DC-DC｜Application

Case 2: Startup Problems Caused by Constant-Current Loads

2023.11.22

Points of this article

・A linear regulator equipped with a foldback current limiting circuit may possibly fail to start if a constant-current load is being applied before startup.

・This problem can be addressed by using a linear regulator having an overcurrent protection circuit with a drooping characteristic.

・When a constant-current load causes current to flow in an electrostatic damage protection diode or a parasitic diode between an IC output pin and ground, there are concerns that degradation of or damage to the IC may result. Hence a Schottky barrier diode is connected between the output pin and ground for protection.

table of contents

Case 2: Startup Problems Caused by Constant-Current Loads

If a constant-current load is connected to a linear regulator IC equipped with a foldback current limiting circuit before the IC is started, the IC may fail to start.

Startup Sequence for Normal Loads (Resistors, Capacitors)

Fig. 1 is an example of a current foldback characteristic. If the IC output is in an overload state and the output current reaches the limiting value, protection is effected by lowering the output current limit in a linear manner to lower the output voltage, confining the IC power consumption below a limit.

Because the foldback current limiting function also operates during startup, the IC startup conforms to the foldback characteristics. To begin with, the startup sequence when the output load are normal resistors and capacitors is explained using Fig. 1 and Fig. 2. Fig. 2 is the startup waveform for the circuit with a 100 µF capacitor and a resistive load, in which a 500 mA output (load) current flows when at 5 V, connected to the regulator output. In the two figures, (A) through (E) represent the same times.

Fig.1. Example of current foldback characteristics

Fig.2. Example of startup waveforms
VCC＝12 V，VOUT＝5 V, COUT＝100 μF, IOUT＝500 mA

At a time before VCC is supplied, the output voltage and the output current are both zero. From the time (A) at which VCC is supplied, startup of the IC begins, and the output begins to rise. Then, because the 100 µF output capacitor is connected to the output, the output current increases suddenly in order to charge the capacitor. This is called an inrush current; if it is not limited, as large a current as can possibly flow will flow instantaneously. In this example, as indicated, the current is limited to approximately 300 mA.

Thereafter, the output current gradually charges the output capacitor while increasing linearly along the foldback curve, and the output voltage similarly rises along the foldback curve accordingly, passing through points (B), (C), and (D).

At the time of point (D), charging of the output capacitor essentially ends, the output voltage reaches the preset value, and the output current also moves toward a steady state.

At the time of (E), the output has risen completely and is in a stabilized state, and the output current is at the normal load value of 500 mA.

In this way, a linear regulator having a foldback current limiting circuit starts up from the zero point and follows the foldback curve. When the load consists of resistors and capacitors, the current may be limited during startup, but so long as a current is being supplied to the output, the output voltage will always rise.

The Case of Constant-Current Loads

Next, the case of constant-current loads is explained. When a constant-current load is applied to the output before the IC is started, current flows in the diode between the IC output pin and ground, and a forward voltage appears; consequently the output pin goes to minus-1VF (about -0.7 V). Here the diode is actually an electrostatic damage protection diode built into the IC, and also a parasitic diode that exists due to the device structure (Fig. 3).

Fig. 3. When a constant-current load is applied before startup, VF occurs across an internal diode, and the output pin goes to -1VF

Fig. 4. Case in which a constant-current load is applied before startup

As an example, let the constant-current load draw 500 mA. Point (A) in Fig. 4 is the point at which startup of the IC begins; as explained earlier, due to the constant-current load, the output voltage is negative. When the IC starts, an output current begins to flow, but because the output voltage is -0.7 V, the supplied current decreases from when the voltage is 0 V, in this example falling to about 200 mA. The load is a constant 500 mA current, so if 200 mA is supplied, the output voltage does not rise, and is latched at point (B), which results in startup failure.

When, in a state in which IC startup is already completed and the preset voltage is reached, the constant-current load is connected, operation continues without problem. This is because the IC is already in the steady state, and the required current, in this example 500 mA, can be supplied without issue. However, if the output is once short-circuited (goes to 0 V), operation returns to point (A) in Fig. 4, and the above-described state in which the constant-current load is applied prior to startup again occurs, so that again there is similar startup failure.

As a measure to deal with this, an IC should be selected that has a large output current value, larger than the constant-current load value, that the IC can supply at startup.

However, a linear regulator provided with a foldback current limiting circuit has characteristics such that the current that can be supplied at 0 V output is set to be small, and in many cases even the small current value is not guaranteed. Therefore, for use under conditions such that the constant-current load is applied prior to startup, this problem can be resolved by using a linear regulator with an overcurrent protection circuit having a drooping characteristic. As shown in Fig. 5, a drooping characteristic is such that the current that can be supplied at 0 V output is close to the maximum output current, so that even in the case of a constant-current load, reliable startup is possible.

Fig. 5. Example of a drooping characteristic of an overcurrent protection circuit

Fig. 6. Output pin reverse voltage protection

While not a matter that is directly related to dealing with startup problems, when current flows in the electrostatic damage protection diode and parasitic diode between the IC output pin and ground due to a constant-current load, there is the possibility of degradation or destruction of the element. In order to prevent this, a Schottky barrier diode should be connected between the output pin and ground, as shown in Fig. 6.

    DC-DC

    Basic

    Design

    Evaluation

    Application

    Product Information

    FAQ

    THIS WEBSITE USES COOKIES.
    By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies.
    Privacy Policy
    OK