Problems encountered in debugging the Southchip SC8721 high-efficiency synchronous buck-boost converter

littleshrimp

Problems encountered in debugging the Southchip SC8721 high-efficiency synchronous buck-boost converter [Copy link]

 

 

I had some time to solder the board today. I used a 0.33R current sampling resistor because I only have this one. I used a 2K resistor for the CSO pin. I used an external resistor to set the voltage to 11V. After power-on testing, the output was normal.

However, the voltage at the CSO current detection pin should be 0.33V when the output is 1A, but the actual measurement is about 0.003V. The voltage will change with the current ratio. The difference is 100 times.

Using the microcontroller through I2C configuration, communication is normal. Try to set VOUT_SET to 50, which corresponds to 1V. Then when the internal voltage setting is enabled, the voltage becomes 5.1V, regardless of whether it is Decrement mode or Increment mode.

  buf[0] = 0x0c;//VOUT_SET_MSB = 0b1100  +  VOUT_SET_LSB =0b10 = 50 * 20mV = 1V
  buf[1] = 0x1a;//FB_SEL = 1,FB_ON = 1,FB_DIR = Increment,VOUT_SET_LSB = 0b10
  HAL_I2C_Mem_Write(&hi2c1, 0x62, 0x03, I2C_MEMADD_SIZE_8BIT,buf, 2, 0x10000);
    
    
    
  buf[0] = 0x0c;//VOUT_SET_MSB = 0b1100  +  VOUT_SET_LSB =0b10 = 50 * 20mV = 1V
  buf[1] = 0x1e;//FB_SEL = 1,FB_ON = 1,FB_DIR = Decrement,VOUT_SET_LSB = 0b10
  HAL_I2C_Mem_Write(&hi2c1, 0xC4, 0x03, I2C_MEMADD_SIZE_8BIT,buf, 2, 0x10000);

The manual mentions the VOUT_REF register but I can't find it in the list below. I'm not sure if my manual version (LiChuang Mall version) is wrong or if there's something wrong with the settings somewhere.

tagetage

Where is the output capacitor???

littleshrimp

tagetage posted on 2021-10-19 20:44 Where is the output capacitor? ? ?

This place

tagetage

I thought it was the input capacitor. Is your board the typical circuit on the datasheet? It doesn't look like it.

littleshrimp

tagetage posted on 2021-10-19 20:53 I thought it was an input capacitor. Is your board the typical circuit on the datasheet? ? It doesn't look like it.

I made this circuit based on the datasheet. I only had a 0.33R sampling resistor so I used this one, and a 2K CSO resistor. I'm not sure if the resistance of this resistor is too small and cannot be driven, resulting in the current monitoring voltage being too low.

The sampling resistor in the manual is 0.01R and the CSO resistor is 64K.

tagetage

The problem lies in the sampling resistor and CSO resistor. The CSO resistor is connected in series and parallel to 64K, and the sampling resistor is too large. According to the reference manual, a piece of iron wire or something like that is 0.01 ohm.

littleshrimp

tagetage posted on 2021-10-20 09:20 The problem lies in the sampling resistor and CSO resistor. The CSO resistor is connected in series and parallel to 64K, and the sampling resistor is too large. According to the reference...

I didn't read the data sheet carefully, but the maximum sampling resistor is 10mR and the minimum is 5mR, and there is no limit on RCSO.

I bought a 10mR resistor, which is on the way. I will try it after receiving it.

tagetage

I guess it's the sampling resistor that's at fault. If the test works, I'll tell you about it here.

littleshrimp

tagetage posted on 2021-10-20 18:09 It is probably the sampling resistor that is at fault. If the test works, I will tell you about it here.

OK

littleshrimp

tagetage posted on 2021-10-20 18:09 I guess it's the sampling resistor that's at fault. If the test works, I'll tell you about it here.

The I2C voltage regulation problem was found. It was because the LOAD register was not configured. After the function was set, this bit needed to be written as 1.

I soldered a 10mR resistor for current detection, but the voltage I measure is still incorrect.

littleshrimp

tagetage posted on 2021-10-20 18:09 I guess it's the sampling resistor that's at fault. If the test works, I'll tell you about it here.

The current monitoring problem was also solved. I connected CSN and CSP in the wrong direction, but it worked fine after I changed it back.

感觉Y

Hello, I want to ask, can this chip work without a microcontroller? Refer to the typical circuit on the datasheet completely, without a microcontroller, SCL, SDA are pulled up by default, can the over-current protection value be set by an external circuit?

littleshrimp

Feeling Y posted on 2021-12-13 11:11 Hello, I would like to ask, can this chip work without a microcontroller? Refer to the typical circuit on the datasheet completely, without connecting a microcontroller, SCL, SDA default on...

It can work without using a microcontroller. The voltage is set by an external resistor and the current can be calculated by the default value of CSO_SET and the following formula