RT8203GA Richtek USA Inc, RT8203GA Datasheet - Page 16

• Manufacturer Part Number: RT8203GA
• Description: IC CTRLR SMPS STPDN DUAL 28SSOP
• Manufacturer: Richtek USA Inc

Specifications of RT8203GA

• Output Isolation: Isolated
• Frequency Range: Adjustable, Selectable
• Voltage - Input: 7 V ~ 24 V
• Voltage - Output: Selectable
• Power (watts): 1.05W
• Operating Temperature: -10°C ~ 85°C
• Package / Case: 28-SSOP (0.150", 3.90mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

RT8203
Application Information
The RT8203 is a dual, Mach Response
valley mode synchronous buck controller. The controller
is designed for low voltage power supplies for notebook
computers. Richtek's Mach Response
specifically designed for providing 100ns “instant-on”
response to load steps while maintaining a relatively
constant operating frequency and inductor operating point
over a wide range of input voltages. The topology
circumvents the poor load transient timing problems of
fixed-frequency current mode PWMs while avoiding the
problems caused by widely varying switching frequencies
in conventional constant-on-time and constant off-time
PWM schemes. The DRV
specifically designed to have better noise immunity for
such a dual output application. The RT8203 includes 5V
(LDO5) and 3.3V (LDO3) linear regulators. LDO5 linear
regulator can step down the battery voltage to supply both
internal circuitry and gate drivers. The synchronous-switch
gate
voltage is above 4.65V, an
LDO5 linear regulator and powers the device form VOUT5.
PWM Operation
The Mach Response
the output filter capacitor's effective series resistance (ESR)
to act as a current sense resistor, so the output ripple
voltage provides the PWM ramp signal. Refer to the
RT8203's function block diagram, the synchronous high
side MOSFET is turned on at the beginning of each cycle.
After the internal one-shot timer expires, the MOSFET is
turned off. The pulse width of this one shot is determined
by the converter's input voltage and the output voltage to
keep the frequency fairly constant over the input voltage
range. Another one shot sets a minimum off-time (400ns
typ). The on-time one shot is triggered if the error
comparator is high, the low side switch current is below
the current limit threshold, and the minimum off-time one
shot has timed out.
PWM Frequency and On-Time Control
The Mach Response
pseudo-constant frequency by feed forwarding the input
and output voltage into the on-time one shot timer. The
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drivers are directly powered from LDO5. When VOUT5
TM
TM
DRV
control architecture runs with
TM
automatic circuit turns off the
TM
mode PWM modulator is
mode controller relies on
TM
TM
DRV
technology is
TM
dual ramp
high side switch on-time is inversely proportional to the
input voltage as measured by the V
the output voltage. There are two benefits of a constant
switching frequency. The first is the frequency can be
selected to avoid noise sensitive regions such as the
455kHz IF band. The second is the inductor ripple-current
operating point remains relatively constant, resulting in easy
design methodology and predictable output voltage ripple.
The frequency for 5V SMPS is set at 100kHz higher than
the frequency for 3V SMPS. This is done to prevent audio-
frequency “beating” between the two sides, which switch
asynchronously for each side. The on-time is given by :
On-Time = K ( V
where K is set by the TON pin-strap connection (Table 1).
The on-times guaranteed in the Electrical Characteristics
tables are influenced by switching delays in the external
high-side power MOSFET. Two external factors that
influence switching frequency accuracy are resistive drops
in the two conduction loops (including inductor and PC
board resistance) and the dead-time effect. These effects
are the largest contributors to the change of frequency
with changing load current. The dead time effect increases
the effective on-time, reducing the
one or both dead times. It occurs only in Forced CCM
Mode (SKIP = high) when the inductor
light or negative load currents. With reversed inductor
current, the inductor’ s EMF causes PHASEx to go high
earlier than normal, extending the on-time by a period equal
to the low-to-high dead time. For loads above the critical
conduction point, the actual switching frequency is :
where V
the inductor discharge path, including synchronous rectifier,
inductor, and PC board resistances; V
the resistances in the charging path; and t
time calculated by the RT8203.
Operation Mode Selection (SKIP)
The RT8203 supports three operation modes: Diode-
Emulation Mode, Ultrasonic Mode, and Forced-CCM Mode.
f =
t
ON
(V
DROP1
OUT
x (V
IN
+ V
is the sum of the parasitic voltage drops in
+ V
DROP1
OUT
DROP2
/ V
)
IN
)
)
DS8203-04 March 2011
switching frequency as
IN
, and proportional to
DROP2
current reverses at
ON
is the sum of
is the on-