IC-WD Datasheet, PDF (6/12 Pages) IC-Haus GmbH – SWITCHED-MODE DUAL VOLTAGE REGULATOR

iC-WD A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
DESCRIPTION OF FUNCTIONS
Rev D1, Page 6/12
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.
cillator frequency is reduced as the level of voltage VH
rises (Fig. 5).
500mV
400mV
300mV
Vsat
V(LVH)
200mV
VB
S
LVH
VH
typ.
100mV
RL
VD
CVH
Figure 1: Principle of operation
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
VB
I_off
− V_sat
− VH
t_f
=
L_VH
I_off
VH + V_D
(1)
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
0V
5.5V
6.0V
6.5V
7.0V
7.5V
VH
Figure 2: Regulating characteristic ∆V_R = f (VH)
1.5V
1.0V
0.5V
max.
typ.
0V
0A
100mA
200mA
300mA
400mA
500mA
I(VHL)
Figure 3: Saturation voltage of switching transistor
1.5V
1.0V
0.5V
max.
typ.
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-
0V
0A
100mA
200mA
300mA
400mA
500mA
I(VHL)
Figure 4: Forward voltage of free-wheeling diode