S-8520A20MC-AVFT2X Datasheet, PDF (19/44 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.9.0_01
S-8520/8521 Series
5. External transistor
Enhancement (Pch) MOS FET or bipolar (PNP) MOS FET can be used for external transistor.
5. 1 Enhancement (Pch) MOS FET type
The EXT pin of the S-8520/8521 Series is capable of directly driving a Pch MOS FET with a gate capacity
around 1000 pF.
When using a Pch MOS FET, 2 to 3% higher efficiency is provided because its switching speed is faster and it
does not cause power dissipation, compared to PNP bipolar transistors.
The important parameters in selecting MOS FETs are the threshold voltage, the breakdown voltage between
gate and source, the breakdown voltage between drain and source, the total gate capacity, the on-resistance,
and the current ratings.
The EXT pin swings from voltages between VIN to VSS. If the input voltage is low, use a MOS FET with the low
threshold voltage. If the input voltage is high, use a MOS FET having the breakdown voltage between gate and
source higher several volts than the input voltage.
Immediately after the power-on or power-off (stopping the step-down operation), the input voltage will be applied
between drain and source of the MOS FET. Use the breakdown voltage between drain and source also higher
several volts than the input voltage.
The total gate capacity and the on-resistance affect efficiency.
Power dissipation when charging and discharging the gate capacity by switching operation affects efficiency, in
the area of low load current, as the total gate capacity is larger and the input voltage is higher. Select a MOS
FET with a small total gate capacity for efficiency at light load.
In the area of large load current, efficiency is affected by power dissipation caused by MOS FET’s on-resistance.
For efficiency at large load, select a MOS FET having as low on-resistance as possible.
As for the current rating, select a MOS FET having the maximum continuous drain current rating higher than IPK.
For reference, this document has the data of efficiency. TM6201 by Toyota Industries Corporation for
applications with an input voltage of 10 V or less, IRF7606 by International Rectifier Corporation Japan for
applications with an input voltage over 10 V (Refer to “ Reference Data”).
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