STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8540/8541 Series
Rev.4.0_00
16 Seiko Instruments Inc.
5. External transistor
The S-8540/8541 series can work with an enhancement (Pch) MOS FET or a bipolar (PNP) transistor as
an external transistor.
5. 1 Enhancement (Pch) MOS FET
The EXT pin can directly drive the Pch MOS FET with a gate capacity of approximate 1200 pF.
When a Pch MOS FET is chosen, efficiency will be 2 to 3 % higher than that achieved by a PNP
bipolar transistor since the MOS FET switching speed is faster than that of the bipolar transistor and
power loss due to the base current is avoided.
The important parameters in selecting a Pch MOS FET are the threshold voltage, breakdown voltage
between gate and source, breakdown voltage between drain and source, total gate capacity, on-
resistance, and the current ratings.
The EXT pin swings from voltage V
IN
to V
SS
. When the input voltage is low, a MOS FET with a low
threshold voltage has to be used so that the MOS FET will turn on as required. When, conversely, the
input voltage is high, select a MOS FET whose gate-source breakdown voltage is higher than the
input voltage by at least several volts.
Immediately after the power is turned on, or the power is turned off (that is, when the step-down
operation is terminated), the input voltage is applied across the drain and the source of the MOS FET.
The transistor therefore needs to have drain-source breakdown voltage that is also several volts
higher than the input voltage.
The total gate capacity and the on-resistance affect the efficiency.
The power loss for charging and discharging the gate capacity by switching operation will affect the
efficiency at low load current region more when the total gate capacity becomes larger and the input
voltage becomes higher. If the efficiency at low load is a matter of concern, select a MOS FET with a
small total gate capacity.
In regions where the load current is high, the efficiency is affected by power loss caused by the on-
resistance of the MOS FET. If the efficiency under heavy load is particularly important in the
application, choose a MOS FET having on-resistance as low as possible.
As for the current rating, select a MOS FET whose maximum continuous drain current rating is higher
than I
PK
.
