U6084B_05 Datasheet, PDF (3/11 Pages) ATMEL Corporation – PWM Power Control with Automatic Duty-cycle Reduction

U6084B
3. Functional Description
3.1 Pin1 – GND
3.1.1
Ground-wire Breakage
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.
3.2 Pin 2 – Enable/Disable
The dimmer can be switched on or off with pin 2, independently of the set duty cycle.
Table 3-1. Pin 2 Function
V₂
> approximately 0.7V or open
< 0.7V or connected to pin 1
Function
Disable
Enable
3.3 Pin 3 – Control Input
The pulse width is controlled by means of an external potentiometer (47 kΩ). The characteristic
(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 (Figure 7-1 on page 9).
Pin 3 is protected against short-circuit to V_Batt and ground GND (V_Batt ≤16.5V).
3.4 Pin 4 – Duty Cycle Reduction
With pin 4 connected according to Figure 7-1 on page 9, the set duty cycle is reduced to
V_Batt ≈ 12.5V. 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 = 12.5V to 16V.
3.4.1
Output Slope Control
The rise and fall time (t_r, t_f) of the lamp voltage can be limited to reduce radio interference. 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 current (Figure 7-1 on page 9).
Calculation:
t_f
=
t_r
=
V_Batt ×
--C----⁴--
I_osc
With V_Batt = 12V, C₄ = 470 pF and I_osc = 40 µA, the controlled slope is
t_f
=
t_r
=
12V ×
4----7---0-----p----F--
40 µA
×
141 µs
3.5 Pin 5 – Attenuation
Capacitor C₄ connected to pin 5 damps oscillation tendencies.
3
4677C–AUTO–09/05