STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

S-8520/8521 Series

13



Operation

1. Switching control method

1. 1 PWM control (S-8520 Series)

The S-8520 Series is a DC-DC converter using a pulse width modulation method (PWM) and features a low

current consumption.

In conventional PFM DC-DC converters, pulses are skipped when the output load current is low, causing a

fluctuation in the ripple frequency of the output voltage, resulting in an increase in the ripple voltage.

The switching frequency does not change, although the pulse width changes from 0 to 100% corresponding to

each load current. The ripple voltage generated from switching can thus be removed easily through a filter

because the switching frequency is constant. And the ripple voltage will be skipped to be low current

consumption when the pulse width is 0% or it is no load, input current voltage is high.

1. 2 PWM / PFM switching control (S-8521 Series)

S-8521 Series is a DC-DC converter that automatically switches between a pulse width modulation method

(PWM) and a pulse frequency modulation method (PFM), depending on the load current, and features low

current consumption.

The S-8521 Series operates under PWM control with the pulse width duty changing from 25 to 100% in a high

output load current area.

The S-8521 Series operates under PFM control with the pulse width duty fixed at 25%, and pulses are skipped

according to the load current. The oscillation circuit thus oscillates intermittently so that the resultant lower self

current consumption prevents a reduction in the efficiency at a low load current. The switching point from PWM

control to PFM control depends on the external devices (coil, diode, etc.), input voltage and output voltage.

The S-8521 Series is an especially highly efficient DC-DC converter at an output current around 100

μA.