LTC4413 datasheet(14/32 Pages) AD

14 / 32 page

LTC4421

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

comparator threshold, the resistor values can be calcu-

lated as shown in Equation 1 through Equation 5.

TOTAL = R1+ R2 + R3 + R4

(1)

R1=

0.5 •R

TOTAL

(

)

OVRISE

(2)

R2 =

OVRISE

UVRISE

– 1

⎛

⎝

⎜

⎞

⎠

⎟ • R1

(3)

R3 =

UVRISE

UVFALL

– 1

⎛

⎝

⎜

⎞

⎠

⎟ •

OVRISE

UVRISE

⎛

⎝

⎜

⎞

⎠

⎟ • R1

(4)

R4 = R1•

OVRISE

0.5

– 1

⎛

⎝

⎜

⎞

⎠

⎟ – R3 – R2

(5)

When setting the resistor values, take into account the

tolerance of the input supply voltage, the tolerance of the

resistors, the ±2% error in the 500mV reference and the

±10nA maximum leakage of the UVR, UVF and OV pins.

To permanently invalidate a channel, connect OV, UVR

and UVF to ground.

During channel turn-on, the relatively large inrush current

causes a voltage drop across the input supply source

resistance and the parasitic resistances of PCB traces and

any cable. This voltage drop can cause UV faults that

trigger a phenomenon called UV motorboating, where

the input supply repeatedly connects and disconnects

from the output. UV motorboating can lead to compo-

nent damage and undesirable/erratic behavior. To pre-

vent UV motorboating, set the VUVRISE and VUVFALL as

far apart as possible to maximize hysteresis and prevent

channel disconnect during the inrush. Ideally, quantify the

worst-case input resistance RSRC,MAX, and set (VUVRISE-

VUVFALL) larger than (ILIM • RSRC,MAX), where ILIM is the

current limit. The OV hysteresis is fixed at 10% above the

OV threshold voltage.

For better accuracy, use one resistive divider per chan-

nel to set the UVF and UVR thresholds, and a second,

separate resistive divider to set the OV threshold. For

ease of calculation, use three individual resistive divider

strings per channel – one for OV, one for UVF and one for

UVR. However, to ensure the UVR threshold is always

higher than the UVF threshold on a given channel,

do not use separate strings for UVR and UVF when

setting their thresholds close together in voltage.

Figure 2 shows these various resistive divider possibili-

ties, using Channel 1 as an example.

Figure 2. Three Resistive Divider Options For Setting the OV, UVR and UVF Threshold Voltages

UVF1

OV1

UVR1

OV1

UVF1

UVR1

UVF1

0.5V

0.5V/0.45V

QUAL

VALID1

VPU

R10

R11

R12

R13

R14

R15

R16

QUAL

TIMER

FEWEST

RESISTORS

ENSURES UVR1

THRESHOLD > UVF1

THRESHOLD

EASIEST TO CALCULATE

VI INPUT SUPPLY

LTC4421

LTC4421 F02

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