ML4812
TYPICAL APPLICATIONS
INPUT INDUCTOR (L1) SELECTION
The central component in the regulator is the input boost
inductor. The value of this inductor controls various
critical operational aspects of the regulator. If the value is
too low, the input current distortion will be high and will
result in low power factor and increased noise at the
input. This will require more input filtering. In addition,
when the value of the inductor is low the inductor dries
out (runs out of current) at low currents. Thus the power
factor will decrease at lower power levels and/or higher
line voltages. If the inductor value is too high, then for a
given operating current the required size of the inductor
core will be large and/or the required number of turns
will be high. So a balance must be reached between
distortion and core size.
One more condition where the inductor can dry out is
analyzed below where it is shown to be maximum duty
cycle dependent.
For the boost converter at steady state:
SUPPLY CURRENT (mA)
25
20
ICC (mA)
15
10
5
0
0
10
20
VCC (V)
30
40
Figure 9a. Total Supply Current vs. Supply Voltage
25
V
OUT
=
V
IN
1
-
D
ON
20
(1)
OPERATING CURRENT
Where D
ON
is the duty cycle [T
ON
/(T
ON
+ T
OFF
)]. The
input boost inductor will dry out when the following
condition is satisfied:
15
10
V
IN
(t )
<
V
OUT
´
(1
-
D
ON
)
or
V
IND RY
=
[1
-
D
ON
(max)]
´
V
OUT
(2)
5
STARTUP
(3)
0
–60 –40 –20
0
20
40
60
80 100 120 140
V
INDRY
: voltage where the inductor dries out.
V
OUT
: output DC voltage.
Effectively, the above relationship shows that the resetting
volt-seconds are more than setting volt-seconds. In energy
transfer terms this means that less energy is stored in the
inductor during the ON time than it is asked to deliver
during the OFF time. The net result is that the inductor
dries out.
0
TEMPERATURE (degrees)
Figure 9b. Supply Current (I
CC
) vs. Temperature
-4
-8
∆V
REF (mV)
5V VREF
9V
–
+
INTERNAL
BIAS
VCC
-12
ENABLE
VREF
VREF
GEN.
-16
-20
-24
0
20
40
60
IREF (mA)
80
100
120
Figure 8. Under-Voltage Lockout Block Diagram
Figure 10. Reference Load Regulation
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
