Application Information: continued
Current Limit
where
R20 = resistance at 20ûC;
The current limit setpoint has to be higher than the normal
full load current. Attention has to be paid to the current
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.
Temperature curves on MOSFET manufacturersÕ 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 over-
load condition.
For future ÒCPUsÓ the full load will be 16A. The internal
current sense comparator current limit voltage limits are:
77mV < VTH < 101mV. Also, there is a 21% total variation
in RSENSE as discussed in the previous section.
We compute the value of the current sensing element
(embedded PCB trace) for the minimum current limit set-
point:
0.00393
ûC
T= operating temperature;
a =
;
R = desired droop resistor value.
For temperature T = 50ûC, the % R change = 12%.
Droop Resistor Tolerance
Tolerance due to sheet resistivity variation
Tolerance due to L/W error
Tolerance due to temperature variation
Total tolerance for droop resistor
In order to determine the droop resistor value the nominal
voltage drop across it at full load has to be calculated. This
voltage drop has to be such that the output voltage at full
load is above the minimum DC tolerance spec:
±8%
1%
12%
21%
VDAC(MIN)-VDC(MIN)
VDROOP(TYP)
=
.
RSENSE(MIN) = RSENSE(TYP) ´ 0.79,
1+RDROOP(TOLERANCE)
Example: for a 450MHz Pentium ¨II, the DC accuracy spec
is 1.93 < VCC(CORE) < 2.07V, and the AC accuracy spec is
1.9V < VCC(CORE) < 2.1V. The CS5132 DAC output voltage is
+2.004V < VDAC < +2.045V. In order not to exceed the DC
accuracy spec, the voltage drop developed across the resis-
tor must be calculated as follows:
RSENSE(MAX) = RSENSE(TYP) ´ 1.21,
VTH(MIN)
77mV
16A
RSENSE(MAX)
=
=
= 4.8m½.
ICL(MIN)
We select,
[VDAC(MIN)-VDC (MIN)
]
VDROOP(TYP)
=
=
1+RDROOP(TOLERANCE)
RSENSE(TYP) = 3.3m½.
We calculate the range of load currents that will cause the
internal current sense comparator to detect an overload
condition.
+2.004V-1.93V
= 61mV.
1.21
With the CS5132 DAC accuracy being 1%, the internal error
amplifierÕs reference voltage is trimmed so that the output
voltage will be 25mV high at no load. With no load, there is
no DC drop across the resistor, producing an output volt-
age tracking the error amplifier output voltage, including
the offset. When the full load current is delivered, a drop of
-50mV is developed across the resistor. Therefore, the regu-
lator output is pre-positioned at 25mV above the nominal
output voltage before a load turn-on. The total voltage
drop due to a load step is ÆV-25mV and the deviation from
the nominal output voltage is 25mV smaller than it would
be if there was no droop resistor. Similarly at full load the
Nominal Current Limit Setpoint
From the overcurrent detection data in the electrical char-
acteristics table:
VTH(TYP) = 86mV,
VTH(TYP)
86mV
RSENSE(NOM)
ICL(NOM)
=
=
= 26A.
3.3m½
regulator output is pre-positioned at 25mV below the nom- Maximum Current Limit Setpoint
inal voltage before a load turn-off. the total voltage increase
From the overcurrent detection data in the electrical char-
acteristics table:
due to a load turn-off is ÆV-25mV and the deviation from
the nominal output voltage is 25mV smaller than it would
be if there was no droop resistor. This is because the output
capacitors are pre-charged to a value that is either 25mV
above the nominal output voltage before a load turn-on or,
25mV below the nominal output voltage before a load turn-
off .
VTH(MAX) = 101mV,
VTH(MAX)
RSENSE(MIN)
VTH(MAX)
ICL(MAX)
=
=
RSENSE(NOM) ´ 0.79
Obviously, the larger the voltage drop across the droop
resistor (the larger the resistance), the worse the DC and
load regulation, but the better the AC transient response.
101mV
3.3m½ ´ 0.79
=
= 38.7A.
16
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
