u6084b ATMEL Corporation, u6084b Datasheet - Page 4

• Manufacturer Part Number: u6084b
• Description: U6084b Pwm Power Control With Automatic Duty-cycle Reduction
• Manufacturer: ATMEL Corporation

3.6
3.6.1
4
Pin 6 – Oscillator
U6084B
Example for Oscillator Frequency Calculation
The oscillator determines the frequency of the output voltage. This is defined by an external
capacitor, C
reached. A second current source is then activated which taps a double current, 2
charging current. The capacitor, C
threshold is reached. The second source is then switched off again and the procedure starts
once more.
where
V
V
V
The above mentioned threshold voltages are calculated for the following values given in the
datasheet.
V
V
V
V
For a duty cycle of 100%, the oscillator frequency, f, is as follows:
Therefore:
For a duty cycle of less than 100%, the oscillator frequency, f, is as follows:
where C
A selection of different values of C
to 2000Hz.
V
V
V
f
f
f
1
1
f
T100
T<100
TL
Batt
T100
T<100
TL
T100
T<100
TL
=
=
,
=
= 0.7,
=
= 11.4V
= Low switching threshold
----------------------------------------------------------- -
2
---------------------------------------------------------------------------------------------------------- -
2
=
2
= 12V, I
---------------------------------------------------------------
2
------------------------------------------------------------------------------------------------------------------------- -
2
= High switching threshold 100% duty cycle
= (12V – 4 mA
and
=
= High switching threshold < 100% duty cycle
= 11.4V
V
=
S
V
4
V
V
8V 3.2V
V
7.6V 3.2V
S
T100
T<100
= 470 pF
2
S
3
= 0.67 and
–
2
S
I
3
40 µA
are fixed values
osc
. It is charged with a constant current, I, until the upper switching threshold is
–
–
=
= 4 mA, R
0.28 = 3.2V
1
–
2
V
=
V
TL
=
V
0.67 = 7.6V
TL
Batt
V
V
Batt
I
Batt
22 nF
osc
–
C
150 )
C
40 µA
22 nF
I
–
2
S
–
3
2
3
I
where C
S
= 0.28
+
I
= 150
S
R
=
4
+
3
R
189Hz
R
4
3
V
3
0.7
Batt
2
2
12V
2
3
, is thus discharged by the current, I, until the lower switching
= 22 nF and I
and C
1
2
C
8V
4
470 pF
4
provides a range of oscillator frequencies from 10Hz
=
osc
185Hz
= 40 µA
4677C–AUTO–09/05
I, from the