IRU3073CQTR【PDF 7/21 页】IC REG DL BCK/LINEAR 16-QSOP

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Sheet目录40

3页 4页 5页 6页 [7页]8页 9页 10页 11页

IRU3073

Rev. 1.0

09/17/03

www.irf.com

If the over-current condition is temporary and goes away

quickly, the IRU3073 will resume its normal operation.

If output is shorted or over-current condition persists,

the output voltage will keep going down until it is below

0.4V. Then the output under-voltage lock out comparator

goes high and turns off both MOSFETs. The operation

waveforms are shown in Figure 6.

Figure 6 - Diagram of over-current operation.

Feedback

voltage

Switching

frequency

High Side MOSFET

turn on time (tON)

Average Inductor

Current

IOUT

DMAX/FS(NOM)

FS(NOM)

0.4V

VREF

<IL>=IOUT

Normal

operation

Over Current

Limit Mode

Shutdown

by UVLO

IO(LIM)

IO(MAX)

VOUT

FS(NOM)3VIN

Operation in current limit is shown in Figure 7, the high

side MOSFET is turned off and inductor current starts to

decrease. Because the output inductor current is higher

than the current limit setpoint (ISET), the over-current com-

parator keeps high until the inductor current decreases

to be below ISET. Then another cycle starts.

During over-current mode, the valley inductor current is:

The peak inductor current is given as:

To avoid undesirable trigger of over-current protection,

this relationship must be satisfied:

iL(VALLEY) = ISET

IL(PEAK) = ISET+(VIN-VOUT)3tON/L ---(3)

ISET=iL(VALLEY)

iL(PEAK)

tON

tOFF

iL(AVG)

Current Limit

Comparator Output

Inductor

Current

HDrv

ISET / IO(NOM) -

DIPK-PK(NOM)

ISET = IO(LIM) - ---(5)

(VIN-VOUT)3VOUT

23fS3L3VIN

( )

(VIN - VOUT)3VOUT

VIN3L3fS

DIPK-PK(LIM) =

IO(LIM) = ISET + ---(4)

DIPK-PK(LIM)

RSET = 3 IO(LIM) - ---(6)

RDS(ON)

IOCSET

[ ( )]

(VIN-VOUT)3VOUT

23fS3L3VIN

Figure 7 - Operation waveforms during current limit.

From Figure 7, the average inductor current during the

current limit mode is:

The inductor's ripple current can be expressed as:

Combination of above equation and (4) results in:

Combination of equations (5) and (2) results in the rela-

tionship between RSET and output current limit:

From the above analysis, the current limit is not only

dependent on the current setting resistor RSET and RDS(ON)

of low side MOSFET but it is also dependent on the

input voltage, output voltage, inductance and switching

frequency as well.

The cycle-by-cycle over-current limit will hold for a cer-

tain amount of time, until the output voltage drops below

0.4V, the under-voltage lock out activates and latches

off the output driver. The operation waveform is shown in

Figure 4. Normal operation will resume after IRU3073 is

powered up again.

Where:

IO(LIM) = The Output Current Limit -typical is 50%

higher than nominal output current.

VIN = Maximum Input Voltage

VOUT = Output Voltage

fS = Switching Frequency

L = Output Inductor

RDS(ON) = RDS(ON) of Low Side MOSFET

IOCSET = OC Threshold Set Current

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