IC-WDCEVALWD2D datasheet(6/12 Pages) ICHAUS

6 / 12 page

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 CVH ﬂows through the coil

LVH in addition to the load current in RL. 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

V(LVH)

LVH

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, ∆VR 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 RVB. When the cut-off current Ioff =

∆

VR/RVB 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 (tr ) and fall times (tf ) depend on the

voltage VH at the inductor. The following approxima-

tion applies:

tr = LVH

Ioff

VB − Vsat − VH

tf = LVH

Ioff

VH + VD

(1)

Vsat = VB − VHL: Saturation voltage of the switching

transistor plus voltage drop at RVB

VD: 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-

cillator frequency is reduced as the level of voltage VH

rises (Fig. 5).

5.5V

6.0V

6.5V

7.0V

7.5V

500mV

400mV

300mV

200mV

100mV

typ.

Figure 2: Regulating characteristic ∆VR = f (VH)

100mA

200mA

300mA

400mA

500mA

I(VHL)

1.5V

1.0V

0.5V

typ.

max.

Figure 3: Saturation voltage of switching transistor

100mA

200mA

300mA

400mA

500mA

I(VHL)

1.5V

1.0V

0.5V

max.

typ.

Figure 4: Forward voltage of free-wheeling diode

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