RT8057A
Application Information
The basic RT8057A 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 :
R1
V
OUT
=
V
REF
x (1
+
)
R2
where V
REF
equals to 0.6V 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
RT8057A
GND
R2
Low Supply Operation
The RT8057A is designed to operate down to an input
supply voltage of 2.7V. 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
RT8057A is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
Under Voltage Protection (UVP)
The output voltage can be continuously monitored for under
voltage protection. When the output voltage is less than
33% of its set voltage threshold after OCP occurs, the
under voltage protection circuit will be triggered to auto
re-softstart.
Input Voltage Over Voltage protection (V
IN
OVP)
When the input voltage (V
IN
) is higher than 6V, V
IN
OVP
will be triggered and the IC stops switching. Once the
input voltage drops below 6V, the IC will return to normal
operation.
Output Over Voltage Protection (V
OUT
OVP)
Figure 1. Setting the Output Voltage
Soft-Start
The RT8057A contains an internal soft-start clamp that
gradually raises the clamp on the FB pin.
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.
When the output voltage exceeds more than 5% of the
nominal reference voltage, the feedback loop forces the
internal switches off within 50μs. Therefore, the output
over voltage protection is automatically triggered by the
loop.
Short Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. A
current runaway detector is used to monitor inductor
current. As current increases beyond the control of current
loop, switching cycles will be skipped to prevent current
runaway from occurring.
Table 1. Inductors
Component
Supplier
TAIYO YUDEN
Series
NR4018
T2R2M
Inductance (μH)
2.2μH
DCR (mΩ)
60
Current Rating (mA)
2700
Dimensions (mm)
4 X 4 X 1.8
www.richtek.com
8
DS8057A-00 March 2011
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
