LP2986ILD-3.3 Datasheet, PDF (13/26 Pages) Texas Instruments – Micropower, 200 mA Ultra Low-Dropout Fixed or Adjustable Voltage Regulator

LP2986
www.ti.com
SNVS137H – MARCH 1999 – REVISED APRIL 2013
Be warned that the ESR of a ceramic capacitor can be low enough to cause instability: a 2.2 µF ceramic was
measured and found to have an ESR of about 15 mΩ.
If a ceramic capacitor is to be used on the LP2986 output, a 1Ω resistor should be placed in series with the
capacitor to provide a minimum ESR for the regulator.
Another disadvantage of ceramic capacitors is that their capacitance varies a lot with temperature:
Large ceramic capacitors are typically manufactured with the Z5U temperature characteristic, which results in the
capacitance dropping by a 50% as the temperature goes from 25°C to 80°C.
This means you have to buy a capacitor with twice the minimum C_OUT to assure stable operation up to 80°C.
ALUMINUM: The large physical size of aluminum electrolytics makes them unattractive for use with the LP2986.
Their ESR characteristics are also not well suited to the requirements of LDO regulators.
The ESR of an aluminum electrolytic is higher than a tantalum, and it also varies greatly with temperature.
A typical aluminum electrolytic can exhibit an ESR increase of 50X when going from 20°C to −40°C. Also, some
aluminum electrolytics can not be used below −25°C because the electrolyte will freeze.
USING AN EXTERNAL RESISTIVE DIVIDER
The LP2986 output voltage can be programmed using an external resistive divider (see Basic Application
Circuits).
The resistor connected between the Feedback pin and ground should be 51.1k. The value for the other resistor
(R1) connected between the Feedback pin and the regulated output is found using the formula:
V_OUT = V_FB × (1 + ( R1 / 51.1k ))
(1)
It should be noted that the 25 µA of current flowing through the external divider is approximately equal to the
current saved by not connecting the internal divider, which means the quiescent current is not increased by using
external resistors.
A lead compensation capacitor (C_F) must also be used to place a zero in the loop response at about 50 kHz. The
value for C _F can be found using:
C_F = 1/(2π × R1 × 50k)
(2)
A good quality capacitor must be used for C_F to ensure that the value is accurate and does not change
significantly over temperature. Mica or ceramic capacitors can be used, assuming a tolerance of ±20% or better
is selected.
If a ceramic is used, select one with a temperature coefficient of NPO, COG, Y5P, or X7R. Capacitor types Z5U,
Y5V, and Z4V can not be used because their value varies more that 50% over the −25°C to +85°C temperature
range.
SHUTDOWN INPUT OPERATION
The LP2986 is shut off by driving the Shutdown input low, and turned on by pulling it high. If this feature is not to
be used, the Shutdown input should be tied to V_IN to keep the regulator output on at all times.
To assure proper operation, the signal source used to drive the Shutdown input must be able to swing above and
below the specified turn-on/turn-off voltage thresholds listed as V_H and V_L, respectively (see Electrical
Characteristics).
Since the Shutdown input comparator does not have hysteresis, It is also important that the turn-on (and turn-off)
voltage signals applied to the Shutdown input have a slew rate which is not less than 40 mV/µs when moving
between the V_H and V_L thresholds.
CAUTION: The regulator output state (either On or Off) can not be specified if a slow-moving AC (or DC) signal
is applied that is in the range between V_H and V_L.
Copyright © 1999–2013, Texas Instruments Incorporated
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