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IRS2980SPBF

LED DRIVER CONTROL IC

International Rectifier 

IRS2980S

Application Information and Additional

Details

The IRS2980S is primarily intended for use in

Buck LED drivers operating with average current

regulation using hysteretic control. The circuit

topology uses a low side MOSFET referenced to

the 0V bus driven by a low side gate driver circuit

within the IRS2980S and an additional fast

recovery freewheeling diode. In order to minimize

switching losses the reverse recovery time of this

diode should be no more than 35nS. The

MOSFET should be selected for low capacitance

to reduce switching losses and low gate charge

(less than 25nC is recommended) to minimize

gate drive current. The LED load is referenced to

the DC bus and not 0V. The IRS2980S

incorporates floating high side current sense

inputs allowing the LED current to be sensed both

when the MOSFET is switched on and off. This

enables hysteretic operation, switching the

MOSFET off when the current rises above an

upper threshold and off when the current falls

below a lower threshold.

The current sense threshold Vcs is nominally

500mV with approximately 100mV of hysteresis

making the upper limit Vcs+50mV and the lower

limit Vcs-50mV. Some overshoot typically occurs

due to propagation delays and a small undershoot

is also possible. These vary depending on di/dt of

the ripple current, which is a function of input and

output voltage, inductor value and frequency as

well as RC filter values (RF and CF). The average

current is maintained at approximately the mid-

point over a wide input and output voltage range

due to the inherent accuracy of hysteretic control.

The LED output current is set by selecting the

value of the current sense resistor RCS. This is

determined by the formula:

operating conditions imposed by the input and

output voltages, output current and inductor value.

The following diagram shows the rise and fall of

the LED current as the MOSFET switches on and

off:

Figure 1: MOSFET gate drive and inductor/LED

current.

The following formulae model the operation of the

IRS2980S based Buck LED driver:

ton

≈ 0.2 ⋅ L ⋅ I LED

VBUS _ DC − VLED

+ RF

⋅CF

+

QG

0.18

+

tdr

toff

≈ 0.2 ⋅ L ⋅ I LED

VLED

+ RF ⋅CF

+

QG

0.26

+

tdf

f SW

=

ton

1

+ toff

(switching frequency)

d = ton (duty cycle)

ton + toff

RCS

=

VCS

I LED

Where ILED is the average LED output current.

The peak to peak ripple ∆ILED will be 20% of the

average LED current ILED due to the hysteretic

operation plus some additional ripple due to circuit

delays. These are caused mainly by the current

sense filter and MOSFET gate drive.

Since the IRS2980S uses hysteretic current

control to switch the Buck MOSFET on and off,

the LED current (which is equal to the inductor

current) is maintained between upper and lower

thresholds. Because of this the switching

frequency and duty cycle vary to meet the

Where,

RF and CF are the current sense filter

components,

L is the inductor value,

QG is the MOSFET gate charge,

tdr and tdf are propagation delays

(These values vary depending on circuit

conditions, tdr decreases with input voltage. tdf is

normally negligible. For a first order approximation

these terms may be ignored.)

www.irf.com

© 2011 International Rectifier

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