LP3855-ADJ Datasheet, PDF (10/16 Pages) National Semiconductor (TI) – 1.5A Fast Response Ultra Low Dropout Linear Regulators

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:
V_OUT = V_ADJ x ( 1 + R1 / R2)
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
significant error to the output voltage accuracy, with 1%
resistors typically adding a total error of approximately 1.4%
to the output voltage (this error is in addition to the tolerance
of the reference voltage at V_ADJ).
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 pro-
grammed to 2.0V or below, the regulator output may mo-
mentarily exceed its programmed output voltage during start
up. Outputs programmed 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 capaci-
tance 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 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 below).
20074370
FIGURE 1. ESR Curve for C_OUT (with 10µF Tantalum
Input Capacitor)
It should be noted that it is possible to operate the part with
an output capacitor whose ESR is below these limits, as-
suming that sufficient ceramic input capacitance is provided.
This will allow stable operation using ceramic output capaci-
tors (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
conditions. This maximum amount of ceramic output capaci-
tance is dependent upon the amount of ceramic input ca-
pacitance used as well as the load current of the application.
This relationship is shown in Figure 2, which graphs the
maximum stable value of ceramic output capacitance as a
function of ceramic input capacitance for load currents of
1.5A.
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20074395
FIGURE 2. Maximum Ceramic Output Capacitance vs
Ceramic Input Capacitance
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