SC221 Datasheet, PDF (9/14 Pages) Polyfet RF Devices – SILICON GATE ENHANCEMENT MODE RF POWER VDMOS TRANSISTOR

SC221 x
Applications Information
General Description
The SC221 is a step-down regulator capable of delivering
a lower voltage from an input supply voltage of 2.7V to
5.5V with high efficiency. Using a unique control architec-
ture, the SC221 is capable of delivering a peak efficiency
upto 90%, while maintaining efficiency over 80% during
light load condition. The converter operates at 20MHz
switching frequency with pulse width modulation (PWM)
at moderate to heavy loads up to 650mA. Under light
load, the converter operates in power save (PSAVE) mode.
voltages. At very light loads, the SC221 maintains high
efficiency by shutting down all but the most essential
circuit blocks, maintaining a typical quiescent current of
about 17.5µA. In this mode, some circuitry used to control
absolute DC accuracy is turned off. In that case, there may
be a small DC shift (tens of mV) between normal opera-
tion and this ‘no-load’ state. However, in the case of a load
transient, the system will turn on the high-side FET within
nanoseconds as a discontinuous pulse is issued. The tran-
sition to PWM mode can occur within that nominal
on-time, giving superior no load to full load transient
response.
Control Scheme
The SC221 operates with a self-oscillating control method
based on the output voltage ripple. This control loop
compares the output voltage to an internal 1V reference
to regulate the output voltage directly, adjusting the
turn-on and turn-off time of the power switches such that
the output voltage at the load is held at a precise value.
This architecture gives a single-cycle response to transient
events.
The transition from PWM mode to PSAVE mode is con-
trolled by sensing the minimum value of the ripple
current. When it drops below a threshold level for 32 con-
secutive cycles, the transition to PSAVE mode is initiated.
Once in PSAVE, the regulation is maintained by modulat-
ing the time between fixed current pulses. When a new
pulse is required by the loop before the existing pulse has
terminated, the loop determines that the load cannot be
maintained in PSAVE mode. This prompts an instanta-
neous switch from PSAVE mode back to PWM mode.
To maintain constant frequency, a phase locked loop (PLL)
is included to synchronize the switching with an internal
clock. This PLL adjusts the effective upper and lower
thresholds of the comparator, thus maintaining a constant
frequency. Under transient conditions the voltage control
loop determines the required switching pattern to best
maintain the output voltage. After a short transient, the
PLL will bring the switching frequency back to normal. At
extreme duty cycles, where the frequency control loop
may not be able to maintain frequency lock even under
steady-state conditions, frequency may fall, but the
output voltage regulation will be maintained by the main
voltage control loop. This unique architecture helps to
achieve the excellent line and load transient response.
Operating Modes
The SC221 operates in two modes over a wide range of
load currents. At moderate to high load, it operates in
PWM mode with its switching frequency held constant.
Under light loads, the converter enters PSAVE mode auto-
matically. With the typical 220nH inductor, the transition
between PWM and PSAVE mode typically occurs in the
range of 100-200mA, depending on the input and output
Enable and Start-up
The SC221 is enabled by applying a voltage higher than
1.2V on EN pin pin, and it is disabled when the applied
voltage is pulled below the logic low threshold. The EN
pin can also be used to set the switching frequency: if a
digital clock is fed to EN, the SC221 will sense this as a
valid enable signal and the external clock will be used
rather than the internal 20MHz oscillator.
The SC221 has an internal soft start circuit that limits the
inrush current during start-up with a stepped current
limit. Over the course of about 40μs, the SC221 is stepped
in increments of one quarter of the nominal current limit
to full current limit, thereby reducing the worst-case
surge current that might otherwise be reflected to the
input current.
Current Limit
The SC221 integrates a current limit feature to protect
itself and the external components during over load con-
dition. When the current in the high-side PMOS switch
exceeds the current limit, the PMOS switch is turned off.
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