L4987 datasheet(8/11 Pages) STMICROELECTRONICS

8 / 11 page

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 (VIN

≤ VOUT +VDROP)

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 VOUT and VFLAG

are the same either when the VIN 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 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:

VFLAG(LOW) =VCE = 0.5 = VSUPPLY -RPULL xIFL

VSUPPLY is chosen by design and, thus is known,

while IFL must be at maximum 10mA. Then

0.5V

≥ VSUPPLY -RPULL x 10mA

The minimum value of RPULL, is, so, determined

by the following equation:

RPULL(min)

≥ VSUPPLY -

0.5

Regarding the maximum value of RPULL note that

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 VFLAG.

The following example shows how to determine

the RPULL max in the case of CMOS logic, no

load and 10

µA (for L4978 it is the maximum value

of IFH) of control flag leakage current.

Becaause of CMOS logic:

VFLAG(HIGH)

≥

VSUPPLY

But:

VFLAG(HIGH) =VSUPPLY -RPULL xIFH

≥

VSUPPLY

so, the maximum value is determined by the

following equation:

RPULL(MAX)

≤

VSUPPLY

Figure 1: Output and Flag Voltage vs Input

Figure 2: Flag Voltage vs Input

L4987 SERIES

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