Table 2. Design Parameters

9.2.2.1 Setting the Output Voltage

The TL5209 develops a 1.242-V reference voltage, V_REF, between the output and the adjust terminal. As shown in Figure 40, this voltage is applied across resistor R1 to generate a constant current. The current I_ADJ from the ADJ terminal could introduce DC offset to the output. Because, this offset is very small (about 50 nA), it can be ignored. The constant current then flows through the output set resistor R2 and sets the output voltage to the desired level. Equation 1 is used for calculating VOUT:

With an R1 resistance of 100 kΩ and a desired output voltage of 3.3 V, the value for R2 can be calculated:

Therefore, with an R2 resistance of 165.7 kΩ, the output voltage can be set to 3.3 V.

The TL5209 adjustable output should not be adjusted above 6.75 V ± 10% due to the internal Zener diode clamping the output voltage above 6.75 V. Although I_ADJ is very small, R2 should be limited to less than 470 kΩ for optimum performance.

9.2.2.2 Input Capacitor

If the input of the regulator is located more than ten inches from the power-supply filter, or if a battery is used to power the regulator, TI recommends a minimum 1-μF input capacitor.

9.2.2.3 Output Capacitor

As with all PNP regulators, an output capacitor is needed for stability. The required minimum size of this output capacitor depends on several factors, one of which is whether a bypass capacitor is used.

• With no bypass capacitor, TI recommends a minimum C_OUT of 1 μF.
• With a bypass capacitor of 470 pF (see Figure 40), TI recommends a minimum C_OUT of 2.2 μF.
• Larger values of C_OUT are beneficial, because they improve the regulator transient response.

Another factor that can determine the minimum size of the output capacitor is the load current. At low loads, a smaller output capacitor is needed for stability.

The equivalent series resistance (ESR) of the output capacitor also can affect regulator stability. C_OUT should have an ESR of ?1 Ω, and it should have a resonant frequency greater than 1 MHz. Too low of an ESR can cause the output to have a low-amplitude oscillation and/or underdamped transient response. Most tantalum or aluminum electrolytic capacitors can be used for the output capacitors. However, care must be taken at low temperatures, because aluminum electrolytics use electrolytes that can freeze at low temperature (? –30°C). Solid tantalum capacitors do not exhibit this problem and should be used below –25°C.

9.2.2.4 Bypass Capacitor

An optional bypass capacitor, C_BYP, can be externally connected to the regulator through the BYP pin for improved noise performance. Connected to the internal voltage divider and the error amplifier of the regulator, this bypass capacitor filters the noise of the internal reference and reduces the noise effects on the error amplifier. The overall result is a significant drop in output noise of the regulator. TI recommends a 470-pF bypass capacitor.

Adding a bypass capacitor has several effects on the regulator that must be taken into account. First, the bypass capacitor reduces the phase margin of the regulator and, thus, the minimum C_OUT needs to be increased to 2.2 μF, as previously mentioned. Second, upon start-up of the regulator, the bypass capacitor has an effect on the regulator turnon time. If a slow ramp-up of the output is needed, larger values of C_BYP should be used. Conversely, if a fast ramp-up of the output is needed, use a smaller C_BYP or none at all.

If a bypass capacitor is not needed, BYP should be left open.