LP3856-ADJ_15 Datasheet, PDF (9/22 Pages) Texas Instruments – 3A Fast Response Ultra Low Dropout Linear Regulators

LP3856-ADJ
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Application Hints
SNVS243E – SEPTEMBER 2003 – REVISED APRIL 2013
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)
(1)
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 programmed to 2.0V or below, the regulator output may
momentarily 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 required 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 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 25 below).
10
1.0
STABLE REGION
COUT > 10PF
0.1
.01
.001
0
1
2
3
LOAD CURRENT (A)
Figure 25. 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,
assuming that sufficient ceramic input capacitance is provided. This will allow stable operation using ceramic
output capacitors (see next section).
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