EL7512 View Datasheet(PDF) - Renesas Electronics
Part Name
Description
Manufacturer
EL7512 Datasheet PDF : 8 Pages
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EL7512
Applications Information
The EL7512 is a step-up regulator, operated at fixed frequency
pulse-width-modulation (PWM) control. The input voltage is
2V-12V and output voltage is 5V-18V. The switching frequency
(up to 1.2MHz) is decided by the resistor connected to RT pin.
Start-Up
After VDD reaches a threshold of about 2V, the start-up
oscillator generates fixed duty-ratio of 0.5-0.7 at a frequency of
several hundred kilohertz. This will boost the output voltage.
When VDD reaches about 3.7V, the PWM comparator takes
over the control. The duty ratio will be decided by the multiple-
input direct summing comparator, Max_Duty signal (about 90%
duty-ratio), and the Current Limit Comparator, whichever is the
smallest.
The soft-start is provided by the current limit comparator. As
the internal 12µA current source charges the external CSS, the
peak MOSFET current is limited by the voltage on the
capacitor. This in turn controls the rising rate of the output
voltage.
The regulator goes through the start-up sequence as well after
the EN signal is pulled to HI.
Steady-State Operation
When the output reaches the preset voltage, the regulator
operates at steady state. Depending on the input/output
conditions and component values, the inductor operates at
either continuous-conduction mode or discontinuous-
conduction mode.
In the continuous-conduction mode, the inductor current is a
triangular waveform and LX voltage a pulse waveform. In the
discontinuous-conduction mode, the inductor current is
completely dried out before the MOSFET is turned on again.
The input voltage source, the inductor, and the MOSFET and
output diode parasitic capacitors forms a resonant circuit.
Oscillation will occur in this period. This oscillation is normal
and will not affect the regulation.
At very low load, the MOSFET will skip pulses sometimes. This
is normal.
Current Limit
The MOSFET current limit is nominally 1.2A and guaranteed
1A. This restricts the maximum output current IOMAX based on
the following formula:
IOMAX
=
1
–
---2--I--L-
V-----I--N--
VO
where:
IL is the inductor peak-to-peak current ripple and is decided
by:
IL
=
-V----I--N--
L
--D----
FS
D is the MOSFET turn-on ratio and is decided by:
D
=
-V----O-----–-----V----I--N--
VO
FS is the switching frequency.
The following table gives typical values:
MAX CONTINUOUS OUTPUT CURRENTS
VIN (V)
2
VO (V)
5
L (µH)
10
FS (kHz)
1000
IOMAX
(mA)
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
5
15
10
1000
260
9
12
10
1000
630
9
15
10
1000
470
12
15
10
1000
670
12
18
11
1000
510
Component Considerations
It is recommended that CIN is larger than 10µF. Theoretically,
the input capacitor has ripple current of IL. Due to high-
frequency noise in the circuit, the input current ripple may
exceed the theoretical value. Larger capacitor will reduce the
ripple further.
The inductor has peak and average current decided by:
ILAVG = 1----I-–-O----D---
ILPK
=
IL
AVG
+
-----I--L-
2
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.
FN7290 Rev 1.00
May 23, 2005
Page 6 of 8
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