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-

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.

VH

VB

Vsat

V(LVH)

VD

S

LVH

CVH

RL

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-

teristic in Fig. 2, is obtained from the voltage VH and

the internal reference voltage and is compared to the

∆

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.

voltage VH at the inductor. The following approxima-

tion applies:

(1)

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

VH

500mV

400mV

300mV

200mV

100mV

0V

typ.

0A

100mA

200mA

300mA

400mA

500mA

I(VHL)

1.5V

1.0V

0.5V

0V

typ.

max.

Figure 3: Saturation voltage of switching transistor

0A

100mA

200mA

300mA

400mA

500mA

I(VHL)

1.5V

1.0V

0.5V

0V

max.

typ.

Figure 4: Forward voltage of free-wheeling diode