LTC2960
APPLICATIONS INFORMATION
V
A
LTC2960-3
R4
681k
DV
CC
V
B
IN
+
R5
48.7k
R6
6.81M
OUT
The falling threshold can be restored to the original value
by reducing the value of R5. Under the assumption that
the addition of R6 has a negligible impact on the rising
threshold, a new R4/R5 ratio can be calculated as shown:
R4
V
R
6.6V
=
– 1=
– 1= 14.71
+
R5 V
TH
+
V
HYS
420mV
(
)
2960 F06
Figure 6. External Hysteresis
Given the ratio of R4/R5, the closest 1% resistor value for
R5 is 46.4k. With the actual resistor values now known,
the final thresholds can be calculated by plugging the
values into the equations above for V
R
and V
F
to obtain:
V
R
=
6.626V, V
F
=
6.010V, V
HYST
=
616mV
The addition of R6 allows OUT to sink or source current
to the summing junction at IN
+
. Neglecting internal switch
resistances and providing that R6 >> R5, the externally
modified hysteresis (referred to V
A
) becomes:
⎛
R4
⎞
V
HEXT
≈
V
HYS(VA)
+
V
B
⎜ ⎟
⎝
R6
⎠
Since the amount of hysteresis is to be doubled, the
second term in the above expression needs to be about
300mV. With a logic supply, V
B
, equal to 3V, the ratio R4/
R6 should be about 0.1. Choosing R6 to be 6.81M satis-
fies the design criteria.
The addition of R6 modifies the rising and falling thresholds
originally determined by R4 and R5. The modified rising
threshold becomes:
⎛
R4 R4
⎞
V
R
=
V
TH
+
V
+
HYS
• ⎜
1+
+
⎟
⎝
R5 R6
⎠
As a result of the added current component through R6
an error term exists that is a function of the pull-up volt-
age, V
B
in Figure 6.
Operation with Supply Transients over 40V and Hot
Swapping
The circuit in Figure 7(a) allows the LTC2960 to withstand
high voltage transients. The magnitude of the voltage
transients that can be absorbed is set by the voltage rat-
ing of RZ. A TT-IRC pulse-withstanding surface mount
1206 resistor with a nominal voltage rating of 200V is
used. The external 30V Zener diode (Z1) and the 143kΩ
current limiting resistor (RZ) protect the V
IN
supply pin
of the LTC2960. Note that there is a speed penalty which
is the time constant determined by RZ and C1, 14.3ms in
this example. If V
IN
is below 30V, there is a voltage drop
across RZ that is dependent on the quiescent current of
the LTC2960 which is nominally less than 150mV but can
be as high as 290mV if
MR
is pulled low. The maximum
voltage drop is determined by the maximum specified I
CC
and
MR
pull-up currents. For conditions where the Zener
conducts current, it can be biased in the microamp range
owing to the low quiescent current of the LTC2960. For a
supply voltage of 150V, the Zener is biased <1mA. When
input pins are used to sense V
IN
, the input pins ADJ/IN
+
/
IN
–
absolute maximum rating of 3.5V must not be exceeded.
V
IN
can be a maximum of 8.75x the lowest programmed
threshold to satisfy this condition. For a maximum V
IN
of
150V, the lowest programmable threshold is >17V.
2960f
(
)
=
(
400mV
+
20mV
)
•
(
1+ 13.98
+
0.1
)
=
6.3336V
It is apparent that the R4/R6 term does not affect the ris-
ing threshold significantly resulting in a change of only
+0.645%. The falling threshold incorporating R6 is:
⎛
R4 R4
⎛
V
TH
– V
B
⎞ ⎞
V
F
=
V
TH
⎜
1+
+
⎜
⎟
⎝
R5 R6
⎝
V
TH
⎠ ⎟
⎠
=
0.4V
•
(
1+ 13.98 – 0.65
)
=
5.732V
11
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
