground (power ground) are wired separately and connected at a single point at the chip GND;
① Switch circuit is short
The red LOOP1 in the figure below shows the current flow direction when the DC-DC high side tube is turned on and the low side tube is turned off; The green LOOP2 indicates the current flow direction when the high side tube is closed and the low side tube is open;
In order to make these two loops as small as possible and introduce less interference, the following principles should be followed:
Inductance shall be close to SW pin as far as possible;
Input capacitance shall be close to VIN pin as far as possible;
The ground of input and output capacitors shall be close to PGND pin as far as possible;
Use copper laying method for wiring;
Why do you do this?
If the wire is too thin and too long, the impedance will be increased, and high current will produce relatively high ripple voltage on this large impedance;
If the wire is too thin and too long, the parasitic inductance will be increased. The switching noise on this inductance will affect the stability of DC-DC and cause EMI problems;
Parasitic capacitance and impedance will increase switching loss and conduction loss, and affect DC-DC efficiency;
② Single point grounding
Single point grounding refers to the single point grounding of signal ground and power ground. There will be relatively large switching noise on the power ground, so it is necessary to avoid interference to sensitive small signals, such as FB feedback pins.
High current ground: L, Cin, Cout, Cboot are connected to the network of high current ground;
Low current ground: Css, Rfb1 and Rfb2 are separately connected to the network of signal ground;
The following figure shows the layout of a development board of TI. The red is the current path when the upper tube is turned on, and the blue is the current path when the lower tube is turned on; The following layout has the following advantages:
① The GND of the input and output capacitors are connected with copper sheet. When placing the parts, place them together as much as possible;
② The current path of DC-DC Ton and Toff is very short;
③ The small signal on the right is grounded at a single point, which is far away from the left high current switching noise;
TI DC-DC PCB layout of a development board
03 Example
The layout of a typical DC-DC BUCK circuit is given below, and the following points are given in the SPEC:
The switching circuit formed by input capacitor, high side MOS transistor and freewheeling diode shall be as small and short as possible;
The input capacitance shall be as close to the Vin Pin pin as possible;
Ensure that all feedback connections are short and direct, and the feedback resistor and compensation element are as close to the chip as possible;
SW is far away from sensitive signals, such as FB;
Connect VIN, SW, especially GND to a large copper area to cool the chip and improve thermal performance and long-term reliability;
Layout guidance
04 Summary
The layout of DC-DC circuit is very important, which directly affects the working stability and performance of DC-DC. Generally, SPEC of DC-DC chip will give layout guidance, which can be referred to for design.
This is the end of today‘s article. I hope it will be helpful to you.
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