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TPS56xx Functions
2-6
Note that V
del
is independent of the output voltage.
To calculate V
del
for this example design, use the component measurements
given in Section 2.2.4.2. They are repeated here for convenience:
L = 1.5 µH
ESR = 10 mΩ
T
del
= 400 ns
Now calculate V
del
:
V
del
+
5
1.5 10
–
6
400 10
–
9
10 10
–
3
+ 13.3
mV
Since V
del
does not depend on the output voltage setting, V
del
is a larger por-
tion of the total output voltage ripple for lower output voltages. So, the hystere-
sis (which is proportional to the output voltage setting) should be set so that
the desired output ripple voltage is obtained for the lowest output voltage. The
total output voltage ripple requirement per Table 1–2 for an output voltage of
1.5 V is 30 mV, the hysteresis is set as:
V
Hysteresis
+ 30
mV
*
V
del
+ 30
mV
* 13.3
mV
+ 16.7
mV
For convenience, and allowing a little margin, the hysteresis will be designed
for 15 mV.
To set the hysteresis, connect two external resistors to form a resistor divider
from VREFB (pin 5) to ANAGND (pin 7) with the center of the divider connected
to VHYST (pin 4). The hysteresis of the comparator is equal to twice the
voltage that is between the VREFB (pin 5) and VHYST (pin 4) pins. Or,
V
Hysteresis
+ 2
(
VREFB–VHYST
)
For this design, 15 mV of hysteresis was chosen for a 1.5 V output voltage.
V
Hysteresis
+ 15
mV
+ 2
(
1.5
V–VHYST
)
Solving for VHYST:
VHYST
+
VREFB–
V
Hysteresis
2
+ 1.5
–
15
mV
2
+ 1.4925
V
Referring to the schematic, Figure 1–3, the two external resistors are R10 and
R14. From the previous section, the total resistance required is 20 kΩ. Since
R10 is very small compared to R14, for simplicity set R14 = 20 kΩ. To calculate
the value of R13:
VHYST
+
VREFB
R
14
R
14 )
R
10
Solving for R10:
R
10 +
VREFB
R
14
VHYST
–R
14 +
(
1.5
V
)(
20
k
W
)
1.4925
V
–
20
k
W + 100 W
The controller hysteresis is now set to 15 mV. This, in addition to the 13.3 mV
due to propagation delays, results in a total ripple voltage of less than the
design goal of 30 mV for an output voltage of 1.5 V.