U6084B Datasheet, PDF (5/10 Pages) ATMEL Corporation – PWM Power Control with Automatic Duty-cycle Reduction

U6084B
Pins 11 and 12 – Short-circuit Protection and Current Sensing
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 the FET is possible only after a power-on reset. The current
source, I_dis, ensures that capacitor C₅ is not charged by parasitic currents. Capacitor C₅
is discharged by I_dis to typ. 0.7 V.
Time delay, t_d, is as follows:
t_d = C₅ × -⁽--V--₍--¹_I--c-¹-_h---^–--_–--0--I--._d-7--_i-_s--V₎-----⁾
With C₅ = 330 nF and V_Batt = 12 V, we have
t_d = 330 nF × -⁽--9---.--8----2-V--0---^–---_µ--0-A--.--7-----V-----⁾ = 150 ms
Current Limitation
The lamp current is limited by a control amplifier that protects the external power transis-
tor. 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 differ-
ence V_T - V_T2 ≈ 10 mV, current 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 supply 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.
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 voltage doubler circuit. This permits a gate voltage supply at a
duty cycle of 100%.
Pin 16 – Supply Voltage,
V_s or V_Batt
Undervoltage Detection
In the event of voltages of approximately V_Batt < 5.0 V, the external FET is switched off
and the latch for short-circuit detection is reset.
A hysteresis ensures that the FET is switched on again at approximately V_Batt ≥ 5.4 V.
Overvoltage Detection
Stage 1
Stage 2
If overvoltages of V_Batt > 20 V (typically) occur, the external transistor is switched off and
switched on again at V_Batt < 18.5 V (hysteresis).
If V_Batt > 28.5 V (typically), the voltage limitation of the IC is reduced from 26 V to 20 V.
The gate of the external transistor remains at the potential of the IC ground, thus pro-
ducing voltage sharing between the FET and lamps in the event of overvoltage pulses
(e.g., load-dump). The short-circuit protection is not in operation. At V_Batt < 23 V, the
overvoltage detection stage 2 is switched off.
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4677B–AUTO–02/04