TS2951_15 Datasheet, PDF (6/9 Pages) Taiwan Semiconductor Company, Ltd – 150mA Ultra Low Dropout Voltage Regulator

TS2951
Taiwan Semiconductor
APPLICATION INFORMATION (CONTINUE)
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, but is relatively inefficient, as increasing capacitor from 1µF to 220µF only decreases the noise from
430µV to 160µVrms 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 0.01µF when doing this, the output capacitor must be increased to
3.3uF to maintain stability. These change the output noise from 430µV/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 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 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 V_OUT=4.75V). The TS2951
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.
Programming The output Voltage
The TS2951 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. There are two external resistor required for this programming. Refer to the below equation for
the programming the output voltage:
Volt=V_REF x (1+R1\R2)+I_FB * R1
The V_REF is 1.235V and I_FB is the feedback bias current, nominally -20nA. The minimum recommended load current
of 1µA forces an upper limit of 1.2MΩon value of R2, If no load presented the I_FB produces error of typically 2% in
V_OUT 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 12µA. The TS2951
typically draw 60µA at no load with pin 2 open-circuit, and this value does not look much.
Document Number: DS_P0000207
Error Output Timing
6
Version: G15