CS5166 datasheet(15/22 Pages) CHERRY

15 / 22 page

rating of the external power components as these are the

first to fail during an overload condition. The MOSFET

continuous and pulsed drain current rating at a given case

temperature has to be accounted for when setting the cur-

rent limit trip point. For example the IRL 3103S (D2 PAK)

MOSFET has a continuous drain current rating of 45A at

VGS = 10V and TC = 100˚C. Temperature curves on MOS-

FET manufacturers’ data sheets allow the designer to

determine the MOSFET drain current at a particular VGS

and TJ (junction temperature). This, in turn, will assist the

designer to set a proper current limit, without causing

device breakdown during an overload condition.

For a 300MHz Pentium ® II CPU the full load is 14.2A. The

internal current sense comparator current limit voltage

limits are: 55mV < VTH < 130mV. Also, there is a 29% total

variation in RSENSE as discussed in the previous section.

We select the value of the current sensing element (embed-

ded PCB trace) for the minimum current limit setpoint:

RSENSE(MAX) =

⇒ RSENSE × 1.29 =

⇒

RSENSE × 1.29 = 3.87mΩ ⇒ RSENSE = 3mΩ

We calculate the range of load currents that will cause the

internal current sense comparator to detect an overload

condition.

From the overcurrent detection data section (pg 3),

Nominal Current Limit Setpoint

VTH(TYP) = 76mV.

ICL(NOM) =

Maximum Current Limit Setpoint

Therefore , ICL(NOM) =

= 25.3A

VTH(MAX) = 110mV.

Therefore,

ICL(MAX) =

= 51.6A

Therefore, the range of load currents that will cause the

internal current sense comparator to detect an overload

condition through a 3mΩ embedded PCB trace is: 14.2A <

ICL < 51.6A, with 25.3A being the nominal overload condi-

tion.

There may be applications whose layout will require the

use of two extra filter components, a 510Ω resistor in series

with the ISENSE pin, and a 0.1µF capacitor between the

ISENSE and VFB pins. These are needed for proper current

limit operation and the resistor value is layout dependent.

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 set-

point is determined when this 510Ω is taken into consider-

ation.

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 510Ω filter resistor

RFB = VFB pin 3.3K filter resistor

IFB = VFB bias current

Minimum current sense resistor (droop resistor) voltage

drop required for current limit when RISENSE is used

VTRIP(MIN) = 55mV + (13µA × 510) – (3.3K × 1µA) = 55mV +

6.6mV – 3.3mV = 58.3mV

Nominal current sense resistor (droop resistor) voltage

drop required for current limit when RISENSE is used

VTRIP(NOM) = 76mV + (30µA × 510) – (3.3K × 0.1µA) =

76mV + 15.3mV – 0.33mV = 90.97mV

Maximum current sense resistor (droop resistor) voltage

drop required for current limit when RISENSE is used

VTRIP(MAX) = 110mV + (50µA × 510) = 110mV + 25.5mV =

135.5mV

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

lated:

RSENSE(MAX) =

= 4.1mΩ

RSENSE(NOM) =

= 3.18mΩ

The range of load currents that will cause the internal cur-

rent sense comparator to detect an overload condition is as

follows:

Nominal Current Limit Setpoint

ICL(NOM) = VTRIP(NOM) / RSENSE(NOM)

Therefore,

ICL(NOM) = 90.97mV / 3.18mΩ = 28.6A

4.1mΩm

1.29

RSENSE(MAX)

1.29

58.3mV

14.2A

110mV

3mΩ × 0.71

110mV

RSENSE × 0.71

110mV

RSENSE(MIN)

76mV

3mΩ

VTH(TYP)

RSENSE(NOM)

55mV

14.2A

VTH(MIN)

ICL(MIN)

Application Information: continued

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