8.3.4 Analog Dimming

The CSH pin can be used to analog dim the LED current by adjusting the current sense voltage (V_SNS). There are several different methods to adjust V_SNS using the CSH pin:

1. External variable resistance: Adjust a potentiometer placed in series with R_CSH to vary V_SNS.
2. External variable current source: Source current (0 µA to I_CSH) into the CSH pin to adjust V_SNS.

Figure 14. Analog Dimming Circuitry

In general, analog dimming applications require a lower switching frequency to minimize the effect of the leading edge blanking circuit. As the LED current is reduced, the output voltage and the duty cycle decreases. Eventually, the minimum on-time is reached. The lower the switching frequency, the wider the linear dimming range. Figure 14 shows how both CSH methods are physically implemented.

Method 1 uses an external potentiometer in the CSH path which is a simple addition to the existing circuitry. However, the LEDs cannot dim completely because there is always some resistance causing signal current to flow. This method is also susceptible to noise coupling at the CSH pin because the potentiometer increases the size of the signal current loop.

Method 2 provides a complete dimming range and better noise performance, though it is more complex. It consists of a PNP current mirror and a bias network consisting of an NPN, 2 resistors and a potentiometer (R_ADJ), where R_ADJ controls the amount of current sourced into the CSH pin. A higher resistance value sources more current into the CSH pin, causing less regulated signal current through R_HSP, effectively dimming the LEDs. V_REF should be a precise external voltage reference, while Q7 and Q8 should be a dual pair PNP for best matching and performance. The additional current (I_ADD) sourced into the CSH pin can be calculated using Equation 9.

Equation 9.

The corresponding LED current ( I_LED) for a specific I_ADD is:

Equation 10.