IC-WDCEVALWD2D datasheet(8/12 Pages) ICHAUS

8 / 12 page

iC-WD A/B/C

SWITCHED-MODE DUAL VOLTAGE REGULATOR

Rev D1, Page 8/12

Ioff

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 − Vsat )/RLVH < Ioff . The switching transis-

tor is switched on continuously and VH reaches: VH =

VB − Vsat − I(VH) × RLVH . 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

VH to VCC or VCCA the size of standby capacitor

CVH 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 CVH, 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 CVCC and CVCCA 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

Pv = I(NER) × (VB − Vfw (LED))

(5)

must be taken into account. A resistor RLED 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.

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