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ZHCSS29R April 2004 – April 2024 TL103W , TL103WA , TL103WB

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7.2.1.2.1 Constant Current Circuit

For the constant current feedback circuit, the amplifier is configured in a low-side current sense configuration. Resistor R6 is used as the current sensing resistor to sense the current flowing between the battery and flyback converter. This is shown in Equation 1, where I_BAT is the output current delivered to the battery. The voltage at the non-inverting input of the amplifier specifies the maximum current (or constant current) that is delivered to the battery.

Equation 1.

V_{B A T -} = I_{B A T} \times R 6

The reference of the TL103Wx is powered by the battery voltage. To be able to achieve a constant current, this reference needs to be provided with 2.5V or greater to provide a fixed 2.5V. The first step in designing a constant current circuit is to specify that 2.5V can be achieved at the battery's minimum voltage. The value of R5 must also be designed so that a sink-current between 0.5mA to 100 mA (for TL103W or TL103WA) is achieved across the specified range of the battery voltage. These two steps are shown below.

Equation 2.

V_{B A T} (m i n) \times \frac{R 4 + R 3}{R 4 + R 3 + R 5 + R 6} \geq 2.5 V

Equation 3.

0.5 m A \leq \frac{V_{B A T} - V_{R E F}}{R 5} \leq 100 m A

For this design R5 is chosen to be 2kΩ. Knowing this and the specified battery range of 6V to 20V, we can calculate that the reference sinks anywhere from 1.75mA to 10mA using Equation 2.

Once a fixed 2.5V reference is achieved, we can use this accurate DC voltage to specify a constant current target on the non-inverting input of the amplifier. This can be done by calculating the voltage at the amplifier's inverting input when a constant current target is achieved. Specifying R6 to be 10mΩ along with a constant current design target of 6A, we find that this voltage to be 60 mV using Equation 1. The voltage at the non-inverting pin of the amplifier is specified by Equation 4.

Equation 4.

V_{I N +} = 2.5 V \times \frac{R 3}{R 4 + R 3}

Using the component values and design targets calculated so far, Equation 2 and Equation 4 can be updated to:

Equation 5.

6 V \times \frac{R 4 + R 3}{R 4 + R 3 + 2 k Ω + 10 m Ω} \geq 2.5 V

Equation 6.

60 m V = 2.5 V \times \frac{R 3}{R 4 + R 3}

Using both Equation 5 and Equation 6, we calculate that R3 needs to be greater than around 34.28Ω. Adding additional headroom to this, we can calculate R3 = 36Ω and R4 = 1464Ω.

Figure 7-1 Constant Current Feedback Circuit