データシートサーチシステム
  Japanese  ▼
ALLDATASHEET.JP

X  

LM1084IS-3.3 データシート(PDF) 7 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1084IS-3.3
部品情報  5A Low Dropout Positive Regulators
Download  13 Pages
Scroll/Zoom Zoom In 100%  Zoom Out
メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1084IS-3.3 データシート(HTML) 7 Page - National Semiconductor (TI)

Back Button LM1084IS-3.3 Datasheet HTML 3Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 4Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 5Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 6Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 7Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 8Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 9Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 10Page - National Semiconductor (TI) LM1084IS-3.3 Datasheet HTML 11Page - National Semiconductor (TI) Next Button
Zoom Inzoom in Zoom Outzoom out
 7 / 13 page
background image
Application Note
GENERAL
Figure 1 shows a basic functional diagram for the LM1084-
Adj (excluding protection circuitry) . The topology is basically
that of the LM317 except for the pass transistor. Instead of a
Darlingtion NPN with its two diode voltage drop, the LM1084
uses a single NPN. This results in a lower dropout voltage.
The structure of the pass transistor is also known as a quasi
LDO. The advantage a quasi LDO over a PNP LDO is its
inherently lower quiescent current. The LM1084 is guaran-
teed to provide a minimum dropout voltage 1.5V over tem-
perature, at full load.
OUTPUT VOLTAGE
The LM1084 adjustable version develops at 1.25V reference
voltage, (V
REF), between the output and the adjust terminal.
As shown in figure 2, this voltage is applied across resistor
R1 to generate a constant current I1. This constant current
then flows through R2. The resulting voltage drop across R2
adds to the reference voltage to sets the desired output
voltage.
The current I
ADJ from the adjustment terminal introduces an
output error . But since it is small (120uA max), it becomes
negligible when R1 is in the 100
Ω range.
For fixed voltage devices, R1 and R2 are integrated inside
the devices.
STABILITY CONSIDERATION
Stability consideration primarily concern the phase response
of the feedback loop. In order for stable operation, the loop
must maintain negative feedback. The LM1084 requires a
certain amount series resistance with capacitive loads. This
series resistance introduces a zero within the loop to in-
crease phase margin and thus increase stability. The equiva-
lent series resistance (ESR) of solid tantalum or aluminum
electrolytic capacitors is used to provide the appropriate zero
(approximately 500 kHz).
The Aluminum electrolytic are less expensive than tantal-
ums, but their ESR varies exponentially at cold tempera-
tures; therefore requiring close examination when choosing
the desired transient response over temperature. Tantalums
are a convenient choice because their ESR varies less than
2:1 over temperature.
The recommended load/decoupling capacitance is a 10uF
tantalum or a 50uF aluminum. These values will assure
stability for the majority of applications.
The adjustable versions allows an additional capacitor to be
used at the ADJ pin to increase ripple rejection. If this is done
the output capacitor should be increased to 22uF for tantal-
ums or to 150uF for aluminum.
Capacitors other than tantalum or aluminum can be used at
the adjust pin and the input pin. A 10uF capacitor is a
reasonable value at the input. See Ripple Rejection section
regarding the value for the adjust pin capacitor.
It is desirable to have large output capacitance for applica-
tions that entail large changes in load current (microproces-
sors for example). The higher the capacitance, the larger the
available charge per demand. It is also desirable to provide
low ESR to reduce the change in output voltage:
∆V= ∆I x ESR
It is common practice to use several tantalum and ceramic
capacitors in parallel to reduce this change in the output
voltage by reducing the overall ESR.
Output capacitance can be increased indefinitely to improve
transient response and stability.
RIPPLE REJECTION
Ripple rejection is a function of the open loop gain within the
feed-back loop (refer to Figure 1 and Figure 2). The LM1084
exhibits 75dB of ripple rejection (typ.). When adjusted for
voltages higher than V
REF, the ripple rejection decreases as
function of adjustment gain: (1+R1/R2) or V
O/VREF. There-
fore a 5V adjustment decreases ripple rejection by a factor of
four (−12dB); Output ripple increases as adjustment voltage
increases.
However, the adjustable version allows this degradation of
ripple rejection to be compensated. The adjust terminal can
be bypassed to ground with a capacitor (C
ADJ). The imped-
ance of the C
ADJ should be equal to or less than R1 at the
desired ripple frequency. This bypass capacitor prevents
ripple from being amplified as the output voltage is in-
creased.
1/(2
π*f
RIPPLE*CADJ)
≤ R
1
LOAD REGULATION
The LM1084 regulates the voltage that appears between its
output and ground pins, or between its output and adjust
pins. In some cases, line resistances can introduce errors to
the voltage across the load. To obtain the best load regula-
tion, a few precautions are needed.
10094665
FIGURE 1. Basic Functional Diagram for the LM1084,
excluding Protection circuitry
10094617
FIGURE 2. Basic Adjustable Regulator
www.national.com
7


