Modifications
1-4
1.3
Modifications
The TPS54350EVM−235 is designed to demonstrate the small size that can
be attained when designing with the TPS54350, so many of the features,
which allow for extensive modifications, have been omitted from this EVM.
1.3.1
Output Voltage Setpoint
Changing the value of R2 can change the output voltage in the range of 0.9 V
to 5 V. The value of R2 for a specific output voltage can be calculated by using
Equation 1−1. Table 1−3 list the values for R2 for some common output
voltages.
Equation 1−1.
R
2
+
1 k
W
0.891 V
V
O
*
0.891 V
Table 1−3. Output Voltage Programming
Output Voltage (V)
R
2
Value (
W
)
1.2
2.87 k
1.5
1.47 k
1.8
976
2.5
549
3.3
374
5.0
221
The minimum output voltage is limited by the minimum controllable on time of
the device, 125 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
O(min)
= 125 nsec
× ƒ
s
×
V
I(max)
1.3.2
Switching Frequency
The switching frequency may be trimmed to any value between 250 kHz and
700 kHz by changing the value of R4. The switching frequency can also be set
to preprogrammed values of 250 kHz by shorting the RT pin to AGND, or
500 kHz by floating the RT pin. Decreasing the switching frequency results in
increased output ripple unless the value of L1 is increased.
1.3.3
Input Filter
An onboard electrolytic input capacitor may be added at C1.
1.3.4
UVLO Programming
The TPS54350 is provided with an internal voltage divider from VIN to AGND.
These start and stop thresholds are given in Table 1−4.