Post Date : Mar 09, 2023 / Last updated : Apr 06, 2023

The current limiting characteristic of general voltage regulators is determined by the current (IOUT) that can be supplied by the IC at startup.
The current that can be supplied at startup (IOUT) varies as the output voltage is increased in accordance with the current limiting characteristic.

This section explains the voltage regulator startup characteristic using the startup waveforms of the previously introduced Type A (Drooping type ⇒ Foldback), Type B (Foldback only), and Type C (Drooping type ⇒ Drooping type).

Even for the same type of current limiter, there are products for which the startup characteristic does not rely solely on the current limiting characteristic due to having a soft start function or inrush current prevention function, so be sure to check the datasheet and actual device characteristics of the product being used before creating the design.

MEASUREMENT METHOD AND CONDITIONS

The voltage regulator output current (IOUT) waveforms shown hereafter are measured using a current probe placed between the voltage regulator’s VOUT terminal and output capacitor CL. This output current is equivalent to the current provided by the IC.

Inserting the current probe after the output capacitor prevents the current supplied by the IC from being measured accurately because the transient current is smoothed by the output capacitor.

Unless otherwise stated, the output current is designed using a variable resistor where CIN=CL=1μF, VIN= design output voltage +1.0V.

1) Current limiter Type A (Drooping type ⇒ Foldback)

This section explains the startup waveform using the waveform for when the XC6219B302 (output voltage 3.0V product) is started up by the CE terminal.

XC6219B302: Current limiting characteristic
XC6219B302: Startup characteristic @RL=20mA/3.0V

Immediately after startup – Foldback current limiter: V1, I1 – V3, I3

The output voltage immediately after startup is 0V.
The current limiting characteristic when the output voltage is 0V is an IOUT of approximately 20mA, so immediately after startup the IC supplies 20mA of current to the output side.

When current is supplied to the output side, the output voltage raises because electric charge is charged to the output capacity.
At the foldback current limiter, when the output voltage increases, the IC increases the current supplied in proportion to the output voltage.

When the output voltage reaches V3 where the foldback and drooping type switching occurs, the IC stops increasing the supplied current and the device switches to the drooping type current limiter.

Drooping type current limiter: V3, I3 – V4, I4

At the drooping type current limiter, the current supplied by the IC is constant. The output voltage rises while the current supplied by the IC stays constant and the set output voltage of V4 is reached.

Startup end

When the set output voltage of V4 is reached, the output voltage no longer needs to increase any further, so the current supplied by the IC decreases. The amount of current supplied by the IC after startup is just the load current.

2) Current limiter Type B (Foldback current limiter only)

This section explains the startup waveform using the waveform when the XC6221A302 (output voltage 3.0V product) is started up by the CE terminal as an example.

XC6221A302: Current limiter characteristic
XC6221A302: Startup characteristic @RL=20mA/3.0V

Immediately after startup – Foldback current limiter: V1, I1 – V3, I3

The output voltage immediately after startup is 0V.

The current limiting characteristic when the output voltage is 0V is an IOUT of approximately 30mA, so the IC supplies 30mA of current to the output side immediately after startup.

When current is supplied to the output side, electric charge is charged to the output capacity causing the output voltage to increase.

At the foldback current limiter, when the output voltage increases, the current supplied by the IC increases proportionately to the output voltage.

When the output voltage reaches V3 where the foldback and drooping type switching occurs, the IC stops increasing the supplied current and the device switches to the drooping type current limiter.

Startup end

When the set output voltage of V4 is reached, the output voltage no longer needs to increase any further, so the current supplied by the IC decreases. The amount of current supplied by the IC after startup is just the load current.

3-1) Current limiter Type C (Drooping type ⇒ Drooping type)

This section explains the startup waveform using the waveform when the XC6223B301 (output voltage 3.0V product) is started up by the CE terminal as an example.

XC6223B301: Current limiting characteristic
XC6223B301: Startup characteristic @RL=20mA/3.0V

Immediately after startup – Drooping type current limiter: V1, I1 – V2, I2

The output voltage immediately after startup is 0V.

The current limiting characteristic when the output voltage is 0V is an IOUT of approximately 50mA, so the IC supplies 50mA of current to the output side immediately after startup. A drooping type current limiter is employed, so the supply current is limited to 50mA until the output voltage reaches 0.8V (refer to V2).

Drooping type current limiter – Drooping type current limiter: V2, I2 – V4, I4

When the output voltage reaches 0.8V (refer to V2), the current limit value of the drooping type current limit value becomes larger.
As the current limit value becomes larger, the current supplied to the output side increases changing the supply current from I2 to I4 and raising the output voltage.

Startup end

When the set output voltage of V4 is reached, the output voltage no longer needs to increase any further, so the current supplied by the IC decreases. The amount of current supplied by the IC after startup is just the load current.

3-2) Current limiter Type C (Drooping type ⇒ Drooping type) + Inrush current prevention function

Finally, this section explains the startup characteristic of XC6223F301 (output voltage 3.0V product) when an inrush current prevention function is added to the product explained in 3-1.

To check the effect of the inrush current prevention function, take the measurement under conditions where the normal state load current is 150mA.

The inrush current prevention of XC6223F301 is a function that keeps the inrush current supplied to the output side within the set value for about 120μs when the output voltage immediately after startup increases to 0.8V (product set to 3.0V). When the inrush voltage prevention function is canceled, the current supplied to the output side is limited by the current limiting characteristic.

XC6223F301: Current limiting characteristic
XC6223F301: Startup characteristic @RL=150mA/3.0V

Immediately after startup – Drooping type current limiter: V1, I1 – V2, I2

The output voltage immediately after startup is 0V.

The current limiting characteristic when the output voltage is 0V is an IOUT of approximately 50mA, so the IC supplies 50mA of current to the output side immediately after startup. A drooping type current limiter is employed, so the supply current is limited to 50mA until the output voltage reaches 0.8V (refer to V2).

Drooping type current limiter – Inrush current prevention function: V2, I2 – V3, I3

When the output current reaches 0.8V (refer to V2), the inrush current prevention function activates.
The inrush current prevention function keeps the inrush current supplied to the output side within the set value for about 120μs.

Under these conditions, an inrush current that would have been 370mA without the inrush current prevention function is kept down to an inrush current of up to 110mA.

Inrush current prevention function – Drooping type current limiter: V3, I3 – V4, I4

When the inrush current prevention function ends, the device moves to the drooping type current limiter.

In this case, the inrush current is canceled when the output voltage is about 2V, and the output voltage increases to the set output voltage while the supply current is transitioning to the current limit value.

Startup end

When the set output voltage of V4 is reached, the output voltage no longer needs to increase any further, so the current supplied by the IC decreases. The amount of current supplied by the IC after startup is just the load current.