LP38502-ADJ Datasheet, PDF (14/27 Pages) Texas Instruments – LP3850x-ADJ, LP3850xA-ADJ 1.5-A Flexcap Low-Dropout Linear Regulatorfor 2.7-V to 5.5-V Inputs

LP38500-ADJ, LP38502-ADJ
SNVS539G – NOVEMBER 2007 – REVISED JUNE 2015
www.ti.com
8.2.2.2 Input Capacitor
All linear regulators can be affected by the source impedance of the voltage which is connected to the input. If
the source impedance is too high, the reactive component of the source may affect the control loop’s phase
margin. To ensure proper loop operation, the ESR of the capacitor used for C_IN must not exceed 0.5 Ω. Any
good quality ceramic capacitor will meet this requirement, as well as many good quality tantalums. Aluminum
electrolytic capacitors may also work, but can possibly have an ESR which increases significantly at cold
temperatures. If the ESR of the input capacitor may exceed 0.5 Ω, it is recommended that a 2.2-µF ceramic
capacitor be used in parallel, as this will assure stable loop operation.
8.2.2.3 Output Capacitor
Any type of capacitor may be used for C_OUT, with no limitations on minimum or maximum ESR, as long as the
minimum amount of capacitance is present. The amount of capacitance can be increased without limit.
Increasing the size of C_OUT typically will give improved load transient response.
8.2.2.4 Setting The Output Voltage
The output voltage of the LP38500/2-ADJ can be set to any value between 0.6V and 5V using two external
resistors shown as R1 and R2 in Figure 19.
Figure 19. Setting Output Voltage
The value of R2 should always be less than or equal to 10 kΩ for good loop compensation. R1 can be selected
for a given V_OUT using the following formula:
V_OUT = V_ADJ (1 + R1/R2) + I_ADJ (R1)
where
• V_ADJ is the adjust pin voltage
• I_ADJ is the bias current flowing into the adjust pin
(2)
8.2.2.5 RFI/EMI Susceptibility
Radio Frequency Interference (RFI) and Electro-Magnetic Interference (EMI) can degrade any integrated circuit's
performance because of the small dimensions of the geometries inside the device. In applications where circuit
sources are present which generate signals with significant high frequency energy content (> 1 MHz), care must
be taken to ensure that this does not affect the device regulator.
If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes
from the output of a switching regulator), good ceramic bypass capacitors must be used at the input pin of the
device to reduce the amount of EMI conducted into the device.
If the LP38500, LP38502-ADJ output is connected to a load which switches at high speed (such as a clock), the
high-frequency current pulses required by the load must be supplied by the capacitors on the device output.
Since the bandwidth of the regulator loop is less than 300 kHz, the control circuitry cannot respond to load
changes above that frequency. This means the effective output impedance of the device at frequencies above
300 kHz is determined only by the output capacitor(s). Ceramic capacitors provide the best performance in this
type of application.
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