RT8055 데이터 시트보기 (PDF) - Richtek Technology
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RT8055 Datasheet PDF : 14 Pages
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Application Information
The basic RT8055 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.
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
RT8055
R2
GND
Figure 1. Setting the Output Voltage
Soft-Start
The RT8055 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 RT8055 is determined by
an external resistor that is connected between the SHDN/
RT pin and GND. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator. The RT resistor value
can be determined by examining the frequency vs. RRT
curve. Although frequencies as high as 2MHz are possible,
the minimum on-time of the RT8055 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 110ns. Therefore, the minimum duty cycle is
equal to 100 x 110ns x f (Hz).
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS8055-05 November 2012
RT8055
3.0
2.5
RRT = 180k for 1.8MHz
2.0
1.5
1.0
0.5
0.0
0
200
400
600
ROSC ((KkΩ))
Figure 2
800
1000
100% Duty Cycle Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on-time. Further reduction of the supply voltage forces
the main switch to remain on for more than one cycle
eventually reaching 100% duty cycle.
The output voltage will then be determined by the input
voltage minus the voltage drop across the internal
P-MOSFET and the inductor.
Low Supply Operation
The RT8055 is designed to operate down to an input supply
voltage of 2.6V. One important consideration at low input
supply voltages is that the RDS(ON) of the P-Channel and
N-Channel power switches increases. The user should
calculate the power dissipation when the RT8055 is used
at 100% duty cycle with low input voltages to ensure that
thermal limits are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant
frequency architectures by preventing sub-harmonic
oscillations at duty cycles greater than 50%. It is
accomplished internally by adding a compensating ramp
to the inductor current signal. Normally, the maximum
inductor peak current is reduced when slope compensation
is added. In the RT8055, however, separated inductor
current signals are used to monitor over current condition.
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www.richtek.com
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