CS5166 datasheet(19/25 Pages) ONSEMI

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CS5166

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This series resistor affects the calculation of the current

limit setpoint, and has to be taken into account when

determining an effective current limit.

The calculations below show how the current limit

setpoint is determined when this 510 Ω is taken into

consideration.

VTRIP + VTH ) (ISENSE

RISENSE) * (RFB

IFB)

Where:

VTRIP = voltage across the droop resistor that trips the

ISENSE comparator.

VTH = internal ISENSE comparator threshold

ISENSE = ISENSE bias current

RISENSE = ISENSE pin 510 Ω filter resistor

RFB = VFB pin 3.3 k filter resistor

IFB = VFB bias current

Minimum current sense resistor (droop resistor) voltage

drop required for current limit when RISENSE is used

VTRIP(MIN) + 55 mV ) (13 mA

510) * (3.3 k

1.0 mA)

+ 55 mV ) 6.6 mV * 3.3 mV + 58.3 mV

Nominal current sense resistor (droop resistor) voltage

drop required for current limit when RISENSE is used

VTRIP(NOM) + 76 mV ) (30 mA

510) * (3.3 k

0.1 mA)

+ 76 mV ) 15.3 mV * 0.33 mV + 90.97 mV

Maximum current sense resistor (droop resistor) voltage

drop required for current limit when RISENSE is used

VTRIP(NOM) + 110 mV ) (50 mA

510)

+ 110 mV ) 25.5 mV + 135.5 mV

The value of RSENSE (current sense PCB trace) is then

calculated:

RSENSE(MAX) + 58.3 mV

14.2 A +

4.1 mW

RSENSE(NOM) +

RSENSE(MAX)

1.29

+ 4.1 mWm

1.29

+ 3.18 mW

The range of load currents that will cause the internal

current sense comparator to detect an overload condition is

as follows:

Nominal Current Limit Setpoint

ICL(NOM) +

VTRIP(NOM)

RSENSE(NOM)

Therefore,

ICL(NOM) + 90.97 mV

3.18 mW +

28.6 A

Maximum Current Limit Setpoint

ICL(MAX) +

VTRIP(MAX)

RSENSE(MAX)

Therefore,

ICL(MAX) +

135 mV

3.18 mW

0.71 +

60 A

Therefore, the range of load currents that will cause the

internal current sense comparator to detect an overload

condition through a 3.0 mΩ embedded PCB trace is: 14.2 A

< ICL 60 A, with 28.6 A being the nominal overload

condition.

Design Rules for Using a Droop Resistor

The basic equation for laying an embedded resistor is:

RAR + ò

or R + ò

where:

A = W × t = cross−sectional area

ρ = the copper resistivity (μΩ − mil)

L = length (mils)

W = width (mils)

t = thickness (mils)

For most PCBs the copper thickness, t, is 35 μm (1.37

mils) for one ounce copper. ρ = 717.86 μΩ−mil

For a Pentium II load of 14.2 A the resistance needed to

create a 43 mV drop at full load is:

Response Droop + 43 mV

IOUT

+ 43 mV

14.2 A +

3.0 mW

The resistivity of the copper will drift with the

temperature according to the following guidelines:

DR + 12% @ TA +) 50°C

DR + 34% @ TA +) 100°C

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