s-8521b51mc-aukt2g Seiko Instruments Inc., s-8521b51mc-aukt2g Datasheet - Page 19

• Manufacturer Part Number: s-8521b51mc-aukt2g
• Description: Step-down, Pwm Control Or Pwm / Pfm Switchable Switching Regulator Controller
• Manufacturer: Seiko Instruments Inc.

STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLOR
Rev.8.3
S-8520 / 8521 Series
_00
5. External transistor
The external transistor of the enhancement (Pch) MOS FET type or bipolar (PNP) typ.
5. 1 Enhancement (Pch) MOS FET type
The EXT pin is capable of directly driving a Pch MOS FET with a gate capacity of some 1000 pF.
When a Pch MOS FET is chosen, because it has a higher switching speed than a PNP type bipolar
transistor and because power losses due to the presence of a base current are avoided, efficiency will
be 2 to 3 % higher than when other types of transistor are employed.
The important parameters to be kept in mind in selecting a MOS FET include the threshold voltage,
breakdown voltage between gate and source, breakdown voltage between drain and source, total
gate capacity, on-resistance, and the current rating.
The EXT pin swings from voltage V
over to voltage V
. If the input voltage is low, a MOS FET with
IN
SS
a low threshold voltage has to be used so that the MOS FET will come on as required. If, 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 when the power is turned off (that is, when the step-down
operation is terminated), the input voltage will be imposed across the drain and the source of the MOS
FET. Therefore, the transistor needs to have a 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 increase,
when the total gate capacity becomes larger and the input voltage rises higher. Therefore the gate
capacity affects the efficiency of power in a low load current region. If the efficiency under light loads
is a matter of particular 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 losses caused due to the
on-resistance of the MOS FET.
Therefore, if the efficiency under heavy loads is particularly
important for your application, choose a MOS FET with as low an on-resistance as possible.
As for the current rating, select a MOS FET whose maximum continuous drain current rating is higher
than I
.
PK
For reference purpose, some efficiency data has been included in this document. For applications
with an input voltage range of 10 V or less, data was obtained by using TM6201 of Toyoda Industries
Corporation. IRF7606, a standard of International Rectifier Corporation, was used for applications
with an input voltage range over 10 V (Refer to " Reference Data ").
Seiko Instruments Inc.
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