9

SC220Q

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 V 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 00-200mA, 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 9µA. 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 .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.

Applications Information

Applications Information