LP3873, LP3876

SNVS220E – NOVEMBER 2002 – REVISED APRIL 2013

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If a load is connected to the IC 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 IC output. Since 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 IC at frequencies above 100 kHz is determined only by the

output capacitor(s).

In applications where the load is switching at high speed, the output of the IC may need RF isolation from the

load. It is recommended that some inductance be placed between the 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, since RFI/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 multi-layer 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.

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 1Hz bandwidth). This type of noise is usually plotted on a curve as a

function of frequency.

Total output Noise or Broad-band 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, LP3873/LP3876 achieves

low noise performance and low quiescent current operation.

The total output noise specification for LP3873/LP3876 is presented in the Electrical Characteristics table. The

Output noise density at different frequencies is represented by a curve under typical performance characteristics.

SHORT-CIRCUIT PROTECTION

The LP3873 and LP3876 are short circuit protected and in the event of a peak over-current condition, the short-

circuit control loop will rapidly drive the output PMOS pass element off. Once the power pass element shuts

down, the control loop will rapidly cycle the output on and off until the average power dissipation causes the

thermal shutdown circuit to respond to servo the on/off cycling to a lower frequency. Please refer to the section

on POWER DISSIPATION/HEATSINKING for power dissipation calculations.

ERROR FLAG OPERATION

The LP3873/LP3876 produces a logic low signal at the Error Flag pin when the output drops out of regulation

due to low input voltage, current limiting, or thermal limiting. This flag has a built in hysteresis. The timing

diagram in Figure 23 shows the relationship between the ERROR flag and the output voltage. In this example,

the input voltage is changed to demonstrate the functionality of the Error Flag.

The internal Error flag comparator has an open drain output stage. Hence, the ERROR pin should be pulled high

through a pull up resistor. Although the ERROR flag pin can sink current of 1mA, this current is energy drain

from the input supply. Hence, the value of the pull up resistor should be in the range of 10k

Ω to 1MΩ. The

ERROR pin must be connected to ground if this function is not used. It should also be noted that when the

shutdown pin is pulled low, the ERROR pin is forced to be invalid for reasons of saving power in shutdown

mode.

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