iC-WD A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 9/12
APPLICATIONS INFORMATION
DIMENSIONING
Using equation (3) it is possible to determine the maxi-
The size of shunt RVB determines the cut-off current mum cut-off current for intermittent flow. The maximum
Ioff . By varying this in combination with the value for value for VB must be inserted:
the inductor LVH, the power input, the efficiency and
the timing can be adapted to the application.
s
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×ILmax (VH). From equation (1) it follows
that:
Tmax
LVH
1
Ioff
=
2·ILmax (VH)
·
VB
max
1
1
+
−V −VH
VH+V
max
sat
D
The shunt RVB can be dimensioned with this informa-
tion. ∆VRmax can be obtained from Fig. 2:
Tmin
1
∆VR
max
LVH
=
×
2ILmax (VH)
max
RVB
=
1
1
+
Ioff
max
VB −V −VH
VH+V
sat
min
D
EXAMPLE
Specified are: VB = 18...36 V, IL
= 100 mA; the maximum inductance can be estimated to:
max
1/125kHz
200mA
1
LVH
=
·
= 178µH
max
1
1
+
18V−1.1V−7.0V
7.0V+1.1V
The inductance selected is 150 µH, for example. Consequently, the maximum required cut-off current and the
shunt are found to be:
s
1/75kHz
150µH
1
Ioff
=
2·100mA
·
= 324mA
max
1
1
+
30V−1.1V−7V
7V+1.1V
400mV
≈ 1.2Ω
324mA
⇒ RVB
=
It is not always possible to dimension the circuit for in- = 220 µH, RVB = 1 Ω is suitable for maximum perfor-
termittent flow, particularly not when high output cur- mance throughout the entire specification range.
rents are required with a low supply voltage. Permit-
ting continuous flow may prove conducive to higher
efficiency and less interference. The inductance se-
SELECTING THE COMPONENTS
lected is to be higher than in the above formula; the
Since the coil must not to become saturated, it should
equations for maximum cut-off current and the shunt
be designed for maximum cut-off current. This can be
can be used with the selected coil.
checked by testing the coil current with a current probe:
In the event of saturation the current rises much more
It is simplest to ascertain the correct dimensioning sharply than with low currents. A low internal resis-
by experiment in a test set-up (Evaluation Board). tance of the coil reduces the losses and increases the
The dimensioning shown in the block diagram (LVH regulator’s efficiency. When the supply voltage is low,
ICHAUS [ IC-HAUS GMBH ]
