LTC2960
APPLICATIONS INFORMATION
V
V
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:
A
B
LTC2960-3
DV
R4
681k
CC
R4
R5
VR
6.6V
=
– 1=
– 1= 14.71
VTH + V+
+
IN
OUT
420mV
(
)
HYS
R5
48.7k
R6
6.81M
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
2960 F06
values into the equations above for V and V to obtain:
R
F
Figure 6. External Hysteresis
VR = 6.626V, V = 6.010V, V
= 616mV
F
HYST
The addition of R6 allows OUT to sink or source current
+
As a result of the added current component through R6
an error term exists that is a function of the pull-up volt-
to the summing junction at IN . Neglecting internal switch
resistances and providing that R6 >> R5, the externally
age, V in Figure 6.
modified hysteresis (referred to V ) becomes:
B
A
R4
R6
⎛
⎞
Operation with Supply Transients over 40V and Hot
Swapping
VHEXT ≈ VHYS(VA) + V
⎜
⎝
⎟
⎠
B
Since the amount of hysteresis is to be doubled, the
second term in the above expression needs to be about
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
300mV. With a logic supply, V , equal to 3V, the ratio R4/
B
R6 should be about 0.1. Choosing R6 to be 6.81M satis-
fies the design criteria.
TheadditionofR6modifiestherisingandfallingthresholds
originally determined by R4 and R5. The modified rising
threshold becomes:
current limiting resistor (RZ) protect the V supply pin
IN
of the LTC2960. Note that there is a speed penalty which
is the time constant determined by RZ and C1, 14.3ms in
⎛
⎞
⎟
⎠
R4 R4
+
this example. If V is below 30V, there is a voltage drop
V = V + V
• 1+
+
IN
⎜
R
(
TH
)
HYS
⎝
R5 R6
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
= 400mV + 20mV • 1+ 13.98 + 0.1
(
) (
)
voltage drop is determined by the maximum specified I
CC
= 6.3336V
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
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:
supply voltage of 150V, the Zener is biased <1mA. When
+
input pins are used to sense V , the input pins ADJ/IN /
IN
–
⎛
⎞
⎛
⎞
R4 R4 VTH – V
IN absolutemaximumratingof3.5Vmustnotbeexceeded.
B
V = VTH 1+
+
F
⎜
⎟
⎠
⎜
⎟
V can be a maximum of 8.75x the lowest programmed
R5 R6 ⎝ VTH
IN
⎝
⎠
threshold to satisfy this condition. For a maximum V of
IN
150V, the lowest programmable threshold is >17V.
= 0.4V • 1+ 13.98 – 0.65 = 5.732V
(
)
2960f
11
Linear Systems [ Linear Systems ]
