EL7515 Просмотр технического описания (PDF) - Renesas Electronics
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EL7515 Datasheet PDF : 9 Pages
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EL7515
Typical Performance Curves (Continued)
VIN = 5V, VO = 12V, IO = 30mA
VIN
VLX
50mV/DIV
10V/DIV
VO
IL
20mV/DIV
0.5A/DIV
0.5µs/DIV
FIGURE 7. STEADY STATE OPERATION (INDUCTOR
DISCONTINUOUS CONDUCTION)
VIN = 5V, VO = 12V, IO = 300mA
VIN
VLX
VO
IL
50mV/DIV
10V/DIV
20mV/DIV
0.5A/DIV
0.5µs/DIV
FIGURE 8. STEADY STATE OPERATION (INDUCTOR
CONTINUOUS CONDUCTION)
VIN = 5V, VO = 12V, IO = 300mA
VIN
VO
IL
0.5ms/DIV
FIGURE 9. POWER-UP
2V/DIV
5V/DIV
0.5A/DIV
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 RT pin.
Start-Up
After VDD 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 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
FN7120 Rev 2.00
August 10, 2007
VIN = 5V, VO = 12V, IO = 50mA TO 300mA
IO
VO
100mA/DIV
0.5V/DIV
0.2ms/DIV
FIGURE 10. LOAD TRANSIENT RESPONSE
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.
Page 6 of 9
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