Detail description STCS1A
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7 Detail description
The STCS1A is a BiCMOS constant current source designed to provide a precise constant
current starting from a varying input voltage source. The main target is to replace discrete
components solution for driving LEDs in low voltage applications such as 5 V, 12 V or 24 V
giving benefits in terms of precision, integration and reliability.
7.1 Current setting
The current is set with an external sensing resistor connected to the FB pin. The feedback
voltage is 100 mV, then a low resistor value can be chosen reducing power dissipation. A
value between 1 for instance, should one need a 700 mA LEDs current, R
F
should be
selected according to the following equation:
For instance, should one need a 700 mA LEDs current, R
F
should be selected according to
the following equation:
R
F
= V
FB
/ I
LEDs
= 100 mV / 700 mA = 142 mΩ
7.2 Enable
When the enable pin is low the device completely off thus reducing current consumption to
less than 1 µA. When in shutdown mode, the internal main switch is off.
7.3 PWM dimming
The PWM input allows implementing PWM dimming on the LED current; when the PWM
input is high the main switch will be on and vice versa. A typical frequency range for the
input is from few Hertz to 50 kHz. The maximum dimming frequency is limited by the
minimum rise/fall time of the current (obtained with C
SLOPE
= 0) which is around 4µs each.
Above 50 kHz the current waveforms starts assuming a triangular shape.
While the PWM input is switching, the overall circuitry remains on, this is needed in order to
implement two important features: short delay time and controlled slope for the current.
Since the PWM pin is controlling just the main switch, the overall circuitry is always on and it
is able to control the delay time between the PWM input signal and the output current in the
range of few µs, this is important to implement synchronization among several light LED
sources.
The rise and fall slope of the current is controlled by the C
SLOPE
capacitor. The rise and fall
time are linear dependent from the C
SLOPE
capacitor value (see graph in typical
characteristics). A controlled rise time has two main benefits: reducing EMI noise and avoid
current spike at turn on.
When C
SLOPE
is left floating, the internal switch is turned on at maximum speed, in this
condition an overshoot can be present on the LED current before the system goes into
regulation.
