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3

RD-0608

2. Constant current control

The IRS2540/1/01/11 is a time-delayed hysteretic Buck controller. During normal operating conditions the output

current is regulated via the IFB pin voltage to a nominal value of 500mV. This feedback signal is compared to an

internal high precision band gap voltage reference. An on-board dV/dt filter has also been used to prevent erroneous

transitioning. This is necessary since the IFB pin is sensitive to noise.

The Buck topology may use a fast recovery diode for the lower switch or a synchronous MOSFET shown as M2 in

Fig 1. On power up as the VCC supply to the IR2540/1/01/11 reaches VCCUV+ the LO output is held high and the

HO output low for a predetermined period of time. This initiates charging of the bootstrap capacitor establishing the

VBS floating supply for the high side output. The chip then begins toggling HO and LO outputs as needed to

regulate the load current monitored and fed back through the current sense resistor RCS. The dead times of

approximately 140ns between the LO and HO gate drive signals prevents shoot through and reduce switching losses,

particularly at higher frequencies.

Fig. 1 IRS2540/1/01/11 Constant Current LED Driver Typical Schematic

(see Fig. 16 for evaluation board full schematic)

Note: Cout and Rout are optional and may be required in some applications

Under normal operating conditions, if VIFB is below VIFBTH, HO will be high and the load receives current from

VBUS through the Buck inductor L1. This simultaneously stores energy in the output stage comprised of L1 and

COUT as VIFB begins to increase. When VIFB crosses VIFBTH, HO transitions low after the delay t_HO_off.

Once HO is low, LO will transition high after the deadtime. The inductor and output capacitor release the stored

energy into the load and VIFB starts decreasing. When VIFB crosses VIFBTH again, LO switches low after the

delay t_LO_off and HO switches high after the delay t_HO_on. The cycle then repeats continuously at a frequency

depending on the load current and the values of the output inductor and capacitor.