STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8540/8541 Series
Rev.4.0_00
10 Seiko Instruments Inc.
Operation
1. Switching control method
1. 1 PWM control (S-8540 Series)
The S-8540 series consists of pulse width modulation (PWM) DC/DC converters. In conventional
pulse frequency modulation (PFM) DC/DC converters, pulses are skipped when they operate at low
output load current, causing the variation in the ripple frequency and the increase in the ripple voltage of
the output voltage both of which constitute inherent drawbacks to those converters.
In the S-8540 series the pulse width varies in the range from 0 to 100% according to the load current,
yet ripple voltage produced by the switching can easily be removed by a filter since the switching
frequency is always constant. These converters thus provide a low-ripple voltage over wide range of
input voltage and load current. And it 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 switchover control (S-8541 Series)
The S-8541 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-8541 series operates under PWM control with the pulse width duty changing from 29 to 100%
when the output load current is high. On the other hand, when the output current is low, the S-8541
series operates under PFM control with the pulse width duty fixed at 29%, 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 when the load current is low. The
switching point from PWM control to PFM control depends on the external devices (coil, diode, etc.),
input voltage, and output voltage. This series is an especially efficient DC-DC converter at an output
current of around 100 μA.
