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3.1 Adjusting Output Voltage (R5 and R6)
V
VOUT
+ V
REF
+
ȧ
ȡ
Ȣ
1 )
R3
R5 R6
R5)R6
ȧ
ȣ
Ȥ
(1)
3.2 Using Remote Sense (J3)
3.3 5-V Input Operation (R10 and R15)
Schematic
The regulated output voltage can be adjusted within a limited range by changing the ground resistor in the
feedback resistor divider (R6 and R5). The output voltage is given by the formula
where
• V
VREF
= 0.700 V
• R3 = 10.0 k Ω
Table 2 contains common values for R6 to generate popular output voltages with R5 open R5 can be
used to increase the accuracy that can be obtained without using more expensive resistors.
TPS40075EVM-001 is stable through these output voltages but the efficiency may suffer as the power
stage is optimized for the 1.5-V output.
Table 2. Adjusting V
1V5_OUT
With R14
V
OUT
(V) R16 (k Ω )
3.3
(1)
2.67
2.5
(1)
3.83
2.2
(1)
4.64
2.0
(1)
8.36
1.8 6.34
1.5 8.66
1.2 14.0
(1)
Due to higher duty cycles associated with higher output voltages or lower input voltages, output
current should be limited to 10A when operating with output voltages greater than 2.0V or input
voltages below 6V to reduce conduction losses in the main switching FET (Q1). Under these
conditions a lower R
(DS)on
FET would normally be selected.
TPS40075EVM-001 provides the user with remote sense capabilities through the connector J3. When
remote sense is used, J3 should be connected at the load to provide more accurate load regulation by
compensating for losses over the terminal connections and load wire connections. When remote sense is
connected the output voltage measured between TP15 and TP16 may show a positive load regulation
characteristic (increasing output voltage with increasing load). This is the result of the controller's
compensation of resistive losses between the local sense voltage (TP15 and TP16) and the remote sense
connection (J3). TP17 and TP18 are connected to the remote sense lines and thus will show the voltage
at the load when remote sense is connected.
Excessive phase shift from inductive components in the load or remote sense connections can cause
instability if care is not taken to minimize these parasitic effects in the remote sense line. A twisted pair of
insulated cables from the load connection to J3 is preferred to minimize noise injection and inductance in
the remote sense line. In a device layout care should be taken to shield the remote sense line from
high-noise, high-current or digital signals to limit noise injection into the feedback path and provide the
most accurate regulation possible.
Due to higher duty cycles associated with higher output voltages or lower input voltages, output current
should be limited to 10 A when operating with output voltages greater than 2.0 V or input voltages below 6
V to reduce conduction losses in the main switching FET (Q1). Under these conditions a lower R
DS(on)
MOSFET would normally be selected.
SLUU254 – DECEMBER 2006 Using the TPS40075: A 12-V to 1.5-V @ 15-A Synchronous Buck Converter 5
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