ML4865
DESIGN CONSIDERATIONS
INPUTVOLTAGE RANGE
900
700
500
300
WITH
EXTERNAL
SCHOTTKY
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.
WITHOUT
EXTERNAL
SCHOTTKY
100
0
OUTPUTCURRENTCAPABILITY
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:
0
2
4
6
8
10
V
(V)
IN
Figure 4. Output Current vs. InputVoltage
100
V
= 12V
OUT
V
= 10V
IN
90
80
70
60
50
V
= 5V
= 2V
IN
IN
V
IN
IOUT(MAX)
@
I
IN
(A)
(1)
VOUT
V
Where, for applications using the internal synchronous
rectifier:
V
IN
with Schottky
without Schottky
IOUT(MAX)
0.05
1
2
V
0.4 0.65
IN 6
7
VOUT
1
10
100
1000
IIN 0.05
0.65
V
0.4
IN
I
(mA)
OUT
Figure 5. Efficiency vs. Output Current
And for applications using an external Schottky:
300
250
V
IN
IOUT(MAX)
0.07
1
V
IN6
0.4
0.025
1
V
0.65
IN6
2
7 2
7
VOUT
IIN 0.07
0.025
V
0.4
IN
200
V
0.65
IN
WITH
EXTERNAL
SCHOTTKY
150
100
50
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.
WITHOUT
EXTERNAL
SCHOTTKY
0
0
2
4
6
8
10
V
(V)
IN
Figure 6. No Load Input Current vs. Input Voltage for the
Circuit of Figure 7
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
