U6084B-MFPG3Y View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

U6084B-MFPG3Y

PWM Power Control with Automatic Duty Cycle Reduction

Atmel Corporation 

3.6 Pin 6 – Oscillator

The oscillator determines the frequency of the output voltage. This is defined by an external

capacitor, C2. It is charged with a constant current, I, until the upper switching threshold is

reached. A second current source is then activated which taps a double current, 2 × I, from the

charging current. The capacitor, C2, is thus discharged by the current, I, until the lower switching

threshold is reached. The second source is then switched off again and the procedure starts

once more.

3.6.1

Example for Oscillator Frequency Calculation

VT100 = VS × α1 = (VBatt – IS × R3) × α1

VT<100 = VS × α2 = (VBatt – IS × R3) × α2

VTL = VS × α3 = (VBatt – IS × R3) × α3

where

VT100 = High switching threshold 100% duty cycle

VT<100 = High switching threshold < 100% duty cycle

VTL = Low switching threshold

α1, α2 and α3 are fixed values

The above mentioned threshold voltages are calculated for the following values given in the

datasheet.

VBatt = 12V, IS = 4 mA, R3 = 150Ω

α1 = 0.7, α2 = 0.67 and α3 = 0.28

VT100 = (12V – 4 mA × 150Ω) × 0.7 ≈ 8V

VT<100 = 11.4V × 0.67 = 7.6V

VTL = 11.4V × 0.28 = 3.2V

For a duty cycle of 100%, the oscillator frequency, f, is as follows:

f

=

--------------------------I--o---s--c--------------------------

2 × (VT100 – VTL) × C2

where C2 = 22 nF and Iosc = 40 µA

Therefore:

f = -------------------------4---0-----µ---A--------------------------- = 189Hz

2 × (8V – 3.2V) × 22 nF

For a duty cycle of less than 100%, the oscillator frequency, f, is as follows:

f = --------------------------------------------------I-o---s--c--------------------------------------------------

2 × (VT<100 – VTL) × C2 + 4 × VBatt × C4

where C4 = 470 pF

f = ------------------------------------------------------4---0-----µ----A------------------------------------------------------- = 185Hz

2 × (7.6V – 3.2V) × 22 nF + 4 × 12V × 470 pF

A selection of different values of C2 and C4 provides a range of oscillator frequencies from 10Hz

to 2000Hz.

4 U6084B

4677C–AUTO–09/05

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