iC-WD
A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 8/12
Ioff
VH to VCC or VCCA the size of standby capacitor
C
VH
should be increased for this type of operation (e.g.
22 µF).
SERIES REGULATORS VCC and VCCA
To obtain the lowest possible interference voltage even
with the small smoothing capacitor C
VH
, two inde-
pendent series regulators with a NPN emitter follower
stage are connected downstream of intermediate volt-
age VH. The Output voltages VCC or VCCA are con-
stant ±5%. The suppression of interference voltage for
the output voltages is best when VH is also no lower
than 6.0 V dynamically (iC-WDA: 4.3 V).
The series regulators are compensated internally,
hence they are stable during no-load operation, with-
out external capacitance. Stability over the entire load
range is ensured by the minimum capacitance values
for C
VCC
and C
VCCA
given in the electrical character-
istics. Current-limited outputs are used as protection
against destruction in the event of a short circuit.
FAULT EVALUATION
The two output voltages VCC and VCCA are moni-
tored. When the voltage drops below the undervoltage
threshold (due to overload, etc.), a message is sent
to the current-limited open-collector output NER (ac-
tive low). The chip temperature is also monitored. In
the event of overtemperature the switching regulator is
turned off and it is not enabled against until the chip
temperature has decreased. This thermal shutdown
of the regulator is indicated by NER = low. Since the
fault output NER is current-limited, an LED can be con-
nected directly for the optical message display, how-
ever the additional power dissipation which occurs
VB
VH
VHL
0
I(LVH)
0
t
r
T = 1/f
osz
Figure 7: Continous flow
SWITCHING REGULATOR: Operation at low supply
voltage
A third operating state occurs when the supply volt-
age VB is scarcely higher than VH. The cut-off cur-
rent can no longer be reached in this case since:
(VB
−
VH
−
V
sat
)/R
LVH
<
I
off
. The switching transis-
tor is switched on continuously and VH reaches:
VH
=
VB
−
V
sat
−
I(VH)
×
R
LVH
. Factoring in this special
feature makes it possible to operate the iC-WD even
at low supply voltage. Operability is still guaranteed
at VB
≈
7.6 V. Nonetheless, the maximum current-
carrying capacity depends on the coil’s internal re-
sistance and supply voltage VB. The transition from
regulator mode to continuously activated transistor is
fluid. To avoid feedback of interference voltage from
P
v
=
I(NER)
×
(VB
−
V
fw
(LED))
(5)
must be taken into account. A resistor R
LED
in se-
ries with the LED can reduce the additional chip power
dissipation in the event of a fault. CMOS- or TTL-
compatible logic inputs can be activated with a pull-up
resistor at NER.
ICHAUS [ IC-HAUS GMBH ]
