TL5209 Datasheet, PDF (18/22 Pages) Texas Instruments – 500-mA LOW-NOISE LOW-DROPOUT VOLTAGE REGULATOR WITH SHUTDOWN

TL5209
500-mA LOW-NOISE LOW-DROPOUT VOLTAGE REGULATOR
WITH SHUTDOWN
SLVS581 – SEPTEMBER 2006
APPLICATION INFORMATION
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Enable/Shutdown
The enable function is only available in the SOIC (D) and TO-263 (KTT) packages. The EN pin is CMOS-logic
compatible. When EN is held high (>2 V), the regulator is active. Likewise, applying a low signal (<0.4 V at
25°C) to EN or leaving it open shuts down the regulator. If the enable/shutdown feature is not needed, EN
should be tied to V_IN.
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, a minimum 1-µF input capacitor is recommended.
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, a minimum C_OUT of 1 µF is recommended.
• With a bypass capacitor of 470 pF (see Figure 6), a minimum C_OUT of 2.2 µF is recommended.
• 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 above 1 MHz. Too low 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 should be used 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.
Bypass Capacitor
An optional bypass capacitor, C_BYP, can be externally connected to the regulator via the BYP pin for improved
noise performance (only for SOIC and TO-263 packages). 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. A
470-pF bypass capacitor is recommended.
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 startup of the regulator, the bypass capacitor has an effect on the
regulator turn-on 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.
Low-Voltage Operation
When using the TL5209-18 and TL5209-25 in voltage-sensitive applications, special considerations are required.
If appropriate output and bypass capacitors are not chosen properly, these devices may experience a temporary
overshoot of their nominal voltages.
At start-up, the full input voltage is initially applied across the regulator pass transistor, causing it to be
temporarily fully turned on. By contrast, the error amplifier and voltage-reference circuits, being powered from
the output, are not powered up as fast. In order to slow down the output ramp and give the error amplifier time to
respond, select larger values of output and bypass capacitors. The longer ramp time of the output allows the
regulator enough time to respond and keeps the output from overshooting its nominal value.
To prevent an overshoot when starting up into a light load (≈100 µA), 4.7-µF and 470-pF capacitors are
recommended for the output and bypass capacitors, respectively. At higher loads, 10-µF and 470-pF capacitors
should be used.
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