8.2.2.1 External Capacitors

Like any low-dropout regulator, external capacitors are required to assure stability. these capacitors must be correctly selected for proper performance.

8.2.2.2 Selecting a Capacitor

It is important to note that capacitance tolerance and variation with temperature must be taken into consideration when selecting a capacitor so that the minimum required amount of capacitance is provided over the full operating temperature range. In general, a good Tantalum capacitor will show very little capacitance variation with temperature, but a ceramic may not be as good (depending on dielectric type). Aluminum electrolytics also typically have large temperature variation of capacitance value.

Equally important to consider is a capacitor's ESR change with temperature: this is not an issue with ceramics, as their ESR is extremely low. However, it is very important in Tantalum and aluminum electrolytic capacitors. Both show increasing ESR at colder temperatures, but the increase in aluminum electrolytic capacitors is so severe they may not be feasible for some applications (see the Capacitor Characteristics section).

8.2.2.3 Capacitor Characteristics

8.2.2.4 RFI and EMI Susceptibility

Radio frequency interference (RFI) and electromagnetic interference (EMI) can degrade the performance of any integrated circuit because of the small dimensions of the geometries inside the device. In applications where circuit sources are present which generate signals with significant high-frequency energy content (> 1 MHz), care must be taken to ensure that this does not affect the IC regulator.

If RFI and EMI noise is present on the input side of the LP396x regulator (such as applications where the input source comes from the output of a switching regulator), good ceramic bypass capacitors must be used at the input pin of the LP396x.

If a load is connected to the LP396x output which switches at high speed (such as a clock), the high-frequency current pulses required by the load must be supplied by the capacitors on the LP396x output. Because the bandwidth of the regulator loop is less than 100 kHz, the control circuitry cannot respond to load changes above that frequency. The means the effective output impedance of the LP396x at frequencies above 100 kHz is determined only by the output capacitors.

In applications where the load is switching at high speed, the output of the LP396x may need RF isolation from the load. It is recommended that some inductance be placed between the LP396x output capacitor and the load, and good RF bypass capacitors be placed directly across the load.

PCB layout is also critical in high-noise environments, because RFI and EMI is easily radiated directly into PC traces. Noisy circuitry should be isolated from clean circuits where possible, and grounded through a separate path. At MHz frequencies, ground planes begin to look inductive and RFI/EMI can cause ground bounce across the ground plane.

In multilayer PCB applications, care should be taken in layout so that noisy power and ground planes do not radiate directly into adjacent layers which carry analog power and ground.

8.2.2.5 Output Adjustment

An adjustable output device has output voltage range of 1.216 V to 5.1 V. To obtain a desired output voltage, the following equation can be used with R1 always a 10-kΩ resistor.

Equation 3.

For output stability, C_F must be between 68 pF and 100 pF.

8.2.2.6 Turnon Characteristics for Output Voltages Programmed to 2.0 V or Below

As V_IN increases during start-up, the regulator output will track the input until V_IN reaches the minimum operating voltage (typically about 2.2 V). For output voltages programmed to 2 V or below, the regulator output may momentarily exceed its programmed output voltage during start up. Outputs programmed to voltages above 2 V are not affected by this behavior.

8.2.2.7 Output Noise

Noise is specified in two ways:

• Spot noise or output noise density is the RMS sum of all noise sources, measured at the regulator output, at a specific frequency (measured with a 1-Hz bandwidth). This type of noise is usually plotted on a curve as a function of frequency.
• Total output noise or broadband noise is the RMS sum of spot noise over a specified bandwidth, usually several decades of frequencies.

Attention should be paid to the units of measurement. Spot noise is measured in units µV/√Hz or nV/√Hz and total output noise is measured in µV_(rms).

The primary source of noise in low-dropout regulators is the internal reference. In CMOS regulators, noise has a low-frequency component and a high-frequency component, which depend strongly on the silicon area and quiescent current. Noise can be reduced in two ways: by increasing the transistor area or by increasing the current drawn by the internal reference. Increasing the area will decrease the chance of fitting the die into a smaller package. Increasing the current drawn by the internal reference increases the total supply current (ground pin current). Using an optimized trade-off of ground pin current and die size, LP396x achieves low noise performance and low quiescent current operation.

The total output noise specification for LP396x is presented in the Electrical Characteristics table. The output noise density at different frequencies is represented by a curve under typical performance characteristics.

8.2.2.8 Shutdown Operation

A CMOS logic level signal at the shutdown (SD) pin will turnoff the regulator. Pin SD must be actively terminated through a 10-kΩ pullup resistor for a proper operation. If this pin is driven from a source that actively pulls high and low (such as a CMOS rail-to-rail comparator), the pull-up resistor is not required. This pin must be tied to the IN pin if not used.

8.2.2.9 Maximum Output Current Capability

LP3961 and LP3964 can deliver a continuous current of 800 mA over the full operating temperature range. A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. Under all possible conditions, the junction temperature must be within the range specified under operating conditions. The total power dissipation of the device is given by:

where I_GND is the operating ground current of the device (specified under the Electrical Characteristics table).

The maximum allowable temperature rise (T_Rmax) depends on the maximum ambient temperature (T_Amax) of the application, and the maximum allowable junction temperature (T_Jmax):

The maximum allowable value for junction to ambient Thermal Resistance, R_θJA, can be calculated using the formula:

LP3961 and LP3964 are available in TO-220, SFM/TO-263, and SOT-223 packages. The thermal resistance depends on amount of copper area or heat sink, and on air flow.