IC-WD Datasheet, PDF (9/12 Pages) IC-Haus GmbH – SWITCHED-MODE DUAL VOLTAGE REGULATOR

iC-WD A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
APPLICATIONS INFORMATION
Rev D1, Page 9/12
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.
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:
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_Lmax (VH). From equation (1) it follows
that:
I_offmax =
2
·
I_Lmax
(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:
L_VHmax
=
T_min
2I_Lmax (VH)
×
1
1
VB_min−V_sat −VH
+
1
VH +V_D
R_VB
=
∆V_Rmax
I_offmax
EXAMPLE
Specified are: VB = 18...36 V, I_Lmax = 100 mA; the maximum inductance can be estimated to:
1/125 kHz
1
L_VHmax =
200 mA
·
1
18 V −1.1 V −7.0 V
+
1
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:
1/75 kHz
1
I_offmax =
2 · 100 mA
150 µH
·
1
30 V −1.1 V −7 V
+
1
7 V +1.1 V
= 324 mA
400 mV
⇒ R_VB = _{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 (L_VH
= 220 µH, R_VB = 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,