RT8055B
Application Information
The basic RT8055B 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 C
IN
and C
OUT
.
Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
V
OUT
=
V
REF
× ⎛
1
+
R1
⎞
⎜
⎟
⎝
R2
⎠
where V
REF
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.
V
OUT
R1
FB
RT8055B
GND
R2
The operating frequency of the RT8055B 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. R
RT
curve. Although frequencies as high as
2MHz are possible, the minimum on-time of the RT8055B
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).
3.0
Switching Frequency (MHz)
1
2.5
R
RT
= 180k for 1.8MHz
2.0
1.5
1.0
0.5
0.0
0
200
400
600
800
1000
Figure 1. Setting the Output Voltage
Soft-Start
The RT8055B contains an internal soft-start clamp that
gradually raises the clamp on the COMP pin.
Power Good Output
The power good output is an open drain output and requires
a pull up resister. When the output voltage is 14% above
or 13% 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. In Soft-Start, PGOOD is
actively held low and is allowed to transition high until the
Soft-Start is finished and the output voltage reaches 90%
of its set voltage.
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.
R
OSC
(K )
(kΩ)
Figure 2
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 RT8055B is designed to operate down to an input
supply voltage of 2.6V. One important consideration at
low input supply voltages is that the R
DS(ON)
of the P-
Channel and N-Channel power switches increases. The
user should calculate the power dissipation when the
RT8055B is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
www.richtek.com
9
DS8055B-03 April 2011
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
