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參數(shù)資料
| 型號(hào)： | L4987 |
| 廠商： | 意法半導(dǎo)體 |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | VERY LOW DROP VOLTAGE REGULATORS WITH INHIBIT AND DROPOUT CONTROL FLAG |
| 中文描述： | 極低降穩(wěn)壓器與抑制和差型控制標(biāo)志 |
| 文件頁數(shù)： | 9/13頁 |
| 文件大?。?/td> | 276K |
| 代理商： | L4987 |
L4987 SERIES
9/13
APPLICATION HINT OF L4987CPT30
HOW TO USE THE CONTROL FLAG
The flag produces a logic "low" whenever the output drops out of regulation. An "out of regulation
condition can result from:
1) Low Input Voltage (V
IN
≤
V
OUT
+ V
DROP
)
2) Current Limiting
3) Thermal Limiting
Figure 1 to 2 show the typical behavior of the output voltage and the control flag versus the input voltage
and the temperature. No hysteresis is implemented; so the response of V
OUT
and V
FLAG
are the same
either when the V
IN
ramps up or down.
The control flag is an open collector which requires an external pull-up resistor. This may be connected to
the regulator output (Figure 9) or some other supply voltage (Figure 10).
Using the regulator output prevents an invalid "high" on the flag which occurs if it is pulled up to an
external voltage while the regulator input voltage is reduced below about 2V (Figure 12).
Concerning the pull-up resistor its value must be properly chosen as suggested below. When "low" as it
is possible to see in figure 6 the control flag voltage is:
V
FLAG(LOW)
= V
CE
= 0.5 = V
SUPPLY
- R
PULL
x I
FL
V
SUPPLY
is chosen by design and, thus is known, while I
FL
must be at maximum 10mA.
Then 0.5V
≥
V
SUPPLY
- R
PULL
x 10mA
The minimum value of R
PULL
, is, so, determined by the following equation:
R
PULL(min)
≥
V
SUPPLY
- 0.5/10 mA
Regarding the maximum value of R
PULL
note that its value depends of the type of logic used (CMOS, TTL
etc.), the transistor leakage current and the presence or not of a load on V
FLAG
.
The following example shows how to determine the R
PULL
max in the case of CMOS logic, no load and
10μA (for L4978 it is the maximum value of I
FH
) of control flag leakage current.
Because of CMOS logic:
V
FLAG(HIGH)
≥
2/3 V
SUPPLY
But:
V
FLAG(HIGH)
= V
SUPPLY
- R
PULL
x I
FH
≥
2/3 V
SUPPLY
so, the maximum value is determined by the following equation:
R
PULL(MAX)
≤ (
1/3 V
SUPPLY
)/10 A
Figure 7 :
Output and Flag Voltage vs Input
Figure 8 :
Flag Voltage vs Input
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