TS2951/A

150mA Ultra Low Dropout Voltage Regulator

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Version: D11

Application Hints (continued)

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

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 1M . If the output is unused this resistor is not required.

Programming The output Voltage

The TS2951/A output voltage is programmable for any value from it reference voltage of 1.235V and it maximum

rating of 30V. For example for 5 volt needs to pin-strapped and using the internal voltage divider by tying pin 1 to 2

and pin 7 to pin 6. Refer to Figure 2, there are two external resistor required for this programming. Refer to the

below equation for the programming the output voltage:

Vout which maybe eliminated at room temperature by trimming R1. To improve the accuracy chooses the value of

R2=100K and this reduces the error by 0.17% and increases the resistor program current by 12uA. The TS2951/A

typically draw 60uA at no load with pin 2 open-circuit, and this value does not look much.