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# How to design CS function and resistance

## How to design CS function and resistance

Level 1
Level 1

How to design CS function and resistance for TDA38812

1 Solution

## Re: How to design CS function and resistance

Moderator
Moderator

Hi @Mugi ,

During the ON time of the Synchronous MOSFET, the inductor current is sensed and mirrored to CS pin with the ratio of GCS. By using an external resistor (RCS) from CS to AGND, the voltage on CS pin (VCS) is proportional to the SW current cycle-by-cycle. The Control MOSFET is only allowed to turn on when VCS is below the internal OCP voltage threshold of 1.2V (during the ON state of Synchronous MOSFET) to limit the SW valley current cycle-by cycle.

The resistor  RCS can be calculated as below.

Where, VCS =1.2 V, GCS = 20 µA/A, VIN - Input voltage, VOUT-Output voltage, L-Output inductance, FSW- Switching frequency and ILIM is the desired DC current limit.

Kindly refer the TDA38812 datasheet for detailed information.

Regards,

Vishnu P

2 Replies

## Re: How to design CS function and resistance

Level 1
Level 1

The inductor current is sensed and mirrored to CS pin with the ratio of G CS . By using an external resistor (R CS ) from CS to AGND, the voltage on CS pin (V CS ) is proportional to the SW current cycle-by-cycle. The Control MOSFET is only allowed tto turn on when V CS  is below the internal OCP voltage threshold of 1.2V (during the ON state of Synchronous MOSFET) to limit the SW valley current cycle-by-

cycle. The resistor R CS  can be calculated as below.

## Re: How to design CS function and resistance

Moderator
Moderator

Hi @Mugi ,

During the ON time of the Synchronous MOSFET, the inductor current is sensed and mirrored to CS pin with the ratio of GCS. By using an external resistor (RCS) from CS to AGND, the voltage on CS pin (VCS) is proportional to the SW current cycle-by-cycle. The Control MOSFET is only allowed to turn on when VCS is below the internal OCP voltage threshold of 1.2V (during the ON state of Synchronous MOSFET) to limit the SW valley current cycle-by cycle.

The resistor  RCS can be calculated as below.

Where, VCS =1.2 V, GCS = 20 µA/A, VIN - Input voltage, VOUT-Output voltage, L-Output inductance, FSW- Switching frequency and ILIM is the desired DC current limit.

Kindly refer the TDA38812 datasheet for detailed information.

Regards,

Vishnu P