STCS1 Detail description
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7 Detail description
The STCS1 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 mA and 1.5 A can be set according to the resistor value, the resulting
output current has a tolerance of ± 10 %.
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 which is around 4 µs 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 a short delay time TD (see Figure 3).
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.
7.4 Diagnostic
When STCS1 is in on mode (EN is high), the device is able to detect disconnection or fail of
the LED string monitoring V
DRAIN
pin. If V
DRAIN
is lower than 75 mV the DISC pin is pulled
low regardless the PWM pin status. This information can be used by the system to inform
that some problem happens in the LEDs.
