Datasheet
2 3
UVTHRESH UV
1 2 3
R R
V V
R R R
+
= ´
+ +
3
ENTHRESH OV
1 2 3
R
V V
R R R
= ´
+ +
G009
1k
10
0.01
1
100
I – Drain-to-Source Current – A
DS
0.1
0.01 0.1 10 1001
Single Pulse
R = 94ºC/W (min Cu)
θJA
1ms
10ms
100ms
1s
DC
Area Limited
by R
DS(on)
V – Drain-to-Source Voltage – V
DS
T FLT
T
10 μA
C = t ,
1.35 V
therefore,
10 μA
C = 7 ms 52 nF
1.35 V
´
´ =
TPS24720
www.ti.com
SLVSAL1C –MARCH 2011–REVISED SEPTEMBER 2013
For this example, select 7 ms to allow for variation of system parameters such as temperature, load, component
tolerance, and input voltage. The timing capacitor is calculated in Equation 12 as 52 nF. Selecting the next-
highest standard value, 56 nF, yields a 7.56-ms fault time.
(12)
Figure 38. CSD16403Q5 SOA Curve
STEP 5. Calculate the Retry-Mode Duty Ratio
In retry mode, the TPS24720 is on for one charging cycle and off for 16 charge/discharge cycles, as can be seen
in Figure 35. The first C
T
charging cycle is from 0 V to 1.35 V, which gives 7.56 ms. The first C
T
discharging
cycle is from 1.35 V to 0.35 V, which gives 5.6 ms. Therefore, the total time is 7.56 ms + 33 x 5.6 ms = 192.36
ms. As a result, the retry mode duty ratio is 7.56 ms/192.36 ms = 3.93%.
STEP 6. Select R
1
, R
2
, and R
3
for UV and OV
Next, select the values of the OV and UV resistors, R
1
, R
2
, and R
3
, as shown in the typical application diagram
on the front page. From the TPS24720 electrical specifications, V
OVTHRESH
= 1.35 V and V
ENTHRESH
= 1.35 V. V
OV
is the overvoltage trip voltage, which in this case is 14 V. V
UV
is the undervoltage trip voltage, which for this
example equals 10.8 V.
(13)
(14)
Assume R
3
is 1 kΩ and use Equation 13 to solve for (R
2
+ R
3
). Use Equation 14 and the (R
2
+ R
3
) from
Equation 13 to solve for R
2
and finally for R
3
. From Equation 13, (R
2
+ R
3
) = 9370.4 Ω. From Equation 14,
R
2
= 296 Ω and R
1
= 9.074 kΩ. Scaling all three resistors by a factor of ten to use less supply current for these
voltage references and using standard 1% resistor values gives R
1
= 90.9 kΩ, R
2
= 2.94 kΩ, and R
3
= 10 kΩ.
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