LTM4641
APPLICATIONS INFORMATION—INPUT PROTECTION FEATURES
Variables to define up-front are as follows:
and it is inferred (in that scenario) that V
closer to 4.1V, when RUN is floating.
would be
HYST
•ꢀ V : V start-up voltage, in volts. This is the custom-
SU IN
ized UVLO rising voltage.
It is the moderately weak pull-up strength of HYST (10kΩ
pull-up to INTV ), and the desire for any loading of
CC
•ꢀ V : V shutdown voltage, in volts. This is the custom-
SD IN
the HYST signal to negligibly alter the HYST logic-high
output voltage level (less than ~50mV), that motivates
a high impedance (~1MΩ) hysteresis-setting resistor to
interface between HYST and UVLO, when custom UVLO
settings are desired.
ized UVLO falling voltage.
•ꢀ V
: The value of the voltage on the HYST pin (in
HYST
volts) when switching action is on and just prior to the
input voltage (V ) falling below V .
IN
SD
•ꢀ R
: The hysteresis-setting resistor. If used, R
is
HYST
HYST
ThecustomizedUVLOstart-upandshutdowninputvoltage
settings can be double-checked with:
recommended to take on a value of 1MΩ or higher, so
that the HYST voltage is negligibly affected by external
loading.
RTUV
VSU =UVOVTH •
+1
(22)
(23)
RBUV ||RHYST
•ꢀ V : The input voltage above which a latchoff input
OV
overvoltage event occurs.
VHYST
RHYST
VSD = VSU –
•RTUV
•ꢀ V : The input voltage above which a nonlatching input
RT
overvoltage event occurs.
To set the input overvoltage (latching and nonlatching)
thresholds,choosefirsthowmuchcurrent,I ,tocontinu-
TOV MOV BOV
string for this function, at ultrahigh line. 10μA to 20µA is
a normal amount to allocate.
Then, R
and R
are given by:
TUV
BUV
DIV
VSU − VSD
VHYST
ally have drawn by the R /R
/R
resistor-divider
RTUV
and
RBUV
=
•RHYST
(20)
(21)
The total resistance of the divider string is then given by:
UVOVTH
VSU −UVOVTH UVOVTH
=
VOV
IDIV
RTOT
=
(24)
–
RTUV
RHYST
Then, the resistors in the input overvoltage divider are
given by:
UVOV is nominally 0.5V, from the Electrical Characteris-
TH
tics Table. The value of V
used in the above equations
HYST
RTOT •UVOVTH
requires more careful consideration. Review Figure 1 and
assess system details of the specific application in which
the LTM4641 is being placed. It is known from the Electri-
cal Characteristics table that when V ≥ 6V that INTV
RBOV
=
,
(25)
VOV
INL
CC
1
1
V
OV
RMOV =UVOVTH •RTOT
•
–
,
(26)
(27)
= 5.3V; and we see the voltage on the HYST pin, when
V
RT
switching action is on, is V , 5.1V—
HYST(SWITCHING_ON)
nominally. Observe that if the RUN pin were driven high
R
TOV
= R – R – R
TOT M B
by 3.3V logic, however, that V would be a Schottky
HYST
It may be tempting to try rearranging these equations
so that R ’s value is fixed, first, and to compute R
diode forward-voltage drop above 3.3V—and V
in
IN
HYST
TOV
MOV
that instance would be 3.6V. If V is targeted below 6V ,
SD
and R
subsequently. However, due to large divide-
BOV
it is necessary to consider that V
’s pull-up voltage,
HYST
down ratio (usually) of ultrahigh line input voltage down
INTV , is decreasing with V . For example, at V
=
CC
INL
INL
to these pins with ~0.5V thresholds, the rounding off of
4.5V input, INTV is nominally 4.3V (V
),
CC
INTVCC(LOWLINE)
R
MOV
and R
to nearest EIA standard values after fixing
BOV
4641f
30
Linear [ Linear ]
