TS2950/TS2951

150mA Ultra Low Dropout Voltage Regulator

4/10

Version: A07

Application Hints

External Capacitors

For the stability of the TS2950/A & TS2951/A is requires a 1.0uF(0.22uF is recommended for 3.0V & 3.3V) or greater

capacitor between output and ground. Oscillation could occur without this capacitor. The tantalum or aluminum

electrolytic works fine; could use the film type work but are not cost efficient. For the operation of below -25

tantalum is recommended since the many aluminum type have electrolytes the freeze at about -30

about 5Ω or less and resonant frequency above 500KHz are most important parameter in the value of Capacitor. The

Capacitors value can be increased without any limit. At lower values of output current, less output capacitance is

required for stability. For the currents below 10mA the value of capacitor can be reduce to 0.33uF and 0.1uF for 1mA.

More output capacitance needed for the 8-pin version at voltage below 5V since it runs the error amplifier at lower gain.

At worst case 3.3uF or greater must be used for condition of 100mA load at 1.250V output.

The TS2950/A & TS2951/A is not like other low dropout regulators will remain stable and regulation with no load in

addition to the internal voltage divider. This feature very important in application, it is like CMOS RAM keep-alive. The

TS2951/A output voltage is set with minimum load of 1uA and external resistors.

If at the input of TS2950/A & TS2951/A connected to battery or between AC filter capacitor and input is 10 inches wire

then 1uF tantalum or aluminum electrolytic capacitor should be connected between input and ground. Instability can

occur if stray capacitor to feedback terminal pin 7 of the TS2951/A. This could cause more problems when using higher

value of external to set the output voltage. To fix this problem the 100pF capacitor between output and feedback and

increasing output capacitance to 3.3uF.

Reducing Output Noise

It could be an advantage to reduce the AC noise present at the output. One method is to reduce the regulator

bandwidth by increasing the value of the output capacitor. This is the only method that noise could reduce on the

TS2950/A & TS2951/A, but is relatively inefficient, as increasing capacitor from 1uF to 220uF only decreases the noise

from 430uV to 160uVrms for a 100KHz bandwidth at 5V output. Noise could be also reduced by fourfold by a bypass

capacitor across R1 since it reduces the high frequency gain from 4 to unity.

Pick Cbypass= 1 / 2πR1 * 200Hz or choose 0.01uF when doing this, the output capacitor must be increased to 3.3uF

to maintain stability. These change the output noise from 430uV/rms for a 100 kHz bandwidth 5V, 3.3V & 3V output.

With the bypass capacitor added, noise no longer scales with output voltage so that improvements are more dramatic

higher output voltage.

Error Detection Comparator Output

The Compactor produces a logic low output whenever the TS2951/A output falls of regulation by more than around 5%.

This is around 60 mV offset divided by the 1.235 reference voltage. Thus trip remains 5% below nominal regardless of

the programmed output voltage of the regulator. The Figure 1 shows the timing diagram depicting the ERROR signal

and the regulator output voltage as the TS2951/A input is ramped up and down. The ERROR signal becomes low at

around 1.3V input,it goes high around 5V input (input voltage at which Vout=4.75V). The TS2951/A dropout voltage

depending on the load, the input voltage trip point around 5V will vary with load current. The output voltage trip point is

around 4.75V, it does not change with load. The error comparator has an open-collector output which requires an

external pull-up resistor. Depending on the system requirement, the resistor maybe returned to 5 or 3.3V output or

other supply voltage depending to the system requirements. For determining the size of the resistor, note that the

output is sinking 400uA, this value adds to battery drain in a low better condition, recommended values 100K to 1MΩ. If

the output is unused this resistor is not required.