iC-WD
A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 7/12
125kHz
typ.
100kHz
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:
I
L
(VH) =
1
t
r
+
t
f
I
2
off
T
(2)
75kHz
fosz
50kHz
25kHz
T
= 1/f
osz
:
Period of internal oscillator (Fig. 5)
5.5V
6.0V
6.5V
7.0V
7.5V
0
VH
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:
For load current I
L
at output VH, the iC-WD ad-
justs the cut-off current I
off
to the following value
(VB
>
VH + V
sat
):
I
off
=
2
·
I
L
(VH)
T
L
VH
1
1
VB−V
sat
−VH
1
+
VH+V
D
(3)
Ioff
I(LVH)
Since only during the charging phase current is drawn
from supply voltage VB, the mean current consumption
is: (VB
>
VH + V
sat
):
I(VB) = I
off
t
r
+ I
0
(VB)
T
(4)
0
VB
I
0
(VB): current consumption without load at VCC,
VCCA (no-load operation)
VHL
VH
0
t
r
t
f
T = 1/f
osz
Figure 6: Intermittent flow
SWITCHING REGULATOR: Intermittent flow
When charging and discharging operation are con-
cluded within a single clock pulse period (t
r
+
t
f
<
T
)
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-
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-
age source and standby capacitor C
VH
is lower. De-
pending on the model and quality of the coil, however,
the low frequent fluctuations may be audible.
ICHAUS [ IC-HAUS GMBH ]
