L4987CPT30 STMicroelectronics, L4987CPT30 Datasheet - Page 8

• Manufacturer Part Number: L4987CPT30
• Description: VERY LOW DROP VOLTAGE REGULATORS WITH INHIBIT AND DROPOUT CONTROL FLAG
• Manufacturer: STMicroelectronics

L4987 SERIES
APPLICATION HINT OF L4987CPT30
How to use the control flag
The flag produces a logic ”low” whenever the
output fall out out of regulation. An ”out of
regulation condition can result from:
1) Low Input Voltage (V
2) Curent Limiting
3) Thermal Limiting
Figure 1 to 2 show the typical behaviour of the
output voltage and the control flag versus the
input voltage and the temperatre. No hysteresis is
implemented; so the response of V
are the same either when the V
down.
The control flag is an open collector which
requires an external pull-up resistor. This may be
connected to the regulator output (Figure 3) or
some other supply voltage (Figure 4).
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 (Figue 5).
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
Figure 1: Output and Flag Voltage vs Input
8/11
FLAG(LOW)
= V
CE
= 0.5 = V
IN
SUPPLY
V
OUT
+ V
IN
- R
OUT
ramps up or
DROP
PULL
and V
)
x I
FL
FLAG
V
while I
0.5V V
The minimum value of R
by the following equation:
R
Regarding the maximum value of R
its value depends of the type of logic used
(CMOS, TTL etc.), the transistor leackage current
and the presence or not of a load on V
The following example shows how to determine
the R
load and 10 A (for L4978 it is the maximum value
of I
Becaause of CMOS logic:
V
But:
V
so, the maximum value is determined by the
following equation:
R
Figure 2: Flag Voltage vs Input
SUPPLY
FLAG(HIGH)
FLAG(HIGH)
PULL(min)
PULL(MAX)
FH
) of control flag leakage current.
PULL
FL
SUPPLY
is chosen by design and, thus is known,
must be at maximum 10mA. Then
max in the case of CMOS logic, no
= V
V
1
3
SUPPLY
2
3
V
SUPPLY
10 A
- R
V
SUPPLY
SUPPLY
PULL
-
10 mA
- R
x 10mA
0.5
PULL
PULL
, is, so, determined
x I
FH
PULL
2
3
FLAG
V
note that
SUPPLY
.