SWITCHED-MODE DUAL VOLTAGE REGULATOR

Rev D1, Page 7/12

5.5V

6.0V

6.5V

7.0V

7.5V

VH

125kHz

100kHz

75kHz

25kHz

0

typ.

50kHz

Figure 5: Oscillator Frequency

The following three operating states of the regulator

are described as a function of the supply voltage and

the load current:

Ioff

0

t

T = 1/f

VH

0

VB

r

osz

Figure 6: Intermittent flow

SWITCHING REGULATOR: Intermittent flow

When charging and discharging operation are con-

and the coil current drops to zero, intermittent flow pre-

vails (Fig. 6). This is the case when the supply voltage

is sufficiently high or the load current sufficiently low.

The current-carrying capacity and power consumption

of the regulator can be easily specified for this operat-

ing mode. Since both the charging and the discharging

current flow in VH, the initial approximation of the mean

current-carrying capacity of VH is:

1

2

T

(2)

T

1

1

+

1

(3)

Since only during the charging phase current is drawn

from supply voltage VB, the mean current consumption

T

(4)

VCCA (no-load operation)

SWITCHING REGULATOR: Continuous flow

If the inductor receives recharge with the next clock

signal before the coil current has run free, no gap is

created in the current. Such continuous flow (Fig. 7)

occurs when the supply voltage is too low or the load

current too high. Since the charging process begins at

various current levels not equal to zero, the timing and

the required cut-off current are difficult to express. In

general, fluctuations occur in the clock frequency at the

time constants of the charging and discharging phase,

which in turn depend on the of supply voltage and the

load current. Since no current gap occurs, the cut-off

current may be lower than during intermittent flow (at

the same load). The losses in the switching transis-

tor, in the free-wheeling diode and due to the internal

resistance of the inductor are consequently lower; the

efficiency of the regulator is thus higher. In addition, in-

terference due to the internal resistance of supply volt-

pending on the model and quality of the coil, however,

the low frequent fluctuations may be audible.