rt9201 Richtek Technology Corporation, rt9201 Datasheet - Page 10

• Manufacturer Part Number: rt9201
• Description: High Efficiency Step-down Dc-dc Converter
• Manufacturer: Richtek Technology Corporation

Application Information
RT9201 is a high-efficiency pulse-width-modulated
(PW M) step-down DC-DC converter. Capable of
delivering 600mA output current over a wide input voltage
range from 2.5 to 5.5V, the RT9201 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 RT9201 and reduces its
quiescent current to 0.1 A.
Pulling EN pin higher than 1.4V enables the RT9201 and
initiates the softstart cycle. A softstart capacitor connected
to SS pin along with the internal 300k
determines V
behavior. A softstart capacitor between 330pF to 1000pF
is recommended for smooth start-up of RT9201.
PWM Operation
During normal operation, the RT9201 regulates output
voltage by switching at a constant frequency and then
transferring the power to the load in each cycle by PWM.
The RT9201 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 PMOSFET. 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 NMOSFET 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.
www.richtek.com
10
RT9201
REF
ramp-up speed and the softstart
resistor
Preliminary
A Zero Detector monitors inductor current by sensing
voltage drop across the NMOSFET synchronous rectifier
when it turns on. The NMOSFET turns off and allows the
converter entering discontinuous conduction mode when
the inductor current decreases to zero. The zero current
defection on threshold is about 50mA.This reduces
conduction loss and increase power conversion efficiency
at light load condition.
PSM Operation
The minimum on-time of RT9201 is approximately 200ns.
Consequently, the converter will enter pulse-skipping-
mode (PSM) during extreme light load condition or when
modulation index (V
reduce switching loss and further increase power
conversion efficiency.
For example : V
Low Dropout Mode Operation
The maximum on-time can exceed one oscillator cycle,
which permits operation up to 100% duty cycle. As the
input voltage drops, the duty cycle increases until the
PMOSFET is held on continuously. Dropout voltage in
100% duty cycle is the output current multiplied by the
on-resistance of the internal switch and inductor, around
300mV (I
When the converter operates around the boundary of
Low-Dropout-Mode and PWM, sub-harmonic oscillation
and large ripple may occur at output voltage as shown in
Figure 1. Figure 2 illustrates the boundary of stable PWM
and Low-Dropout-Mode operations for typical output
voltages. Above each boudary is the stable PWM mode
region with respect to that output voltage. To prevent sub-
harmonic oscillation, please make sure that V
locate in the stable PWM operation region. If the input
voltage range must cross the boundary region, increasing
output capacitors with low ESR can alleviate the output
ripple voltage.
OUT
= 600mA).
IN
= 3.8V, V
OUT
/V
IN
OUT
) is extreme low. This could
= 2.5V & I
DS9201-04 August 2004
LOAD
IN
= 4mA
and V
OUT