RT8078A Ver la hoja de datos (PDF) - Richtek Technology
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RT8078A Datasheet PDF : 12 Pages
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RT8078A
Application Information
The RT8078A is a single-phase buck PWM converter. It
provides single feedback loop, current mode control with
fast transient response. An internal 0.6V reference allows
the output voltage to be precisely regulated for low output
voltage applications. A fixed switching frequency (1MHz)
oscillator and internal compensation are integrated to
minimize external component count.
Main Control Loop
During normal operation, the internal high side power
switch (P-MOSFET) is turned on at the beginning of each
clock cycle. Current in the inductor increases until the
peak inductor current reaches the value defined by the
output voltage (VCOMP) of the error amplifier. The error
amplifier adjusts its output voltage by comparing the
feedback signal from a resistive voltage divider on the FB
pin with an internal 0.6V reference. When the load current
increases, it causes a reduction in the feedback voltage
relative to the reference. The error amplifier raises its output
voltage until the average inductor current matches the new
load current. Once the high side power MOSFET shuts
off, the synchronous power switch (N-MOSFET) turns on
until the beginning of the next clock cycle.
Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT
= VREF × ⎛⎜⎝1 +
R1 ⎞
R2 ⎟⎠
where VREF equals 0.6V (typ.).
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8078A
R2
GND
Figure 1. Setting the Output Voltage
www.richtek.com
8
Soft-Start
The IC contains an internal soft-start function to prevent
large inrush current and output voltage overshoot when
the converter starts up. Soft-start automatically begins
once the chip is enabled. During soft-start, the internal
soft-start capacitor becomes charged and generates a
linear ramping up voltage across the capacitor. This voltage
clamps the voltage at the FB pin, causing the duty pulse
width to increase slowly and in turn reduce the output
surge current. Finally, the internal 0.6V reference takes
over the loop control once the internal ramping-up voltage
becomes higher than 0.6V. The minimum soft-start time
for this IC is set at 500μs.
Power Good Output
The power good output is an open-drain output and requires
a pull up resistor. When the output voltage is 85% below
its set voltage, PGOOD will be pulled low. It is held low
until the output voltage returns to within the allowed
tolerances once more. During soft-start, PGOOD is actively
held low and only allowed to transition high after soft-start
is over and the output voltage has reached 90% of its set
voltage.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
ΔIL
=
⎡
⎢⎣
VOUT
f ×L
⎤
⎥⎦
×
⎡
⎢1
⎣
−
VOUT
VIN
⎤
⎥
⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Highest efficiency operation is achieved by reducing
ripple current at low frequency, but a large inductor is
required to attain this goal.
For ripple current selection, the value of ΔIL = 0.4(IMAX) is a
reasonable starting point. The largest ripple current occurs
at the highest VIN. To guarantee that the ripple current
stays below a specified maximum value, the inductor
should be chosen according to the following equation :
⎡
L= ⎢
VOUT
⎤⎡
⎥ × ⎢1−
VOUT
⎤
⎥
⎢⎣ f × ΔIL(MAX) ⎥⎦ ⎢⎣ VIN(MAX) ⎥⎦
DS8078A-01 June 2011
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