Texas Instruments Evaluation Board for the LP3878-ADJ LP3878-ADJEVAL/NOPB LP3878-ADJEVAL/NOPB Datenbogen
Produktcode
LP3878-ADJEVAL/NOPB
0.001
0.01
0.1
1
10
ESR
(
:
)
0
200
400
600
800
LOAD CURRENT (mA)
STABLE REGION
SNVS311B – MAY 2005 – REVISED APRIL 2013
Figure 31. Stable Region For Output Capacitor ESR
NOTE
Important: The output capacitor must maintain its ESR within the stable region over the
full operating temperature range of the application to assure stability.
full operating temperature range of the application to assure stability.
The output capacitor ESR forms a zero which is required to add phase lead near the loop gain crossover
frequency, typically in the range of 50kHz to 200 kHz. The ESR at lower frequencies is of no importance. Some
capacitor manufacturers list ESR at low frequencies only, and some give a formula for Dissipation Factor which
can be used to calculate a value for a term referred to as ESR. However, since the DF formula is usually at a
much lower frequency than the range listed above, it will give an unrealistically high value. If good quality X5R or
X7R ceramic capacitors are used, the actual ESR in the 50 kHz to 200 kHz range will not exceed 25 milli Ohms.
If these are used as output capacitors for the LP3878-ADJ, the regulator stability requirements are satisfied.
frequency, typically in the range of 50kHz to 200 kHz. The ESR at lower frequencies is of no importance. Some
capacitor manufacturers list ESR at low frequencies only, and some give a formula for Dissipation Factor which
can be used to calculate a value for a term referred to as ESR. However, since the DF formula is usually at a
much lower frequency than the range listed above, it will give an unrealistically high value. If good quality X5R or
X7R ceramic capacitors are used, the actual ESR in the 50 kHz to 200 kHz range will not exceed 25 milli Ohms.
If these are used as output capacitors for the LP3878-ADJ, the regulator stability requirements are satisfied.
It is important to remember that capacitor tolerance and variation with temperature must be taken into
consideration when selecting an output capacitor so that the minimum required amount of output capacitance is
provided over the full operating temperature range. (See
consideration when selecting an output capacitor so that the minimum required amount of output capacitance is
provided over the full operating temperature range. (See
The output capacitor must be located not more than 0.5" from the output pin and returned to a clean analog
ground.
ground.
NOISE BYPASS CAPACITOR:
The 10 nF capacitor on the Bypass pin significantly reduces noise on the regulator output and is required for loop
stability. However, the capacitor is connected directly to a high-impedance circuit in the bandgap reference.
stability. However, the capacitor is connected directly to a high-impedance circuit in the bandgap reference.
Because this circuit has only a few microamperes flowing in it, any significant loading on this node will cause a
change in the regulated output voltage. For this reason, DC leakage current through the noise bypass capacitor
must never exceed 100 nA, and should be kept as low as possible for best output voltage accuracy.
change in the regulated output voltage. For this reason, DC leakage current through the noise bypass capacitor
must never exceed 100 nA, and should be kept as low as possible for best output voltage accuracy.
The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic
capacitors with either NPO or COG dielectric typically have very low leakage. 10 nF polypropolene and
polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low
leakage current.
capacitors with either NPO or COG dielectric typically have very low leakage. 10 nF polypropolene and
polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low
leakage current.
Copyright © 2005–2013, Texas Instruments Incorporated
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