LP2950-N, LP2951-N
SNVS764Q –JANUARY 2000–REVISED DECEMBER 2017
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9.2 Typical Applications
9.2.1 1-A Regulator with 1.2-V Dropout
Figure 36. 1-A Regulator with 1.2-V Dropout
9.2.1.1 Design Requirements
For this design example, use the parameters listed in Table 1 as the input parameters.
Table 1. Design Parameters
DESIGN PARAMETER
Input voltage
DESIGN REQUIREMENT
6.5 V, ±10%, provided by the DC-DC converter switching at 1 MHz
Output voltage
5 V, ±1%
100 mA (maximum), 1 mA (minimum)
< 200 µVRMS
Output current
RMS noise, 10 Hz to 100 kHz
PSRR at 1 KHz
> 50 dB
9.2.1.2 Detailed Design Procedure
At 100-mA loading, the dropout of the LP2950-N/LP2951-N has 600 mV maximum dropout over temperature,
thus an 1500-mV headroom is sufficient for operation over both input and output voltage accuracy. The efficiency
of the LP2950-N/LP2951-N in this configuration is VOUT / VIN = 76.9%. To achieve the smallest form factor, the
TO-92 package is selected. Input and output capacitors are selected in accordance with the Capacitor
Recommendation section. Ceramic capacitances of 1 µF for the input and one 2.2-µF capacitors for the output
are selected. With an efficiency of 73.3% and a 100-mA maximum load, the internal power dissipation is 150
mW, which corresponds to a 18.9°C junction temperature rise for the TO-92 package. With an 85°C maximum
ambient temperature, the junction temperature is at 103.9°C. To minimize noise, a bypass capacitance (CBYPASS
)
of 0.01-µF is selected between pin 7 to pin 1 for LP2951-N.
9.2.1.2.1 Output Capacitor Requirements
A 1-µF (or greater) capacitor is required between the output and ground for stability at output voltages of 5 V or
higher. At lower output voltages, more capacitance is required (2.2 µF or more is recommended for 3-V and
3.3-V versions). Without this capacitor the device oscillates. Most types of tantalum or aluminum electrolytic work
fine here; even film types work but are not recommended for reasons of cost. Many aluminum electrolytics have
electrolytes that freeze at about −30°C, so solid tantalums are recommended for operation below −25°C. The
important parameters of the capacitor are an ESR of about 5 Ω or less and a resonant frequency above 500 kHz.
The value of this capacitor may be increased without limit.
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