EL7515 Просмотр технического описания (PDF) - Renesas Electronics
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EL7515 Datasheet PDF : 9 Pages
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EL7515
At very low load, the MOSFET will skip pulses sometimes.
This is normal.
Current Limit
The MOSFET current limit is nominally 1.4A and guaranteed
1A. This restricts the maximum output current IOMAX based
on Equation 1:
IOMAX
=
1
–
---2--I--L-
V-----I--N--
VO
(EQ. 1)
where:
• IL is the inductor peak-to-peak current ripple and is
decided by Equation 2:
IL
=
-V----I--N-- -D---
L fS
(EQ. 2)
• D is the MOSFET turn-on ratio and is decided by
Equation 3:
D
=
-V----O-----–-----V----I--N--
VO
(EQ. 3)
• fS is the switching frequency
The following table gives typical values:
TABLE 1. MAX CONTINUOUS OUTPUT CURRENTS
VIN
VO
L
FS
IOMAX
(V)
(V)
(ΜH)
(kHz)
(mA)
2
5
10
1000
360
2
9
10
1000
190
2
12
10
1000
140
3.3
5
10
1000
600
3.3
9
10
1000
310
3.3
12
10
1000
230
5
9
10
1000
470
5
12
10
1000
340
9
12
10
1000
630
12
15
10
100
670
Component Considerations
It is recommended that CIN is larger than 10µF.
Theoretically, the input capacitor has a ripple current of IL.
Due to high-frequency noise in the circuit, the input current
ripple may exceed the theoretical value. A larger capacitor
will reduce the ripple further.
The inductor has peak and average current decided by
Equations 4 and 5:
ILPK
=
IL
A
V
G
+
-----I--L-
2
(EQ. 4)
ILAVG = 1----I-–-O----D---
(EQ. 5)
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.
The output diode has an average current of IO, and peak
current the same as the inductor's peak current. A Schottky
diode is recommended and it should be able to handle those
currents.
The output voltage ripple can be calculated as Equation 6:
VO
=
---I--O-----------D-----
FS CO
+
ILPK
ESR
(EQ. 6)
Where:
• CO is the output capacitance.
• The ESR is the output capacitor ESR value.
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 which is given by Equation 7:
ICORMS =
1
–
D
D
+
---------I--L---2-----
IL
A
V
2
G
1--1--2--
ILAVG
(EQ. 7)
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 300k is recommended.
FN7120 Rev 2.00
August 10, 2007
Page 7 of 9
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