6
FN7120.2
August 10, 2007
Applications Information
The EL7515 is a step-up regulator, operated at fixed
frequency pulse-width-modulation (PWM) control. The input
voltage is 1.8V to 13.2V and output voltage is 4.5V to 17V.
The switching frequency (up to 1.2MHz) is decided by the
resistor connected to R
T
pin.
Start-Up
After V
DD
reaches a threshold of about 1.7V, the start-up
oscillator generates fixed duty-ratio of 0.5 to 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 form a resonant circuit.
Oscillation will occur in this period. This oscillation is normal
and will not affect the regulation.
FIGURE 7. STEADY STATE OPERATION (INDUCTOR
DISCONTINUOUS CONDUCTION)
FIGURE 8. STEADY STATE OPERATION (INDUCTOR
CONTINUOUS CONDUCTION)
FIGURE 9. POWER-UP
FIGURE 10. LOAD TRANSIENT RESPONSE
Typical Performance Curves (Continued)
V
IN
= 5V, V
O
= 12V, I
O
= 30mA
V
IN
V
LX
V
O
I
L
50mV/DIV
10V/DIV
20mV/DIV
0.5A/DIV
0.5µs/DIV
V
IN
= 5V, V
O
= 12V, I
O
= 300mA
V
IN
V
LX
V
O
I
L
50mV/DIV
10V/DIV
20mV/DIV
0.5A/DIV
0.5µs/DIV
V
IN
= 5V, V
O
= 12V, I
O
= 300mA
V
IN
V
O
I
L
2V/DIV
5V/DIV
0.5A/DIV
0.5ms/DIV
V
IN
= 5V, V
O
= 12V, I
O
= 50mA TO 300mA
I
O
V
O
100mA/DIV
0.5V/DIV
0.2ms/DIV
EL7515
