TSM108 datasheet(8/13 Pages) STMICROELECTRONICS

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TSM108

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7. Standby Mode

In order to reduce to a minimum the current con-

sumption of the TSM108 when in inactive phase,

the Standby mode (!Stby pin of TSM108) imposes

a complete OFF state of the P-Channel MOSFET,

as well as a complete shut off of the main func-

tions of the TSM108 (operational amplifier, PWM

generator and oscillator, UVLO and OVLO) and

therefore reduces the consuption of the device.

The !STBY command is TTL compatible.

8.Power Transistor: P-channel MOSFET or

PNP Transistor?

The TSM108 can drive, with minor external com-

ponents change, either a P-channel MOSFET, or

a PNP transistor. The user can choose the type of

external power element, nevertheless, here fol-

lows a few considerations which will help to take

this decision

The following figures shows two different sche-

matics where both driving abilities of TSM108 are

shown. The third schematic shows how to improve

the switch off commutation when using a bipolar

PNP transistor.

P-channel MOSFET

PNP transistor

The most immediate way to choose between a

P-channel MOSFET or a PNP transistor is to con-

sider the ratio between the output power of the ap-

plication and the expected components price: the

lower the power, the more suitable the PNP tran-

sistor is; the higher the power, the more suitable

the P-channel MOSFET is. As an example, for a

DC/DC adaptor built for 12V/6V, the recommend-

ed limit to choose between P-channel Mosfet and

PNP transistor is around 200mA.

Below 200mA, the price/performance ratio of the

PNP transistor is very attractive, whereas above

200mA, the P-channel Mosfet takes the advan-

tage.

9. Calculation of the Passive Elements

Let's consider the following characteristics for a

Cigarette Lighter Cellular Phone Battery Charger:

Vin = 12V - input voltage of the converter

Vout = 6V - output voltage of the converter

F = 100kHz - switching frequency of the converter

adjustable with an external capacitor

Iout = 625mA - output current limitation

TSM108

MOSFET P

TSM108

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