U6084B Datasheet, PDF (3/10 Pages) ATMEL Corporation – PWM Power Control with Automatic Duty-cycle Reduction

U6084B
Functional Description
Pin1 – GND
Ground-wire Breakage
Pin 2 – Enable/Disable
To protect the FET in case of ground-wire breakage, a 820 kΩ resistor between gate
and source is recommended to provide proper switch-off conditions.
The dimmer can be switched on or off, with pin 2, independently of the set duty cycle.
Pin 3 – Control Input
Pin 4 – Duty Cycle
Reduction
Output Slope Control
Table 1. Pin 2 Function
V₂
Approximately > 0.7 V or open
< 0.7 V or connected to pin 1
Function
Disable
Enable
The pulse width is controlled by means of an external potentiometer (47 kΩ). The char-
acteristic (angle of rotation/duty cycle) is linear. The duty cycle be varied from 0 to
100%. It is possible to further restrict the duty cycle with resistors R₁ and R₂
(see Figure 3 on page 8).
Pin 3 is protected against short-circuit to V_Batt and ground GND (V_Batt ≤16.5 V).
With pin 4 connected according to Figure 3 on page 8, the set duty cycle is reduced to
V_Batt ≈ 12.5 V. This causes a power reduction in the FET and in the lamps. In addition,
the brightness of the lamps is largely independent of the supply voltage range,
V_Batt = 12.5 to 16 V.
The rise and fall time (t_r, t_f) of the lamp voltage can be limited to reduce radio interfer-
ence. This is done with an integrator which controls a power MOSFET as source
follower. The slope time is controlled by an external capacitor C₄ and the oscillator cur-
rent (see Figure 3 on page 8).
Calculation:
t_f
=
t_r
=
V_Batt ×
--C----⁴--
I_osc
With V_Batt = 12 V, C₄ = 470 pF and I_osc = 40 µA, we thus obtain a controlled slope of
t_f
=
t_r
=
12 V ×
4----7---0-----p----F--
40 µA
×
141 µs
Pin 5 – Attenuation
Pin 6 – Oscillator
Capacitor C₄ connected to pin 5 damps oscillation tendencies.
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 cur-
rent, 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.
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4677B–AUTO–02/04