iC-WD A/B/C
SWITCHED-MODE DUAL VOLTAGE REGULATOR
Rev D1, Page 7/12
125kHz
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:
typ.
100kHz
75kHz
50kHz
1
IL(VH) = Ioff
2
tr +tf
T
(2)
25kHz
T = 1/fosz: Period of internal oscillator (Fig. 5)
0
5.5V
6.0V
6.5V
7.0V
7.5V
VH
For load current IL at output VH, the iC-WD ad-
justs the cut-off current Ioff to the following value
(VB > VH+Vsat):
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:
s
T
LVH
1
Ioff
=
2·IL(VH)
(3)
1
sat
1
+
VB−V −VH
VH+V
D
Ioff
Since only during the charging phase current is drawn
from supply voltage VB, the mean current consumption
is: (VB > VH+Vsat):
tr
T
I(VB) = Ioff +I0(VB)
(4)
0
I0(VB): current consumption without load at VCC,
VB
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-
VH
0
t r
tf
osz
T = 1/f
Figure 6: Intermittent flow
SWITCHING REGULATOR: Intermittent flow
When charging and discharging operation are con- tor, in the free-wheeling diode and due to the internal
cluded within a single clock pulse period (tr + tf < T) resistance of the inductor are consequently lower; the
and the coil current drops to zero, intermittent flow pre- efficiency of the regulator is thus higher. In addition, in-
vails (Fig. 6). This is the case when the supply voltage terference due to the internal resistance of supply volt-
is sufficiently high or the load current sufficiently low. age source and standby capacitor CVH is lower. De-
The current-carrying capacity and power consumption pending on the model and quality of the coil, however,
of the regulator can be easily specified for this operat- the low frequent fluctuations may be audible.
ICHAUS [ IC-HAUS GMBH ]
