IRS25411PBF View Datasheet(PDF) - International Rectifier

Part Name

Description

MFG CO.

IRS25411PBF

LED BUCK REGULATOR CONTROL IC

International Rectifier 

IRS254(0,1)(S)PbF

Application Information and Additional

Details

Operating Mode

The IRS254(01,11) operates as a time-delayed

hysteritic buck controller. During normal operating

conditions the output current is regulated via the IFB

pin voltage (nominal value of 500 mV). This feedback

is compared to an internal high precision bandgap

voltage reference. An on-board dV/dt filter has also

been used to ignore erroneous transitioning.

Once the supply to the IC 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 IC then begins

toggling HO and LO outputs as needed to regulate the

current.

determined as follows. When the inductance value

is large enough to maintain a low ripple on IFB, Iout,avg

can be calculated:

Iout(avg) = VIFBTH RCS

(A)

(B)

Fig.2 (A) Storing Energy in Inductor

(B) Releasing Inductor Stored Energy

Iout

HO

LO

HO 50%

LO

IFB

IFBTH

50%

t_HO_off

DT1

50%

t_HO_on

DT2

50%

50%

t_LO_on

t_LO_off

Fig.1 IRS254(01,11) Control Signals, Iavg=1.2 A

Fig.3 IRS254(0,1) Time Delayed Hysterisis

As long as VIFB is below VIFBTH, HO is on, modulated by

the watchdog timer described below, which maintains

charge for the floating high side on the bootstrap

capacitor. The load is receiving current from VBUS,

which simultaneously stores energy in the inductor, as

VIFB increases, unless the load is open circuit. Once

VIFB crosses VIFBTH, the control loop switches HO off

after the delay tHO,OFF. When HO switches off, LO will

turn on after the deadtime (DT), the inductor then

releases its stored energy into the load and VIFB starts

decreasing. When VIFB drops below VIFBTH again, the

control loop switches HO on after the delay tHO,ON and

LO off after the delay tHO,ON + DT. The switching

continues to regulate the current at an average value

The control method is hysteretic with a free running

frequency, which enables average current regulation

in constrast to a fixed frequency scheme providing

peak current regulation only. This reduces the part

count since there is no need for frequency setting

components and also provides an inherently stable

system, which acts as a dynamic current source.

A deadtime of approximately 140 ns between the two

gate drive signals is incoporated to prevent shoot-

through. The deadtime has been adjusted to maintain

precise current regulation, while still preventing

shoot-through.

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