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L4938E

ADVANCED VOLTAGE REGULATOR

STMicroelectronics 

L4938E - L4938ED - L4938EPD

Connecting a resistor divider R1E, R2E to the ADJ,

OUT2 pin the output voltage 2 can be pro-

grammed to the value of

VOUT2

=

VOUT 1

1



+

R1E(R2E + RADJ)

R2E ⋅ RADJ





with RADJ = 60K to 150K and VOUT1 = 4.95 to

5.05V.

For an exact calculation the temperature coeffi-

cient (Tc -2000pprm) of the internal resistor

(RADJ) must be taken into account. Pin ADJ in this

mode should not have a capacitive burden be-

cause this would reduce the phase margin of the

regulator loop.

Reset circuit

The reset circuit supervises the standby output

voltage. The reset output (RES) is defined from

VOUT ≥ 1V.

Even if VS is lacking, the reset generator is sup-

plied by the output voltage VOUT1.

The reset threshold of 4.7V is defined with the in-

ternal reference voltage (note 9) and standby out-

put divider, when pin PR is left open. The reset

threshold voltage can be programmed in the

range from 3.8V to 4.7V by connecting an exter-

nal resistor from pin PR to GND.

The value of the programming resistor RPR can

be calculated with:

RPR =

22K

4.7K

VRT

−1

−

92.9K,3.8V

≤

VRT ≤

4.7V

The reset pulse delay time tRD, is defined with the

charge time of an external capacitor CT:

tRDmin

=

CT ⋅ 0.6V

1µA

(note

5)

tRDnom

=

CT ⋅ 1.4V

1µA

(note

6)

The reaction time of the reset circuit originates

from the noise immunity. Standby output voltage

drops below the reset threshold only a bit longer

than the reaction time results in a shorter reset

delay time. The nominal reset delay time will be

generated for standby output voltage drops longer

than approximately 50µs.

The minimum rset time is generated if reset con-

dition only occures for a short time triggering a re-

set pulse but not completely discharging CT. The

reset can be related to output2 on request. If

higher charge currents for the reset capacitor are

required a resistors from Pin CT to OUT1, may be

used to increase the current. We recommended

the use of 10KΩ to 5V as an output pull up.

Sense Comparator

The sense comparator compares an input signal

with an internal voltage reference of typical 1.23V.

The use of an external voltage divider makes this

comparator very flexible in the application. It can

be used to supervise the input voltage either be-

fore or after the protection diode and to give addi-

tional information to the microprocessor like low

voltage warnings. We recommended the use of

10KΩ to 5V as an output pull up.

Note 9:

The reference is alternatively supplied from VS or VOUT1. If one supply

is present, the reference is operating.

Thermal Protection

Both outputs are provided with an overtempera-

ture shut down regulation power dissipation down

to uncritical values.

Output 2 will shut down approximately 10K before

output 1.

Under normal conditions shut down of output 2

will allow the chip to cool down again. Thus out-

put 1 will be unaffected.

The thermal shut down reduces the output volt-

ages until power dissipation and the flow of ther-

mal energy out of the chip balance.

Transient Sensitivity

In proper operation (VOUT > 4.5V) the reference is

supplied by VOUT1 thus reducing sensitivity to in-

put transients.

Precise Data will be issued as soon as samples

are available.

6/12

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