RT8062 查看數據表（PDF） - Richtek Technology
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RT8062 Datasheet PDF : 11 Pages
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RT8062
Application Information
The basic IC application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by CIN and COUT.
Main Control Loop
During normal operation, the internal top power switch
(P-MOSFET) is turned on at the beginning of each clock
cycle. Current in the inductor increases until the peak
inductor current reach 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 resistor divider on the FB pin with an internal 0.8V
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. When the
top 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 divider
according to the following equation :
VOUT
=
VREF
×
(1+
R1
R2
)
where VREF equals to 0.8V typical.
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8062
R2
GND
Figure 1. Setting the Output Voltage
Soft-Start
The IC contains an internal soft-start clamp that gradually
raises the clamp on the COMP pin.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequency improves efficiency by
reducing internal gate charge and switching losses but
requires larger inductance and/or capacitance to maintain
low output ripple voltage.
The operating frequency of the IC is determined by an
external resistor , ROSC, that is connected between the
RT pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator. The practical switching
frequency can range from 200kHz to 2MHz. However, when
the RT pin is floating, the internal frequency is set at 2MHz.
Determine the RT resistor value by examining the curve
below. Please notice the minimum on time is about 90ns.
2.4
2.0
1.6
1.2
0.8
0.4
0.0
0
300 600 900 1200 1500 1800 2100
RRT (kΩ)
Figure 2. Switching Frequency vs. RRT Resistor
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
⎤
⎥
⎦
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
8
is a registered trademark of Richtek Technology Corporation.
DS8062-07 November 2012
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