iC-WD
A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 9/12
APPLICATIONS INFORMATION
DIMENSIONING
The size of shunt
R
VB
determines the cut-off current
I
off
. By varying this in combination with the value for
the inductor
L
VH
, 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
L
VH
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
I
off
= 2
×
I
L
max
(VH). From equation (1) it follows
that:
L
VH
max
=
T
min
×
2I
L
max
(VH)
1
1
VB
min
−V
sat
−VH
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:
I
off
max
=
2
·
I
L
max
(VH)
T
max
·
L
VH
1
1
VB
max
−V
sat
−VH
1
+
VH+V
D
The shunt
R
VB
can be dimensioned with this informa-
tion.
∆VR
max
can be obtained from Fig. 2:
+
1
VH+V
D
R
VB
=
∆V
R
max
I
off
max
EXAMPLE
Specified are: VB = 18...36 V,
I
L
max
= 100 mA; the maximum inductance can be estimated to:
1/125
kHz
·
200
mA
1
1
18
V
−1.1
V
−7.0
V
1
+
7.0
V
+1.1
V
L
VH
max
=
= 178
µH
The inductance selected is 150 µH, for example. Consequently, the maximum required cut-off current and the
shunt are found to be:
1/75
kHz
·
150
µH
1
1
30
V
−1.1
V
−7
V
1
+
7
V
+1.1
V
I
off
max
=
2
·
100
mA
= 324
mA
⇒
R
VB
=
400
mV
≈
1.2
Ω
324
mA
= 220 µH, R
VB
= 1
Ω
is suitable for maximum perfor-
mance throughout the entire specification range.
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 (L
VH
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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