SC220Q 데이터 시트보기 (PDF) - Semtech Corporation
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SC220Q Datasheet PDF : 14 Pages
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SC220 x
SC220Q
Applications Information
General Description
The SC220/Q is a step-down regulator capable of deliver-
ing a lower voltage from an input supply voltage of 2.7V
to 5.5V with high efficiency. Using a unique control archi-
tecture, the SC220/Q is capable of delivering a peak
efficiency of 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.
Control Scheme
The SC220/Q operates with a self-oscillating control
method based on the output voltage ripple. This control
loop compares the output voltage to an internal 1V refer-
ence 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.
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/Q 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
voltages. At very light loads, the SC220/Q maintains high
efficiency by shutting 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 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.
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.
Enable and Start-up
The SC220/Q 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 fre-
quency: if a digital clock is fed to EN, the SC220/Q will
sense this as a valid enable signal and the external clock
will be used rather than the internal 20MHz oscillator.
The SC220/Q 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/Q 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/Q 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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