IC-WD datasheet(9/12 Pages) ICHAUS

9 / 12 page

iC-WD A/B/C

SWITCHED-MODE DUAL VOLTAGE REGULATOR

Rev D1, Page 9/12

APPLICATIONS INFORMATION

DIMENSIONING

The size of shunt RVB determines the cut-off current

Ioff . By varying this in combination with the value for

the inductor LVH , the power input, the efﬁciency and

the timing can be adapted to the application.

Normally the supply voltage range and the maximum

output current for VCC and VCCA is speciﬁed. Deﬁne

whether or not only intermittent ﬂow is desired. The

maximum inductance LVH can be estimated on the fol-

lowing basis: In the worst situation, charging and dis-

charging process last exactly one period, which is the

case at minimum supply power. The cut-off current ad-

justs to Ioff = 2 × IL

max (VH ). From equation (1) it follows

that:

LVH

max =

Tmin

2IL

max (VH )

VBmin−Vsat −VH

VH+VD

Using equation (3) it is possible to determine the maxi-

mum cut-off current for intermittent ﬂow. The maximum

value for VB must be inserted:

Ioff

max =

2 · IL

max (VH )

Tmax

LVH

VBmax −Vsat −VH

VH+VD

The shunt RVB can be dimensioned with this informa-

tion. ∆VRmax can be obtained from Fig. 2:

RVB =

∆

max

Ioff

max

EXAMPLE

Speciﬁed are: VB = 18...36 V, IL

max = 100 mA; the maximum inductance can be estimated to:

LVH

max =

1/125 kHz

200 mA

18 V −1.1 V −7.0 V +

7.0 V +1.1 V

= 178 µH

The inductance selected is 150 µH, for example. Consequently, the maximum required cut-off current and the

shunt are found to be:

Ioff

max =

2 · 100 mA

1/75 kHz

150 µH

30 V −1.1 V −7 V +

7 V +1.1 V

= 324 mA

⇒ RVB =

400 mV

324 mA

≈ 1.2 Ω

It is not always possible to dimension the circuit for in-

termittent ﬂow, particularly not when high output cur-

rents are required with a low supply voltage. Permit-

ting continuous ﬂow may prove conducive to higher

efﬁciency and less interference. The inductance se-

lected is to be higher than in the above formula; the

equations for maximum cut-off current and the shunt

can be used with the selected coil.

It is simplest to ascertain the correct dimensioning

by experiment in a test set-up (Evaluation Board).

The dimensioning shown in the block diagram (LVH

= 220 µH, RVB = 1 Ω is suitable for maximum perfor-

mance throughout the entire speciﬁcation range.

SELECTING THE COMPONENTS

Since the coil must not to become saturated, it should

be designed for maximum cut-off current. This can be

checked by testing the coil current with a current probe:

In the event of saturation the current rises much more

sharply than with low currents. A low internal resis-

tance of the coil reduces the losses and increases the

regulator’s efﬁciency. When the supply voltage is low,

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