In battery system circuit, a LVD is used for detecting and monitoring the low voltage when the battery voltage is getting lower and lower. When the battery voltage go down to a certain extend, it can warn users to change batteries (the system can still work for a certain time when in low voltage). In the market, there are some LVD IC with stable performances, such as, HT70XX. But the cost of this LVD IC is quite high. Therefore, in this article, we抎 like to introduce a simple way to realize this detection function for reference.
See figure 1, there are two batteries (1.5V), a stabilizer that maintain the operation voltage at 3.3V, and a LVD in this circuit. The LVD is used for detecting when the voltage is at 2.4V or lower. In normal operation voltage, the LVD is in low level. While in low voltage, the LVD will be in high level.
Theory Analysis
The use of dynatron (0.7V) is essential in this circuit. It can enlarge the current and enable the voltage on LVD to rise from low to high if only the size of R1/R2 is determined.
However, the actual conduct voltage of the dynatron is usually lower than 0.7V, and its b-e terminal distributes part of current. Hence, it is hard to determine the sizes of R1/R2 by calculating. Instead, we could take some tests to get some parameters to adjust their sizes.
1. First, determine the size of R2. We could use the resistors from 0K to100K for test, and then find the suitable size resistor as R2. Note that the size of R2 should not be too large. Otherwise, the malfunction may occur when the system voltage is unstable.
2. Next, determine the size of R1. According to R1 = VLVD * R2, we should first determine R2, and then adjust battery voltage to generate a jump on LVD terminal from low to high. At that time, this battery voltage is the lowest in those parameters. Since part of current is shared by dynatron抯 b-e terminal, the actual value of voltage is slightly lower than the expected.
3. Finally, fix the Vbattery to get the low voltage value, such as 2.4V. Then we could increase R1 to adjust until the low voltage appears. Since R2 is fixed, if R1 increases, the detecting voltage will be increased accordingly, or vise versa.
In order to enable the low voltage alarm when the voltage drops below 2.4V, refer to the following steps:
1. First, select R2 = 10K
2. Next, select R1 = 24K. Then have a test for it, find that the low voltage is around 2V
3. Finally, fix the Vbattery at 2.4v. Then gradually increase the size of R1. When R1=38K, the low voltage appears.
4. Determine parameters
Thinking about this circuit
The above-mentioned circuit is simple, but it consumes decades of 礎 current. To reduce the consumption of current, we could use the method of cutting off the current loop.
