U6084B_05 Datasheet, PDF (5/11 Pages) ATMEL Corporation – PWM Power Control with Automatic Duty-cycle Reduction

U6084B
3.7 Pins 7, 8, 10 and 15
Not connected.
3.8 Pin 9 – Status Short Circuit Latch
The status of the short-circuit latch can be monitored via pin 9 (open collector output).
Table 3-2. Pin 9 Function
Pin 9
L
H
Function
Short-circuit detected
No short-circuit detected
3.9 Pins 11 and 12 – Short-circuit Protection and Current Sensing
3.9.1
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 duty cycle is
switched over to 100% and capacitor C₅ is charged by a current source of 20 µA (I_ch – I_dis). The
external FET is switched off after the cut-off threshold (V_T11) is reached. Renewed switching on
of the FET is possible only after a power-on reset. The current source, I_dis, ensures that capaci-
tor C₅ is not charged by parasitic currents. Capacitor C₅ is discharged by I_dis to typically 0.7V.
Time delay, t_d, is as follows:
t_d
=
C₅ ×
-⁽--V----¹---¹----^–----0----.-7----V-----⁾
(I_ch – I_dis)
With C₅ = 330 nF and V_Batt = 12V, we have
t_d = 330 nF × -⁽--9---.--8--2-V---0---^–--_µ--0-A---.-7----V----⁾ = 150 ms
3.9.2
Current Limitation
The lamp current is limited by a control amplifier that protects the external power transistor. The
voltage drop across an external shunt resistor acts as the measured variable. Current limitation
takes place for a voltage drop of V_T1 ≈ 100 mV. Owing to the difference V_T – V_T2 ≈ 10 mV, cur-
rent limitation occurs only when the short-circuit detection circuit has responded.
After a power-on reset, the output is inactive for half an oscillator cycle. During this time, the sup-
ply voltage capacitor can be charged so that current limitation is guaranteed in the event of a
short-circuit when the IC is switched on for the first time.
3.10
Pins 13 and 14 – Charge Pump and Output
Pin 14 (output) is suitable for controlling a power MOSFET. During the active integration phase,
the supply current of the operational amplifier is mainly supplied by capacitor C₃ (bootstrapping).
Additionally, a trickle charge is generated by an integrated oscillator (f₁₃ ≈ 400 kHz) and a volt-
age doubler circuit. This permits a gate voltage supply at a duty cycle of 100%.
5
4677C–AUTO–09/05