Application Information: continued
rating of the external power components as these are 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.
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
V
GS = 10V and TC = 100˚C. Temperature curves on MOS-
The calculations below show how the current limit set-
point is determined when this 510Ω is taken into consider-
ation.
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.
VTRIP = VTH + (ISENSE × RISENSE) – (RFB × IFB
)
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.
Where VTRIP = voltage across the droop resistor that trips
the ISENSE comparator
V
TH = internal ISENSE comparator threshold
SENSE = ISENSE bias current
RISENSE = ISENSE pin 510Ω filter resistor
We select the value of the current sensing element (embed-
ded PCB trace) for the minimum current limit setpoint:
I
VTH(MIN)
55mV
14.2A
R
FB = VFB pin 3.3K filter resistor
RSENSE(MAX)
=
⇒ RSENSE × 1.29 =
⇒
ICL(MIN)
IFB = VFB bias current
RSENSE × 1.29 = 3.87mΩ ⇒ RSENSE = 3mΩ
Minimum current sense resistor (droop resistor) voltage
drop required for current limit when RISENSE is used
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),
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
Nominal Current Limit Setpoint
VTH(TYP) = 76mV.
VTH(TYP)
VTRIP(NOM) = 76mV + (30µA × 510) – (3.3K × 0.1µA) =
76mV + 15.3mV – 0.33mV = 90.97mV
ICL(NOM) =
RSENSE(NOM)
Maximum current sense resistor (droop resistor) voltage
drop required for current limit when RISENSE is used
Maximum Current Limit Setpoint
76mV
Therefore , ICL(NOM)
VTH(MAX) = 110mV.
Therefore,
=
= 25.3A
VTRIP(MAX) = 110mV + (50µA × 510) = 110mV + 25.5mV =
3mΩ
135.5mV
The value of RSENSE (current sense PCB trace) is then calcu-
lated:
RSENSE(MAX)
58.3mV
14.2A
=
= 4.1mΩ
ICL(MAX)
=
RSENSE(MAX)
1.29
4.1mΩm
1.29
110mV
RSENSE(MIN)
110mV
110mV
RSENSE(NOM)
=
=
= 3.18mΩ
=
=
= 51.6A
RSENSE × 0.71 3mΩ × 0.71
The range of load currents that will cause the internal cur-
rent sense comparator to detect an overload condition is as
follows:
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
Nominal Current Limit Setpoint
ICL(NOM) = VTRIP(NOM) / RSENSE(NOM)
Therefore,
ICL(NOM) = 90.97mV / 3.18mΩ = 28.6A
15
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