6
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 V
DD
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 V
DD
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 I
OMAX
based
on the following formula:
where:
∆I
L
is the inductor peak-to-peak current ripple and is
decided by:
D is the MOSFET turn-on ratio and is decided by:
F
S
is the switching frequency.
The following table gives typical values:
Component Considerations
It is recommended that C
IN
is larger than 10µF.
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.
The inductor has peak and average current decided by:
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 average current of I
O
, and peak
current the same as the inductor's peak current. Schottky
I
OMAX
1
∆I
L
2
--------–
V
IN
V
O
---------
×=
MAX CONTINUOUS OUTPUT CURRENTS
V
IN
(V) V
O
(V) L (µH) F
S
(kHz)
I
OMAX
(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
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
∆I
L
V
IN
L
---------
D
F
S
-------
×=
D
V
O
V
IN
–
V
O
------------------------=
I
LPK
I
LAVG
∆I
L
2
--------+=
I
LAVG
I
O
1D–
-------------=
EL7512
