SC220(2012) Просмотр технического описания (PDF) - Semtech Corporation
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SC220 Datasheet PDF : 14 Pages
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SC220
Applications Information
General Description
The SC220 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 SC220 is capable of delivering a peak efficiency
of 90%, while maintaining efficiency over 80% during light
load condition. The converter operates at 20MHz switch-
ing frequency with Pulse Width Modulation (PWM) with
moderate to heavy output load current of up to 650mA,
which is referred as PWM mode. Under light-load condi-
tion, the converter operates in Power Save (PSAVE)
mode.
Control Scheme
The SC220 operates with a self-oscillating control method
based on the output voltage ripple. This control loop com-
pares 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 is held at a precise value. This architecture
gives a single-cycle response to transient events.
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 SC220 operates in two modes over a wide range of
load currents. Under moderate to heavy load conditions,
it operates in PWM mode with the switching frequency
held constant. Under light-load condition, the converter
enters PSAVE mode automatically. 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 voltages. Under very light load
condition, the SC220 maintains high efficiency by shut-
ting down all but the most essential circuit blocks,
maintaining a typical quiescent current of about 19μA. 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 millivolts) between normal operation
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.
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
consecutive cycles, the transition to PSAVE mode is initi-
ated. Once in PSAVE mode, the regulation is maintained
by modulating 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 instantaneous switch from PSAVE mode back
to PWM mode.
Enable and Start-up
The SC220 is enabled by applying a voltage higher than
1.2V on the EN pin, and it is disabled when the applied
voltaga 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 SC220 will sense this as a
valid enable signal and the external clock will be used
rather than the internal 20MHz oscillator.
The SC220 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 SC220 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 SC220 integrates a current limit feature to protect
itself and the external components under overload con-
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