RT8005
Application Information
RT8005 is a pulse-width-modulated (PWM) step-down DC-
DC converter. Capable of delivering 1A output current over
a wide input voltage range from 2.4V to 5.5V. The RT8005
is ideally suited for portable electronic devices that are
powered from 1-cell Li-ion battery or from other power
sources within the range such as cellular phones, PDAs
and handy terminals.
Chip Enable/Disable and Soft Start
Four operational modes are available: PWM, PSM, Low-
Drop-Out and shut-down modes. Pulling EN pin lower than
0.4V shuts down the RT8005 and reduces its quiescent
current to 1µA. Pulling EN pin higher than 1.5V enables
the RT8005 and initiates the softstart cycle. RT8005 has
internal soft-start that can reduce the Inrush Current during
the rising of Output Voltage.
PWM Operation
During normal operation, the RT8005 regulates output
voltage by switching at a constant frequency transferring
the power to the load in each cycle by PWM. The RT8005
uses a slope-compensated, current-mode PWM controller
capable of achieving 100% duty cycle. At each rising edge
of the internal oscillator, the Control Logic cell sends a
PWM ON signal to the Driver cell to turn on internal P-
MOSFET. This allows current to ramp up through the
inductor to the load, and stores energy in a magnetic field.
The switch remains on until either the current-limit is
tripped or the PWM comparator signals for the output in
regulation. After the switch is turned off, the inductor
releases the magnetic energy and forces current through
the N-MOSFET synchronous rectifier to the output-filter
capacitor and load. The output-filter capacitor stores charge
when the inductor current is above the average output
current and releases charge when the inductor current is
below the average current to smooth the output voltage
across the load. A Zero Detector monitors inductor current
by sensing v oltage drop across the N-MOSFET
synchronous rectifier when it turns on. The N-MOSFET
turns off and allows the converter entering discontinuous
conduction mode when the inductor current decreases to
zero. The zero current detection on threshold is about
80mA.This reduces conduction loss and increase power
conversion efficiency at light load condition.
DS8005-08 August 2007
PSM Operation
Consequently, the converter will enter pulse-skipping mode
(PSM) during extreme light load condition or when
modulation index (V
OUT
/V
IN
) is extreme low. This could
reduce switching loss and further increase power
conversion efficiency.
Over Current Protection
The RT8005 continuously monitors the inductor current
by sensing the voltage across the P-MOSFET when it
turns on. When the inductor current is higher than current
limit threshold (1.8A typical), OCP activates and forces
the P-MOSFET turning off to limit inductor current cycle
by cycle.
Output Voltage Setting and Feedback Network
The output voltage can be set from V
REF
to V
IN
by a voltage
divider as: the internal V
REF
is 0.5V with 1.5% accuracy.
In practical application, keep R1 = 10kΩ respectively and
choose appropriate R2 according to the required output
voltage.
Inductor Selection
The output inductor is suggested as the table of suggested
inductors for optimal performance. Make sure that the
inductor will not saturate over the operation conditions
including temperature range, input voltage range, and
maximum output current. If possible, choose an inductor
with rated current higher than 2A so that it will not saturate
even under short circuit condition.
Input Capacitor Selection
The input capacitor can filter the input peak current and
noise at input voltage source. The capacitor with low ESR
(effective series resistance) provides the small drop voltage
to stabilize the input voltage during the transient loading.
For input capacitor selection, the ceramic capacitors larger
than 2.2µF is recommend. The capacitor must conform to
the RMS current requirement. The maximum RMS ripple
current is calculated as :
I
RMS
=
I
OUT(MAX)
V
OUT
(V
IN
- V
OUT
)
V
IN
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