9

SC220

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-

Applications Information

Applications Information