iC-WD
A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 6/12
DESCRIPTION OF FUNCTIONS
Fig. 1 illustrates the operating principle of the switching
converter in simplified form. When the switch S closes
in steady-state condition, a linearly increasing charg-
ing current for the capacitor C
VH
flows through the coil
L
VH
in addition to the load current in R
L
. The energy
from the supply VB is stored in the coil’s magnetic field.
When the switch opens, the current flows via the diode
through the coil; its energy content is supplied to ca-
pacitor and load.
Vsat
VB
S
V(LVH)
LVH
VH
100mV
cillator frequency is reduced as the level of voltage VH
rises (Fig. 5).
500mV
400mV
∆VR = VB − VBR
300mV
200mV
typ.
VD
CVH
RL
0V
5.5V
6.0V
6.5V
7.0V
VH
7.5V
Figure 2: Regulating characteristic
∆V
R
=
f
(VH)
Figure 1: Principle of operation
1.5V
The block diagram on page 1 shows the iC-WD with
typical wiring. The internally generated clock pulse
closes the switch between VBR and VHL and the cur-
rent in the coil rises (charging phase). A control vari-
able,
∆V
R
in accordance with the regulating charac-
teristic in Fig. 2, is obtained from the voltage VH and
the internal reference voltage and is compared to the
voltage at shunt R
VB
. When the cut-off current
I
off
=
∆V
R
/R
VB
is reached, the switch opens and the coil
current runs free via the integrated power diode (dis-
charge phase). When the next clock signal occurs, this
charging and discharging process is repeated. Fig. 6
shows the resulting current and voltage characteristics.
The current rise (t
r
) and fall times (t
f
) depend on the
voltage VH at the inductor. The following approxima-
tion applies:
t
r
=
L
VH
I
off
VB
−
V
sat
−
VH
t
f
=
L
VH
I
off
VH
+
V
D
(1)
max.
1.0V
Vsat = VB − VHL
typ.
0.5V
0V
0A
100mA
200mA
300mA
400mA
I(VHL)
500mA
Figure 3: Saturation voltage of switching transistor
1.5V
max.
1.0V
VD = − VHL
V
sat
=
VB
−
VHL:
Saturation voltage of the switching
transistor plus voltage drop at
R
VB
V
D
:
Forward voltage of the free-wheeling diode
The current dependencies of the saturation and diode
forward voltage (Fig. 3 and 4) are ignored here, as are
the losses due to the internal resistance of the coil.
The regulator operates at a constant frequency under
load. To prevent VH from rising without load, the os-
typ.
0.5V
0V
0A
100mA
200mA
300mA
400mA
I(VHL)
500mA
Figure 4: Forward voltage of free-wheeling diode
ICHAUS [ IC-HAUS GMBH ]
