EL7515
At very low load, the MOSFET will skip pulses sometimes.
This is normal.
The inductor should be chosen to be able to handle this
current. Furthermore, due to the fixed internal
compensation, it is recommended that maximum inductance
of 10µH and 15µH to be used in the 5V and 12V or higher
output voltage, respectively.
Current Limit
The MOSFET current limit is nominally 1.4A and guaranteed
1A. This restricts the maximum output current I
on the following formula:
based
OMAX
The output diode has average current of I , and peak
O
current the same as the inductor's peak current. Schottky
diode is recommended and it should be able to handle those
currents.
V
∆I
IN
L
×
---------
I
=
1 – --------
OMAX
V
O
2
The output voltage ripple can be calculated as:
where:
I
× D
• ∆I is the inductor peak-to-peak current ripple and is
O
L
∆V = --------------------- + I
× ESR
LPK
O
F
× C
O
decided by:
S
V
D
F
S
IN
--------- ------
∆I
=
×
L
Where:
L
• C is the output capacitance.
O
• D is the MOSFET turn-on ratio and is decided by:
• The ESR is the output capacitor ESR value.
V
– V
IN
V
O
O
D = -----------------------
Low ESR capacitors should be used to minimize the output
voltage ripple. Multilayer ceramic capacitors (X5R and X7R)
are preferred for the output capacitors since they have a low
ESR and small packages. Tantalum capacitors also can be
used, but they take more board space and have higher ESR.
A minimum of 22µF output capacitor is sufficient for high
output current application. For lower output current, the
output capacitor can be smaller, like 4.7µF. The capacitor
should always have enough voltage rating. In addition to the
voltage rating, the output capacitor should also be able to
handle the RMS current is given by:
• F is the switching frequency
S
The following table gives typical values:
TABLE 1. MAX CONTINUOUS OUTPUT CURRENTS
V
V
L
F
I
IN
O
S
OMAX
(mA)
(V)
2
(V)
5
(µH)
(kHz)
1000
1000
1000
1000
1000
1000
1000
1000
1000
100
10
10
10
10
10
10
10
10
10
10
360
190
140
600
310
230
470
340
630
670
2
9
2
12
5
2
∆I
1
12
L
3.3
3.3
3.3
5
------
I
=
(1 – D) × D + ------------------- ×
× I
CORMS
LAVG
2
I
LAVG
9
12
9
Output Voltage
An external resistor divider is required to divide the output
voltage down to the nominal reference voltage. The current
drawn by the resistor network should be limited to maintain
the overall converter efficiency. The maximum value of the
resistor network is limited by the feedback input bias current
and the potential for noise being coupled into the feedback
pin. A resistor network less than 300kΩ is recommended.
The boost converter output voltage is determined by the
relationship:
5
12
12
15
9
12
Component Considerations
It is recommended that C is larger than 10µF.
IN
Theoretically, the input capacitor has ripple current of ∆I .
L
Due to high-frequency noise in the circuit, the input current
ripple may exceed the theoretical value. Larger capacitor will
reduce the ripple further.
R
2
V
= V × 1 + ------
FB
OUT
R
1
where V slightly changes with V . The curve is shown in
FB
DD
The inductor has peak and average current decided by:
this data sheet.
∆I
L
I
= I
+ --------
LPK
LAVG
2
RC Filter
The maximum voltage rating for the V
pin is 12V and is
I
DD
O
I
= -------------
recommended to be about 10V for maximum efficiency to
drive the internal MOSFET. The series resistor R in the RC
LAVG
1 – D
4
FN7120.1
7
May 13, 2005
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