同様の部品番号 - LM1084IS-3.3

メーカー部品番号データシート部品情報
logo
Texas Instruments
LM1084IS-3.3/NOPB TI1-LM1084IS-3.3/NOPB Datasheet
2Mb / 22P
[Old version datasheet]   LM1084 5A Low Dropout Positive Regulators
More results

同様の説明 - LM1084IS-3.3

メーカー部品番号データシート部品情報
logo
Microsemi Corporation
LX8384X-XX MICROSEMI-LX8384X-XX Datasheet
142Kb / 11P
   5A Low Dropout Positive Regulators
LX8384-00CDD MICROSEMI-LX8384-00CDD Datasheet
149Kb / 11P
   5A Low Dropout Positive Regulators
LX8384X-XX MICROSEMI-LX8384X-XX_06 Datasheet
289Kb / 11P
   5A Low Dropout Positive Regulators
LX8384-XX MICROSEMI-LX8384-XX Datasheet
216Kb / 8P
   5A LOW DROPOUT POSITIVE REGULATORS
LX8554-XX MICROSEMI-LX8554-XX Datasheet
198Kb / 7P
   5A EXTREMELY LOW DROPOUT POSITIVE REGULATORS
logo
Texas Instruments
LM1084 TI1-LM1084 Datasheet
2Mb / 22P
[Old version datasheet]   LM1084 5A Low Dropout Positive Regulators
logo
Linear Technology
LT1085IT-12 LINER-LT1085IT-12 Datasheet
187Kb / 12P
   7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators
LT1083 LINER-LT1083_15 Datasheet
291Kb / 22P
   7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators
logo
Linear Integrated Syste...
LT1083 LINEAR-LT1083_1 Datasheet
142Kb / 12P
   3A, 5A, 7.5A Low Dropout Positive Fixed Regulators
logo
Linear Technology
LT1083 LINER-LT1083_05 Datasheet
203Kb / 16P
   7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators
More results


Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13


データシート ダウンロード

Go To PDF Page


リンク URL




プライバシーポリシー
ALLDATASHEET.JP
ALLDATASHEETはお客様のビジネスに役立ちますか?  [ DONATE ] 

Alldatasheetは   |   広告   |   お問い合わせ   |   プライバシーポリシー   |   リンク交換   |   メーカーリスト
All Rights Reserved©Alldatasheet.com


Mirror Sites
English : Alldatasheet.com  |   English : Alldatasheet.net  |   Chinese : Alldatasheetcn.com  |   German : Alldatasheetde.com  |   Japanese : Alldatasheet.jp
Russian : Alldatasheetru.com  |   Korean : Alldatasheet.co.kr  |   Spanish : Alldatasheet.es  |   French : Alldatasheet.fr  |   Italian : Alldatasheetit.com
Portuguese : Alldatasheetpt.com  |   Polish : Alldatasheet.pl  |   Vietnamese : Alldatasheet.vn
Indian : Alldatasheet.in  |   Mexican : Alldatasheet.com.mx  |   British : Alldatasheet.co.uk  |   New Zealand : Alldatasheet.co.nz
Family Site : ic2ic.com  |   icmetro.com