TPS54225-Q1: Current Limit for given Inductor

May

Part Number: TPS54225-Q1

Hi Team,

We received inquiry from customer asking if there is a given equation to calculate the range of the expected current limit for a given inductor size? Because in datasheet (Electrical Characteristics = Current Limit) states that current limit for 2.2uH is in the range of 2.5A to 4.5A.

Thank you and looking forward for your kind response.

Regards,

Maynard

over 3 years ago

0 Tahar Allag over 3 years ago

TI__Mastermind 28720 points

Hello Maynard,

The over current limit is implemented using a cycle-by-cycle valley detect control circuit. During the on time of the high-side FET switch, the switch current increases at a linear rate determined by Vin, Vout, the on-time, and the output inductor value. During the on time of the low-side FET switch, this current decreases linearly. If the measured voltage is above the voltage proportional to the current limit, then the device constantly monitors the low-side FET switch voltage, which is proportional to the switch current, during the low-side on-time.

The inductor does not affect the current limit of the device. However, the inductor affects the inductor peak current, so when choosing an inductor, the ILpeak and current limit need to be taken in consideration.

Equation 5 and 6 in the datasheet provide the hwo to calculate the Ipeak based on the FSW, L, Vin and VOUT.

Thanks!

Tahar

0 May over 3 years ago in reply to Tahar Allag

TI__Genius 9880 points

Hi Tahar,

Thank you for your kind response.

Our customer has a followup question. Could you please clarify if the current limit stated on page 3 (2.5A to 4.5A) is the output current or the valley inductor current? In other words; do I subtract half of the peak to peak inductor current from the stated current limit to calculate the valley inductor current at the limit point , OR, do I add half of the peak to peak inductor current to the stated current limit to calculate the output current at the limit point?

Thank you!

Regards,

Maynard

0 Tahar Allag over 3 years ago in reply to May

TI__Mastermind 28720 points

Hi Maynard,

The current limit is the EC table in the datasheet is the valley inductor current limit.

The figure attached below shows the switch-node voltage, load current and inductor current waveforms for valley current limit.

Instead of monitoring the current during the on-time, the current is monitored during the off-time. As noted, the current decreases during the off-time. At the end of the switching cycle, the switch current is compared to the valley current limit value. In the attached waveform, During Time C, the converter is operating at the current limit. When the current is above the limit, the off-time is extended until the switch current equals the valley current limit; then the next switching cycle is allowed to start.

Thanks !

Tahar