Texas Instruments TPS54673EVM-225, User Manual, Page: 14 / 42

Modifications
1-4
1.3 Modifications
The TPS54x73EVM-225 is designed to demonstrate the small size that can
be attained when designing with the TPS54x73, so many of the features which
allow for extensive modifications have been ommited from this EVM.
Changing the value of R4 can change the output voltage in the range of 0.9
V to 3.3 V. The value of R4 for a specific output voltage can be calculated by
using equation 1 . Table 1–4 lists the values for R4 for some common output
voltages.
R4
+
10 k
W
0.891 V
V
O
*
0.891 V
Table 1–4. Output Voltage Programming
Output Voltage (V) R4 Value (k )
0.9 1000
1.2 28.7
1.5 14.7
1.8 9.76
2.5 5.49
3.3 3.74
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 fre-
quency. The approximat minimum output voltage can be calculated using:
V
OUTMIN
+
200 ns f
s
V
INMIN
The switching frequency may be trimmed to any value between 280 kHz and
700 kHz by changing the value of R6. 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 given in Figure 1–1.
Figure 1–1. Frequency Trimming Resistor Selection Graph
250
300
350
400
450
500
550
600
650
700
750
60 70 80 90 100 110 120 130 140 150 160 170 180
Switching Frequency – kHz
R – Resistance – k
Ω
(1)
(2)