l4987 STMicroelectronics, l4987 Datasheet - Page 9

• Manufacturer Part Number: l4987
• Description: Very Low Drop Voltage Regulators With Inhibit And Dropout Control Flag
• Manufacturer: STMicroelectronics

6
6.1
Application hint of L4987CPT33
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:
Figure 3.
versus the input voltage and the temperature. No hysteresis is implemented; so the
response of V
The control flag is an open collector which requires an external pull-up resistor. This may be
connected to the regulator output
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 2 V
13.).
Concerning the pull-up resistor its value must be properly chosen as suggested below.
When "low" as it is possible to see in
V
V
Then 0.5 V ≥ V
The minimum value of R
R
Regarding the maximum value of R
(CMOS, TTL etc.), the transistor leakage current and the presence or not of a load on
V
The following example shows how to determine the R
no load and 10 µA (for L4987 it is the maximum value of I
Because of CMOS logic:
V
but:
V
so, the maximum value is determined by the following equation:
R
FLAG(LOW)
SUPPLY
FLAG
FLAG(HIGH)
FLAG(HIGH)
PULL(min)
PULL(MAX)
1) Low input voltage (V
2) Current limiting
3) Thermal limiting
.
is chosen by design and, thus is known, while I
to
≥ V
≤ (1/3 V
Figure 4.
= V
≥ 2/3 V
= V
OUT
SUPPLY
SUPPLY
CE
SUPPLY
and V
= 0.5 = V
SUPPLY
SUPPLY
show the typical behavior of the output voltage and the control flag
- 0.5/10 mA
- R
- R
FLAG
PULL
PULL
)/10 mA
PULL
IN
SUPPLY
, is, so, determined by the following equation:
are the same either when the V
≤ V
x 10 mA
x I
OUT
(Figure
FH
PULL
- R
+ V
≥ 2/3 V
Figure 7.
PULL
note that its value depends of the type of logic used
DROP
11.) or some other supply voltage
x I
SUPPLY
)
FL
the control flag voltage is:
PULL
FL
FH
max in the case of CMOS logic,
must be at maximum 10 mA.
) of control flag leakage current.
IN
ramps up or down.
(Figure
(Figure
12.).
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