SC221 datasheet(9/14 Pages) SEMTECH

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SC221 x

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.

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.

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

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.

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 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.

Applications Information

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