Atec Superior-Electric-L116b - Manual (Page 13)

Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are required to assure stability. These capacitors must be correctly selected
for proper performance.
INPUT CAPACITOR: An input capacitor of at least 1µF is required (ceramic recommended). The capacitor must be located not
more than one centimeter from the input pin and returned to a clean analog ground.
OUTPUT CAPACITOR: An output capacitor is required for loop stability. It must be located less than 1 centimeter from the device
and connected directly to the output and ground pins using traces which have no other currents flowing through them.
The minimum amount of output capacitance that can be used for stable operation is 1µF. Ceramic capacitors are recommended
(the LP38690/2 was designed for use with ultra low ESR capacitors). The LP38690/2 is stable with any output capacitor ESR
between zero and 100 Ohms.
ENABLE PIN (LP38692 only): The LP38692 has an Enable pin (EN) which allows an external control signal to turn the regulator
output On and Off. The Enable On/Off threshold has no hysteresis. The voltage signal must rise and fall cleanly, and promptly,
through the ON and OFF voltage thresholds. The Enable pin has no internal pull-up or pull-down to establish a default condition
and, as a result, this pin must be terminated either actively or passively. If the Enable pin is driven from a source that actively pulls
high and low, the drive voltage should not be allowed to go below ground potential or higher than V
IN
. If the application does not
require the Enable function, the pin should be connected directly to the V
IN
pin.
Foldback Current Limiting: Foldback current limiting is built into the LP38690/2 which reduces the amount of output current the
part can deliver as the output voltage is reduced. The amount of load current is dependent on the differential voltage between
V
IN
and V
OUT
. Typically, when this differential voltage exceeds 5V, the load current will limit at about 450 mA. When the V
IN
-
V
OUT
differential is reduced below 4V, load current is limited to about 1500 mA.
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.
Capacitor Characteristics
CERAMIC
For values of capacitance in the 10 to 100 µF range, ceramics are usually larger and more costly than tantalums but give superior
AC performance for bypassing high frequency noise because of very low ESR (typically less than 10 m
Ω). However, some dielectric
types do not have good capacitance characteristics as a function of voltage and temperature.
Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or Y5V capacitor can
lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V also exhibit a severe temperature
effect, losing more than 50% of nominal capacitance at high and low limits of the temperature range.
X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically maintain a capacitance
range within ±20% of nominal over full operating ratings of temperature and voltage. Of course, they are typically larger and more
costly than Z5U/Y5U types for a given voltage and capacitance.
TANTALUM
Solid Tantalum capacitors have good temperature stability: a high quality Tantalum will typically show a capacitance value that
varies less than 10-15% across the full temperature range of -40°C to +125°C. ESR will vary only about 2X going from the high to
low temperature limits.
REVERSE VOLTAGE
A reverse voltage condition will exist when the voltage at the output pin is higher than the voltage at the input pin. Typically this will
happen when V
IN
is abruptly taken low and C
OUT
continues to hold a sufficient charge such that the input to output voltage becomes
reversed. A less common condition is when an alternate voltage source is connected to the output.
There are two possible paths for current to flow from the output pin back to the input during a reverse voltage condition.
1) While V
IN
is high enough to keep the control circuity alive, and the Enable pin (LP38692 only) is above the V
EN(ON)
threshold,
the control circuitry will attempt to regulate the output voltage. If the input voltage is less than the programmed output voltage, the
control circuit will drive the gate of the pass element to the full ON condition. In this condition, reverse current will flow from the
output pin to the input pin, limited only by the R
DS(ON)
of the pass element and the output to input voltage differential. Discharging
an output capacitor up to 1000
μF in this manner will not damage the device as the current will rapidly decay. However, continuous
reverse current should be avoided. When the Enable pin is low this condition will be prevented.
2) The internal PFET pass element has an inherent parasitic diode. During normal operation, the input voltage is higher than the
output voltage and the parasitic diode is reverse biased. However, when V
IN
is below the value where the control circuity is alive,
or the Enable pin is low (LP38692 only), and the output voltage is more than 500 mV (typical) above the input voltage the parasitic
diode becomes forward biased and current flows from the output pin to the input pin through the diode. The current in the parasitic
diode should be limited to less than 1A continuous and 5A peak.
If used in a dual-supply system where the regulator output load is returned to a negative supply, the output pin must be diode
clamped to ground to limit the negative voltage transition. A Schottky diode is recommended for this protective clamp.
LP38690
LP38692
Copyright © 1999-2012, Texas Instruments Incorporated 13