U6084B Datasheet, PDF (3/8 Pages) ATMEL Corporation – PWM Power Control with Automatic Duty-cycle Reduction

U6084B
charged with a constant current, I, until the upper switch-
ing 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.
Example for Oscillator Frequency Calculation
+ a + * a V_T100
V_S
1 ^(VBatt
I_S
R₃)
1
+ a + * a _¦ V_{T 100}
V_S
2 ^(VBatt
I_S
R₃)
2
+ a + * a V_TL V_S
3 ^(VBatt
I_S
R₃)
3
where
+ V_T100 High switching threshold (100% duty cycle)
+ t _t V_{T 100} High switching threshold ( 100% duty cycle)
+ V_TL Low switching threshold
a a a ₁, ₂ and ₃ are fixed constant.
The above mentioned threshold voltages are calculated
for the following values given in the data sheet.
W V_Batt = 12 V, I_S = 4 mA, R₃ = 150 ,
a a a ₁ = 0.7, ₂ = 0.67 and ₃ = 0.28.
+ * W V_T100 (12 V 4 mA 150 )
+ + _t V_{T 100} 11.4 V 0.67 7.6 V
+ + V_TL 11.4 V 0.28 3.2 V
[ 0.7 8 V
For a duty cycle of 100%, an oscillator frequency, f, is
as follows:
+f ₂
*^Iosc
(V_T100 V_TL)
++ m ^C2
, where C₂
and I_osc
22 nF
40 A
Therefore:
+ _* ^m f
40 A
2 (8 V 3.2 V)
+ _{22 nF} 189 Hz
For a duty cycle of less than 100%, the oscillator fre-
quency, f, is as follows:
+f ₂
* ) ^Iosc
t ^(VT 100 ^VTL^{) C}2 ⁴
V_Batt
C₄
whereas C₄ = 470 pF
+ ǒ * Ǔ ) 40 mA
2 7.6 V 3.2 V 22 nF 4 12 V 470 pF
+ 185 Hz
A selection of different values of C₂ and C₄, provides a
range of oscillator frequency, f, from 10 to 2000 Hz.
Pins 7, 8, 10 and 15
Not connected.
Pin 9, Status Short Circuit Latch
The status of the short-circuit latch can be monitored via
Pin 9 (open collector output).
Pin 9
L
H
Function
Short-circuit detected
No short-circuit detected
Pins 11 and 12, Short-Circuit Protection
and Current Sensing
1. Short-Circuit Detection and Time Delay, t_d
The lamp current is monitored by means of an external
 ^{shunt resistor. If the lamp current exceeds the threshold}
for the short-circuit detection circuit (V_T2 90 mV), the
m ^{duty cycle is switched over to 100% and the capacitor C5}
is charged by a current source of 20 A (I_ch – I_dis). The
external FET is switched off after the cut-off threshold
(V_T11) is reached. Renewed switching on the FET is pos-
sible only after a power-on reset. The current source, I_dis,
ensures that the capacitor C₅ is not charged by parasitic
currents. The capacitor C₅ is discharged by I_dis to typ.
0.7 V.
Time delay, t_d, is as follows:
+ @ * ń * t_d C₅ (V_T11 0.7 V) (I_ch I_dis)
With C₅ = 330 nF and V_Batt = 12 V, we have
+ @ * ń m t_d 330 nF (9.8 V 0.7 V) 20 A
+ 150 ms.
TELEFUNKEN Semiconductors
3 (8)
Rev. A1, 14-Feb-97