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TSM108
(Rev.:2001)

AUTOMOTIVE SWITCH MODE VOLTAGE AND CURRENT CONTROLLER

STMicroelectronics 

TSM108

PRINCIPLE OF OPERATION AND APPLICATION HINTS

Description of a DC/DC step down battery

charging application

1. Voltage and Current Controller

TSM108 is designed to drive a P-Channel

MOSFET transistor in Switch Mode Step Down

Converter applications. Its two integrated

operational amplifiers ensure accurate Voltage

and Current Regulation.

The Voltage Control dedicated operational

amplifier acts as an error amplifier and compares

a part of the output voltage (external resistor

bridge) to an integrated highly precise voltage

reference (Vref).

The Current Control dedicated operational

amplifier acts as an error amplifier and compares

the drop voltage through the sense resistor to an

integrated low value voltage reference (Vs).

These two amplified errors are ORed through

diodes, and the resulting signal (“max of”) is a

reference for the PWM generator to set the

switching duty cycle of the P-Channel MOSFET

transistor.

The PWM generator comprises an oscillator (saw

tooth) and a comparator which gives a variable

duty cycle from 0 to 95%. This PWM signal is the

direct command of the output Push Pull stage to

drive the Gate of the P-Channel MOSFET.

Thanks to this architecture, the TSM108 is ideal to

be used from a DC power supply to control the

charging Voltage and Current of a battery in

applications such as Automotive accessories for

Portable Phone charging and power supplies.

2. Voltage Control

The Voltage Control loop is to be set thanks to an

external resistor bridge connected between the

output positive line and the Ground reference. The

middle point is to be connected to the Vctrl pin of

TSM108, and, if R1 is the upper resistor, and R2,

the lower resistor of the bridge, the values of R1

and R2 should follow:

u eq1: Vref = Vout x R2 / (R1 + R2)

When under Constant Voltage Control mode, the

output voltage is fixed thanks to the R1/R2 resistor

bridge.

The total value of R1 + R2 resistor bridge will

determine the necessary bleeding current to keep

the Voltage Control loop effective, even under “no

load” conditions.

The voltage compensation loop is directly

accessible from the pins Vcomp and Vref

(negative input of the Voltage Control dedicated

operational amplifier). The compensation network

is highly dependant of the conditions of use of the

TSM108 (switching frequency, external

components (R, L, C), MOSFET, output

capacitor...).

3. Current Control

The Current control loop is to be set thanks to the

Sense resistor which is to be placed in series on

the output positive line. The output side of the

Sense resistor should be connected to the Ictrl pin

of TSM108, and the common point between

Rsense and the filtering self L should be

connected to the Vs pin of TSM108. If Ilim is the

value of the charging current limit The value of

Rsense should verify:

u eq2: Vs = Rsense x Ilim

When under Constant Current Control mode, the

output current is fixed thanks to the Rsense

resistor (under output short circuit conditions,

please refer to this corresponding section).

The wattage calibration (W) of the sense resistor

should be chosen according to:

u eq2a: W > Rsense x Ilim2

The current compensation loop is directly

accessible from the pins Icomp and Ictrl (negative

input of the Current Control dedicated operational

amplifier.

The compensation network is highly dependant of

the conditions of use of the TSM108 (switching

frequency, external components (R, L, C),

MOSFET, output capacitor...).

4. PWM frequency

The internal oscillator of TSM108 is a saw tooth

waveform that can be frequency adjusted.

In automotive accessory battery charging

applications, it is recommended to set the

switching frequency at a typical 100kHz in order to

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