LP38501-ADJ_15 Datasheet, PDF (16/27 Pages) Texas Instruments – LP3850x-ADJ, LP3850xA-ADJ 3-A FlexCap Low Dropout Linear Regulator for 2.7-V to 5.5-V Inputs

LP38501-ADJ, LP38503-ADJ
SNVS522I – AUGUST 2007 – REVISED AUGUST 2015
www.ti.com
8.2.2 Detailed Design Procedure
8.2.2.1 External Capacitors
The LP38501-ADJ and LP38503-ADJ require that at least 10-µF (±20%) capacitors be used at the input and
output pins located within one cm of the device. Larger capacitors may be used without limit on size for both C_IN
and C_OUT. Capacitor tolerances such as temperature variation and voltage loading effects must be considered
when selecting capacitors to ensure that they provide the minimum required amount of capacitance under all
operating conditions for the application.
In general, ceramic capacitors are best for noise bypassing and transient response because of their ultra low
ESR. It must be noted that if ceramics are used, only the types with X5R or X7R dielectric ratings must be used
(never Z5U or Y5F). Capacitors which have the Z5U or Y5F characteristics have a drop in capacitance of as
much as 50% if their temperature increases from 25°C to 85°C. In addition, the capacitance drops significantly
with applied voltage: a typical Z5U or Y5F capacitor can lose as much as 60% of its rated capacitance if only half
of the rated voltage is applied to it. For these reasons, only X5R and X7R ceramics must be used.
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 meets 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 assures 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 gives improved load transient response.
8.2.2.4 Setting The Output Voltage
The output voltage of the LP38501-ADJ and LP38503-ADJ can be set to any value between 0.6 V and 5 V using
two external resistors shown as R1 and R2 in Figure 25.
Figure 25. Setting Output Voltage
The value of R2 must 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)
16
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LP38501-ADJ LP38503-ADJ