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

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.

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

ΔV

IN

ΔV

O

50mV/DIV

10V/DIV

20mV/DIV

0.5A/DIV

0.5µs/DIV

50mV/DIV

10V/DIV

20mV/DIV

0.5A/DIV

0.5µs/DIV

2V/DIV

5V/DIV

0.5A/DIV

0.5ms/DIV

100mA/DIV

0.5V/DIV

0.2ms/DIV

EL7515