LTM4641
APPENDICES
To nudge the OV
setpoint downward, to a new OOV
100kΩ, low T.C.R. resistor. Using tolerances of 0.1%
and a T.C.R. of 25ppm/ꢀC can provide a considerable
improvementinaccuracyoverthedefaultdividernetwork,
PGM
inception threshold voltage at OV
—using an
PGM(NEW)
R
resistor, only—calculate:
BOVPGM
over temperature. Next, decide the new value of V
OVPGM
1
RBOVPGM
=
desired—OV
—withinapracticalwindowof0.6V<
(37)
PGM(NEW)
<0.9V.Then,computeR
1V –OVPGM(NEW)
1
–
OV
accordingto:
PGM(NEW)
TOVPGM
OVPGM(NEW) •499kΩ 1MΩ
RTOVPGM
=
The new OV
threshold can then be double-checked by
PGM
1
(42)
OVPGM(NEW)
1
1V • 1MΩ||R
499kΩ+1MΩ||R
(
)
BOVPGM
–
OVPGM(NEW)
=
(38)
499kΩ
1V –OVP
• 1MΩ||R
BOVPGM
(
)
(
)
GM(NEW)
(
)
BOVPGM
The new OV
setting can be double-checked by:
When lowering the OV
BOVPGM
setpoint with application of
PGM
PGM
R
only, it is not necessary to apply a C
OVPGM
1V • 1MΩ||R
499kΩ||RTOVPGM +1MΩ||RBOVPGM
(
)
BOVPGM
OVPGM(NEW)
=
(43)
capacitor, because: for an extreme OVPGM(NEW) setting
(
)
of 600mV, which is not practical since that is the voltage
of V during normal regulation, the time-constant of the
FB
PGM
Then, use the next smallest standard value of C
available, computed by:
OVPGM
OV
network would have changed by less than 2μs
from its default value.
COVPGM(NEW)
=
To nudge the OV trip threshold upward to set a new
PGM
15.6µs
499kΩ||1MΩ||RTOVPGM ||RBOVPGM
—using
(39)
(44)
OOV inception threshold voltage at OV
–47pF
PGM(NEW)
(
)
an R
resistor only—calculate:
TOVPGM
1
For example, the OV
nominal value of 666mV—but with better accuracy—by
using 0.1% precision resistors with 25ppm/ꢀC T.C.R.
setpoint can be kept at its
PGM(NEW)
R
=
TOVPGM
OV
1
PGM(NEW)
–
499kΩ
1V – OV
•1MΩ
(
)
PGM(NEW)
for R
OV
= 100k and R
= 49.9k, and bypassing
BOVPGM
TOVPGM
toSGNDwithC
=470pF.TheresultingV
OVPGM
The new OV
setting can then be double-checked by:
PGM
OVPGM
PGM
OOV setpoint threshold becomes better than 1.8%, over
1V •1MΩ
OVPGM(NEW)
=
temperature. The vast majority of the remaining variation
(40)
499kΩ||R
+1MΩ
(
)
TOVPGM
in the threshold setting comes variation of the 1V —a
REF
1.5% reference, over temperature.
If R
is computed in Equation 39 to be smaller than
TOVPGM
10kΩ, connect OV
to 1V
and do not apply any
The extreme values of the OOV setpoint voltage, plus
PGM
REF
C
capacitor; this will yield an OOV setting of 167%
the OVP
term—which is the offset voltage of the fast
OVPGM
ERR
of nominal. Otherwise, use the next smallest standard
comparator( 12mVmaximum,overtemperature)—gives
guidanceonwhattheminimumandmaximumvoltageV
value of C
available, computed by:
OVPGM
FB
can be at which the CROWBAR output would swing logic
high and invoke latchoff overvoltage protection.
15.6µs
499kΩ||1MΩ||R
COVPGM
=
–47pF
(41)
(
)
TOVPGM
One must take care to set the OV
level and not too aggressively. If OV
the system will demonstrate nuisance output overvoltage
latchoff behavior. The output voltage of any switching
voltage to a practical
PGM
PGM
The default V
setpoint is 665mV 2.26%, over
is set too low,
OVPGM
temperature. To tighten the OV
setpoint, begin by
PGM
choosing R
to be a commonly available precision
BOVPGM
4641f
60
Linear [ Linear ]
