SNVS574G July 2008 – July 2019 LM3421 , LM3423
PRODUCTION DATA.
The controller includes a high voltage, low dropout bias regulator. When power is applied, the regulator is enabled and sources current into an external capacitor (CBYP) connected to the VCC pin. The recommended bypass capacitance for the VCC regulator is 2.2 µF to 3.3 µF. The output of the VCC regulator is monitored by an internal UVLO circuit that protects the device from attempting to operate with insufficient supply voltage and the supply is also internally current limited. Figure 20 shows the typical start-up waveforms.
First, CBYP is charged to be above VCC UVLO threshold (approximately 4.2 V). The CVCC charging time (tVCC) can be estimated using Equation 19.
CCMP is then charged to 0.9 V over the charging time (tCMP), which can be estimated using Equation 20.
Once CCMP = 0.9 V, the part starts switching to charge CO until the LED current is in regulation. The CO charging time (tCO) can be roughly estimated using Equation 21.
The system start-up time (tSU) is defined using Equation 22.
In some configurations, the start-up waveform overshoots the steady state COMP pin voltage. In this case, the LED current and output voltage overshoots also, which can trip the overvoltage or protection, causing a race condition. The easiest way to prevent this is to use a larger compensation capacitor (CCMP), thereby slowing down the control loop.