U6084B_05 Datasheet, PDF (4/11 Pages) ATMEL Corporation – PWM Power Control with Automatic Duty-cycle Reduction

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
V_T100 = V_S × α₁ = (V_Batt – I_S × R₃) × α₁
V_T<100 = V_S × α₂ = (V_Batt – I_S × R₃) × α₂
V_TL = V_S × α₃ = (V_Batt – I_S × R₃) × α₃
where
V_T100 = High switching threshold 100% duty cycle
V_T<100 = High switching threshold < 100% duty cycle
V_TL = 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.
V_Batt = 12V, I_S = 4 mA, R₃ = 150Ω
α₁ = 0.7, α₂ = 0.67 and α₃ = 0.28
V_T100 = (12V – 4 mA × 150Ω) × 0.7 ≈ 8V
V_T<100 = 11.4V × 0.67 = 7.6V
V_TL = 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 × (V_T100 – V_TL) × C₂
where C₂ = 22 nF and I_osc = 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 × (V_T<100 – V_TL) × C₂ + 4 × V_Batt × 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