6
A6155
Block Diagram
Pin Description
Pin Name
Function
1
EN
Push-pull active low enable output
2
RES
Open drain active low reset output.
RES must be pulled up to V
OUTPUT
even if unused
3
TCL
Watchdog timer clear input signal
4V
SS
GND terminal
5
INPUT
Voltage regulator input
6
OUTPUT Voltage regulator output
7R
R
EXT input for RC oscillator tuning
8V
IN
Voltage comparator input
Table 5
Functional Description
Voltage Regulator
The A6155 has a 5 V ± 2%, 100 mA, low dropout volt-
age regulator. The low supply current (typ.155 µA) mak-
es the A6155 particularly suited to automotive systems then
remain energized 24 hours a day. The input voltage range is
2.3 V to 26 V for operation and the input protection includes
both reverse battery (20 V below ground) and load dump
(positive transients up to 60 V). There is no reverse current
flow from the OUTPUT to the INPUT when the INPUT equals
V
SS. This feature is important for systems which need to
implement (with capacitance) a minimum power supply
hold-up time in the event of power failure. To achieve good
load regulation a 22 µF capacitor (or greater ) is needed on
the INPUT (see Fig.9). Tantalum or aluminium electrolytics
are adequate for the 22 µF capacitor; film types will work but
are relatively expensive. Many aluminium electrolytics have
electrolytes that freeze at about –30 °C, so tantalums are
recommended for operation below –25 °C. The important
parameters of the 22 µF capacitor are an effective series
resistance of ≤ 5 Ω and a resonant frequency above
500 kHz..
A 10 µF capacitor (or greater) and a 100 nF capacitor
are required on the OUTPUT to prevent oscillations due
to instability. The specification of the 10 µF capacitor is
as per the 22 µF capacitor on the INPUT (see previous
paragraph).
The A6155 will remain stable and in regulation with no
external load and the dropout voltage is typically con-
stant as the input voltage fall to below its minimum level
(see Table 2). These features are especially important in
CMOS RAM keep-alive applications.
Care must be taken not to exceed the maximum junction
temperature (+ 85 °C). The power dissipation within the
A6155 is given by the formula:
P
TOTAL = (VINPUT – VOUTPUT) * IOUTPUT + (VINPUT) * ISS
The maximum continuous power dissipation at a given
temperature can be calculated using the formula:
P
MAX = ( 85 °C – TA) / Rth(j-a)
where R
th(j-a) is the termal resistance from the junction to
the ambient and is specified in Table 2. Note the R
th(j-a)
given in Table 2 assumes that the package is soldered
to a PCB. The above formula for maximum power dissi-
pation assumes a constant load (ie. ≥100 s). The
transient thermal resistance for a single pulse is much
lower than the continuous value. For example the A6155
in DIP8 package will have an effective thermal resistance
from the junction to the ambient of about 10°C/W for a
single 100 ms pulse.
VIN Monitoring
The power-on reset and the power-down reset are
generated as a response to the external voltage level
applied on the V
IN input. The VDD voltage at which reset is
asserted or released is determined by the external
voltage divider between V
DD and VSS, as shown on Fig. 9.
A part of V
DD
is compared to the internal voltage
reference. To determine the values of the divider, the
leakage current at V
IN must be taken into account, as
Voltage
Regulator
Voltage
Reference
Voltage
Reference
INPUT
V
IN
R
OUTPUT
Enable
Logic
Reset
Control
Timer
V
REF
Comparator
TCL
Fig. 8
EN
RES
Current
Controlled
Oscillator
Open drain
output RES
Switch
R1∼ 1 MΩ
Controller
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