TSM108

8/13

6.4. OVLO increase:

If the OVLO level needs to be increased (OV2), an

additional resistor (Rovl2) must be connected

between OV and Gnd following the equation.

(iv)

where Rovl//Rovl2 means that Rovl2 is in parallel

to Rovl

Solving iv. we obtain:

Vref x (Rovh + Rovl))

As an example, if OV2 needs to be set to 40V,

Rovl2 = 87k

Ω

7. Standby Mode

In order to reduce to a minimum the current

consumption 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

functions

of the

TSM108

(operational

amplifier, PWM generator and oscillator, UVLO

and

OVLO)

and

therefore

reduces

the

consumption of the TSM108 to the Istby value.

This !STBY command is TTL compatible, which

means that it can be directly commanded from

whatever logic signal.

8.Power Transistor: P-MOSFET or PNP

Transistor?

The TSM108 can drive, with minor external

components

change,

either

a

P-channel

MOSFET, or a PNP transistor. The choice of the

transistor is completely to the user’s responsibility,

nevertheless, here follows a few elements which

will help to decide which is the most adapted

transistor to drive depending on the application

characteristics

in

terms

of

power

and

performances.

The

following

figures

shows

two

different

schematics 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- MOSFET? PNP Transistor?

The most immediate way to choose from a

P-channel MOSFET or a PNP transistor is to

consider the ratio between the output power of the

application and the expected components price:

the lower the power, the more suitable the PNP

transistor 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 recommended limit to choose from one to the

other is situated around 200mA.

Below 200mA, the price/performance ratio of the

PNP transistor is very attractive, whereas above

200mA,

the

P-channel

Mosfet

takes

the

advantage.

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

9.1. Inductor

The minimum inductor value to choose should

apply to

Lmin = (1 - D) R / 2F

where R = Vout / Iout = 9.6

Ω

and where D = Vout / Vin = 0.5

Therefore, Lmin = 24µH.

L1

GD

D1

TSM108

Q1

MOSFET P

Q1

L1

GD

D1

TSM108

Q1

L1

GD

D1

TSM108