What are the points of DC-DC's PCB design?

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2022-03-17 18:44:07 134

Compared with LDO, the circuit of DC-DC is much more complicated, with higher noise and higher requirements for layout and layout. The quality of layout directly affects the performance of DC-DC, so it is very important to know the layout of DC-DC.

1. Bad layout

EMI, DC-DC SW pin will have higher dv/dt, and higher dv/dt will cause greater EMI interference;
Ground wire noise, bad grounding wiring, will produce relatively large switching noise on the ground wire, and these noises will affect other parts of the circuit;
A voltage drop is generated on the wiring, and the wiring is too long, which will cause a voltage drop on the wiring and reduce the efficiency of the whole DC-DC;A voltage drop is generated on the wiring, and the wiring is too long, which will cause a voltage drop on the wiring and reduce the efficiency of the whole DC-DC;

2. General principles

The switching current circuit should be as short as possible;
The signal ground and the high-current ground (power ground) are routed separately, and are connected at a single point at the chip GND;

① Short switching circuit

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 is the current flow direction when the high-side tube is closed and the low-side tube is opened;

To make these two circuits as small as possible and introduce less interference, the following principles should be observed:

Try to get the inductor close to SW pin;
The input capacitor should be as close to VIN pin as possible;
The ground of the input capacitor is as close to the PGND pin as possible；
Route by laying copper;

Why do you want to do this?

If the line is too thin and too long, the impedance will be increased, and the high current will produce a relatively high ripple voltage on this impedance;
If the line is too thin and too long, it will increase the parasitic inductance, which will be coupled with switching noise, which 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 single-point grounding between signal ground and power ground. There will be relatively large switching noise on the power ground, so it is necessary to try to avoid interference to sensitive small signals, such as FB feedback pins.

High-current ground: L, Cin, Cout and Cboot are connected to the network of high-current ground;
Small current ground: Css, Rfb1, Rfb2 are separately connected to the network of signal ground;

The following diagram shows the layout of a development board of TI, with red as the current path when the upper tube is open and blue as the current path when the lower tube is open; The following layout has the following advantages:

① The GND of the input capacitor is connected with copper skin, and when decorating, the ground of the two should be put together as much as possible;The GND of the input capacitor is connected with copper skin, and when decorating, the ground of the two should be put together as much as possible;

②The current path of DC-DC Ton and Toff is very short;

③ The small signal on the right side is grounded at a single point, which is far away from the noise of the high current switch on the left side.

TI某开发板的DC-DC PCB layout

3. example

The layout of a typical DC-DC BUCK circuit is given as follows, and the following points are given in SPEC:

The switching circuit formed by input capacitor, high-side MOS transistor and freewheeling diode is as small and short as possible;
The input capacitor should be as close to Vin Pin as possible;
Ensure that all feedback connections are short and direct, and the feedback resistors and compensation elements are as close to the chip as possible;
SW away from sensitive signals, such as FB;
Connect VIN, SW, especially GND to a large copper area to cool the chip and improve the thermal performance and long-term reliability.

DC-DC BUCKtypical circuit

4. Summarize

The layout of DC-DC circuit is very important, which directly affects the stability and performance of DC-DC. Generally, the SPEC of DC-DC chip will give layout guidance, which can be used for reference in design.

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What are the points of DC-DC's PCB design?

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