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IRS2980SPBF

LED DRIVER CONTROL IC

International Rectifier 

pins by a 1mA current source pulling down on VS

so that a supply voltage is produced across the

external capacitor CHVS to supply the high side

circuitry. A value of 22nF is recommended for

CHVS. The internal bias supply also dissipates

some power, which can be calculated from the

formula:

PBIAS = (VBUS _ DC _ AV −10) ⋅1mA

In order for the high side current sense circuitry to

function, a minimum bus voltage of 30V is

required to provide adequate bias supply current

and standoff voltage.

Thermal Considerations

Since the IRS2980S dissipates some power

during normal operation, temperature rise of the

IC die must be considered as part of the design

process.

The SO8 IC package has a maximum power

rating (PD) of 625mW, therefore the sum of PREG

and PBIAS should not exceed this value.

The junction temperature should remain below

125°C to ensure operation within specifications.

The junction temperature is normally 10°C above

the case temperature for an SO8 package

therefore the case temperature should not exceed

115°C at maximum ambient.

The junction to ambient thermal resistance of the

package (RθJA) is 128°C/W. This would give an

80°C temperature rise without any thermal relief at

the maximum PD value of 625mW.

ΔT = PD ⋅ RθJA

In order to reduce the junction temperature rise

thermal relief should be added around the

IRS2980S on the PCB. With adequate thermal

relief the die temperature rise can be greatly

reduced. The recommended method is to place

an area of copper on the opposite side of the PCB

to the IC in the same position with several vias

added underneath the IC to conduct heat through

to the other side.

IRS2980S

Figure 4: Thermal relief example

In addition if the IRS2980S is used in an

application where the circuit is encapsulated in

thermally conductive filler the die temperature rise

is also greatly reduced.

Figure 4 shows an example of thermal relief

placed around the IRS2980S in a high voltage

application. The top side copper layer is shown as

red and the bottom side is blue.

With thermal relief ∆T becomes:

ΔT

=

PD

⋅

⎡

⎢⎣

RθJA⋅ RθHS

RθJA + RθHS

⎤

⎥⎦

Where RθHS is the thermal resistance of the

thermal relief area or heat sink, which will

normally be lower than RθJA.

Dimming

The IRS2980S includes a PWM dimming

oscillator that generates a linear ramp waveform

at the RAMP pin with the frequency determined by

an external capacitor to COM (CRAMP). A DC

voltage is applied to the ADIM pin which is

compared to this ramp to produce a gating signal

that enables and disables the high frequency

switching of the MOSFET gate drive. By varying

the ADIM voltage the duty cycle is adjusted

allowing brightness to be adjusted from zero to

100%. This is accomplished by operating the LED

driver in burst mode and varying the duty cycle of

the bursts. The LED current during dimming is

shown in figure 5:

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© 2011 International Rectifier

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