ML4865
DESIGN CONSIDERATIONS
INPUT VOLTAGE RANGE
The input voltage range determines whether an external
Schottky diode is necessary or optional. If the input
voltage is 6V or lower, the ML4865 can be operated as a
stand alone boost regulator with a shutdown that fully
isolates the input from the output. Adding an optional
Schottky diode extends the input voltage range up to 10V,
and improves the efficiency and the output current
capability. However, the external diode now provides a
leakage path from the input to the output during
shutdown.
900
WITH
EXTERNAL
SCHOTTKY
700
IOUT (mA)
500
300
OUTPUT CURRENT CAPABILITY
The maximum current available at the output of the
regulator is related to the maximum inductor current by
the ratio of the input to output voltage and the full load
efficiency. The maximum inductor current is dependent on
the input voltage. The full load efficiency may be as low
as 65% when the ML4865 is used without a Schottky
diode and can exhibit an input voltage dependence when
an external diode is used. The maximum output current
can be determined by using the typical performance
curves shown in Figures 4 and 5, or by calculation using
the following empirical equation:
I
OUT( MAX)
V
@
IN
I
IN
h
(A)
V
OUT
(1)
100
0
0
2
4
WITHOUT
EXTERNAL
SCHOTTKY
6
VIN (V)
8
10
Figure 4. Output Current vs. Input Voltage
100
VIN = 10V
90
VOUT = 12V
EFFICIENCY (%)
80 VIN = 5V
Where, for applications using the internal synchronous
rectifier:
I
OUT( MAX)
V
@
IN
´
0.05
´
V
IN
+
0.4
´
0.65
V
OUT
70
VIN = 2V
21
6
7
60
with Schottky
without Schottky
50
1
10
IOUT (mA)
100
1000
I
IN
=
0.05
´
V
IN
+
0.4
h
=
0.65
And for applications using an external Schottky:
I
OUT( MAX)
@
V
IN
0.07
V
IN
+
0.4
0.025
V
IN
+
0.65
V
OUT
Figure 5. Efficiency vs. Output Current
300
21
6
7 21
6
7
IIN (mA)
250
I
IN
=
0.07
´
V
IN
+
0.4
200
WITH
EXTERNAL
SCHOTTKY
h
=
0.025
´
V
IN
+
0.65
The curves and the equations are based on the operating
circuit shown in Figure 7. It is recommended to verify the
current capability and efficiency for the components
selected.
150
100
50
WITHOUT
EXTERNAL
SCHOTTKY
0
2
4
VIN (V)
6
8
10
0
Figure 6. No Load Input Current vs. Input Voltage for the
Circuit of Figure 7
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
