Characteristics and Application of Integrated Voltage Regulator TL499AC

The widely used voltage stabilizers are divided into linear series negative feedback voltage stabilizers and switching voltage stabilizers. These two types of voltage stabilizers have their own characteristics and are usually made into monolithic voltage stabilizer integrated circuits. However, the uniqueness of the integrated voltage stabilizer TL499AC is that it has two complete circuits: an adjustable linear series voltage stabilizer and a boost switching voltage stabilizer. In addition to the external feedback sampling resistor (used to adjust the output voltage) and a few components such as inductors and capacitors, the other components necessary for the two types of voltage stabilizers are integrated inside the integrated circuit, so it is very convenient to use. Its main features and performance parameters are as follows:

●The input range of the series regulator is 4.5～32V;

●Switching regulator input range 1.1～10V;

●The output voltage is adjustable from 2.9 to 30V;

●When the series regulator loses its voltage regulation function due to low input voltage, the switching regulator will automatically start working, thus ensuring that the power supply will not be interrupted under the condition of backup power supply.

● Built-in over-current, over-voltage, over-heat and other protections, with high reliability.

2. Internal structure and function of TL499AC

TL499AC is an 8-pin dual in-line packaged monolithic integrated circuit that combines a series regulator and a switching regulator. The pin arrangement is shown in Figure 1, and the internal functional block diagram is shown in Figure 2. The internal circuit of TL499AC is divided into two parts: one is the series regulated power supply part composed of A2 and its related circuits. The other is the boost switching regulated power supply part composed of A1 and its related circuits, in which T1 is the main switch tube, and T2 is the mirror current detection tube of T1. Since the emitter current of T2 is proportional to the emitter current of T1 (the proportionality coefficient is determined by the emitter resistance of the two tubes), the operating current of the main switch tube T1 can be indirectly monitored by measuring the emitter current (voltage) of T2 to achieve overcurrent protection. Because the series regulator and the switching regulator share the same feedback terminal and output terminal, the two regulators cannot work at the same time under normal circumstances. As can be seen from Figure 2, due to the existence of E1, and the two regulators share the same reference voltage E2 and the same feedback network, the output of the series regulator is slightly higher than the output voltage of the switching regulator under normal circumstances, and the difference is about 2% of the output voltage. Therefore, when both the series regulator and the switching regulator have input voltages, the output voltage of the series regulator is higher than the output voltage of the switching regulator, thus forcing the switching regulator to be in a dormant state, and the switching regulator only consumes microampere current at this time. Once the input voltage of the series regulator is reduced to insufficient to maintain the output voltage stability due to power outages and other reasons, the switching regulator will automatically take over the work. This feature makes this circuit particularly suitable for making DC uninterruptible power supplies.

3. TL499AC maximum rated parameters

●Series regulator input voltage: 35V;

●Switching regulator input voltage: 10V;

●Built-in switch tube/isolation diode current: 1A

●Continuous total power consumption: TA≤25℃: 1W;

When TA>25℃: derating factor: 8mW/℃;

●Working environment temperature range: -20℃～85℃;

●Storage temperature range: -65℃～150℃.

4. Typical application circuit

The actual voltage stabilization circuit composed of TL499AC is shown in Figure 3. In this circuit, VCC (this voltage is usually obtained by rectifying and filtering the AC power) supplies power to the load through a series voltage regulator. Therefore, VCC should be at least 2V higher than the output voltage under normal conditions. E is the backup power supply, which is generally a rechargeable battery in the circuit requiring uninterrupted power supply. When the AC power supply is normal, it is trickle charged (in the circuit of Figure 3, a resistor is connected between the 1st and 3rd pins of TL499AC). When the power fails, the switching power supply immediately starts automatically and supplies power to the load from the backup power supply.

The value ranges of the components in Figure 3 are shown in Table 1. The maximum output current of the series regulator is 100mA (internal limit). The maximum output current of the switching regulator is not only related to the input and output voltages, but also to the value of R1. The resistance of this resistor directly affects the maximum operating current of the switch tube. The larger the resistance, the smaller the maximum operating current of the switch tube, and therefore the smaller the load current capacity. The specific parameters are listed in Table 2. The values ​​of the output current IO in Table 2 correspond to the values ​​when R1=150Ω/1000Ω, respectively, in mA.

According to the characteristics of TL499AC, this circuit is particularly suitable for application in the following occasions:

(1) Occasions that require uninterrupted power supply, such as power failure data storage in computer systems and detachable data acquisition cards.

(2) Portable electronic products that require a high power supply voltage but low current. For example, a rechargeable AC/DC dual-purpose digital multimeter can use this circuit to save the trouble of frequent battery replacement.

(3) Where high and low voltage power supply is required. For example, in a portable digital tuner radio, the lower voltage of the battery pack is directly supplied to the amplifier part of the radio, and the boosted voltage (12V) can be used by the digital tuner.

5. Precautions for use

Since the switching regulator generates a certain amount of electromagnetic radiation interference when it is working, the regulator part should be shielded for applications with higher requirements (this is especially important for radio equipment). When designing the circuit board, the power supply part and the small signal amplifier part should be properly separated, the ground wire should be routed separately, and the inductor L should also be wound with an integral small magnetic ring.

Capacitor C1 in Figure 3 can be omitted if the requirements are not high, but the dynamic performance of the circuit will be reduced and the output voltage ripple will increase slightly. The value of R1 should not be less than 150Ω, otherwise, the overcurrent protection value of the switch tube may be set too large, causing the protection function to fail or damaging the internal switch tube.

When the voltage difference between the input and output of the series regulator is greater than 8V, the maximum output current will decrease due to the power consumption limit of the adjustment tube. In addition, if the load current is greater than the current capacity of the series regulator, the series regulator and the switching regulator will work at the same time when the output voltage drops, which may cause the temperature of the integrated circuit to be too high, causing the circuit to work abnormally.