lp3855et-adj National Semiconductor Corporation, lp3855et-adj Datasheet - Page 9
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lp3855et-adj
Manufacturer Part Number
lp3855et-adj
Description
1.5a Fast Response Ultra Low Dropout Linear Regulators
Manufacturer
National Semiconductor Corporation
Datasheet
1.LP3855ET-ADJ.pdf
(16 pages)
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Part Number:
lp3855et-adj/NOPB
Manufacturer:
NS/国半
Quantity:
20 000
Application Hints
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the resistors R1 and R2 (see
Typical Application Circuit). The output is also dependent on
the reference voltage (typically 1.216V) which is measured at
the ADJ pin. The output voltage is given by the equation:
This equation does not include errors due to the bias current
flowing in the ADJ pin which is typically about 10 nA. This error
term is negligible for most applications. If R1 is > 100kΩ , a
small error may be introduced by the ADJ bias current.
The tolerance of the external resistors used contributes a sig-
nificant error to the output voltage accuracy, with 1% resistors
typically adding a total error of approximately 1.4% to the out-
put voltage (this error is in addition to the tolerance of the
reference voltage at V
TURN-ON CHARACTERISTICS FOR OUTPUT VOLTAGES
PROGRAMMED TO 2.0V OR BELOW
As Vin increases during start-up, the regulator output will track
the input until Vin reaches the minimum operating voltage
(typically about 2.2V). For output voltages programmed to
2.0V or below, the regulator output may momentarily exceed
its programmed output voltage during start up. Outputs pro-
grammed to voltages above 2.0V are not affected by this
behavior.
EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are re-
quired to assure stability. these capacitors must be correctly
selected for proper performance.
INPUT CAPACITOR: An input capacitor of at least 10µF is
required. Ceramic or Tantalum may be used, and capacitance
may be increased without limit
OUTPUT CAPACITOR: An output capacitor is required for
loop stability. It must be located less than 1 cm from the device
and connected directly to the output and ground pins using
traces which have no other currents flowing through them
(see PCB Layout section).
The minimum amount of output capacitance that can be used
for stable operation is 10µF. For general usage across all load
currents and operating conditions, the part was characterized
V
OUT
C
Load Transient Response
C
OUT
IN
= V
ADJ
= 2 x 10µF CERAMIC
= 2 x 10µF CERAMIC
ADJ
).
x ( 1 + R1 / R2)
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using a 10µF Tantalum input capacitor. The minimum and
maximum stable ESR range for the output capacitor was then
measured which kept the device stable, assuming any output
capacitor whose value is greater than 10µF (see Figure 1 be-
low).
FIGURE 1. ESR Curve for C
It should be noted that it is possible to operate the part with
an output capacitor whose ESR is below these limits, assum-
ing that sufficient ceramic input capacitance is provided. This
will allow stable operation using ceramic output capacitors
(see next section).
OPERATION WITH CERAMIC OUTPUT CAPACITORS
LP385X voltage regulators can operate with ceramic output
capacitors if the values of the input and output capacitors are
selected appropriately. The total ceramic output capacitance
must be equal to or less than a specified maximum value in
order for the regulator to remain stable over all operating con-
ditions. This maximum amount of ceramic output capacitance
is dependent upon the amount of ceramic input capacitance
used as well as the load current of the application. This rela-
C
Load Transient Response
C
OUT
IN
= 2 x 10µF CERAMIC
= 2 x 10µF CERAMIC
Capacitor)
OUT
(with 10µF Tantalum Input
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