ST1CC40 Datasheet, PDF (20/37 Pages) STMicroelectronics – 3 A monolithic step-down current source with synchronous rectification

Application information
7
Application information
ST1CC40
7.1
7.1.1
7.1.2
Component selection
Sensing resistor
In closed loop operation the ST1CC40 feedback pin voltage is 100 mV so the sensing
resistor calculation is expressed as:
Equation 18
R_S = ¹----⁰-_I--_L⁰---_E--^m-_D----^V---
Since the main loop (see Section 6.1) regulates the sensing resistor voltage drop, the
average current is regulated into the LEDs. The integration period is at minimum 5 * T_SW
since the system bandwidth can be dimensioned up to F_SW/5 at maximum.
The system performs the output current regulation over a period which is at least five times
longer than the switching frequency. The output current regulation neglects the ripple
current contribution and its reliance on external parameters like input voltage and output
voltage variations (line transient and LED forward voltage spread). This performance can
not be achieved with simpler regulation loops like a hysteretic control.
For the same reason the switching frequency is constant over the application conditions,
that helps to tune the EMI filtering and to guarantee the maximum LED current ripple
specifications in the application range. This performance cannot be achieved using constant
on/off-time architecture.
Inductor and output capacitor selection
The output capacitor filters the inductor current ripple that, given the application conditions,
depends on the inductor value. As a consequence, the LED current ripple, that is the main
specification for a switching current source, depends on the inductor and output capacitor
selection.
Figure 12. Equivalent circuit
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DocID18279 Rev 5