Texas Instruments TPS54980EVM-022, User Manual

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Modifications
1-4
1.3 Modifications
The TPS54980EVM-022 is designed to demonstrate the small size that can
be attained when designing with the TPS54980, however many of the
features, which allow for extensive modifications, have been omitted from this
EVM.
1.3.1 Changing Output Voltage
By changing the value of R
2
, the output voltage can be set to a value in the
range of 0.9 V to 2.5 V. The value of R
2
for a specific output voltage can be
calculated by using Equation 1−1. Table 1−3 lists the values for R
2
for some
common output voltages.
Equation 1−1.
R
2
+
10 k
W
0.891 V
V
O
*
0.891 V
Table 1−3. Output Voltage Programming
Output Voltage (V) R
2
Value (k
W)
0.9 1000
1.2 28.7
1.5 14.7
1.8 9.76
2.5 5.49
The minimum output voltage is limited by the minimum controllable on-time of
the device, 200 ns, and is dependent upon the duty cycle and operating
frequency. The approximate minimum output voltage can be calculated using
Equation 1−2:
Equation 1−2.
V
OUTMIN
+
200 nsec
ƒ
s
V
INMAX
1.3.2 Switching Frequency
Switching frequency can be trimmed to any value between 280 kHz and
700 kHz by changing the value of R4. Decreasing the switching frequency
results in increased output ripple unless the value of L1 is increased. A plot of
the value of RT versus the switching frequency is shown in Figure 1−1.