Texas Instruments TPS769 Series, Руководство пользователя, Страница: 10 / 25

Output Voltages Less Than V
REF
1-4 Introduction
1.3 Output Voltages Less Than V
REF
Applications requiring a lower output voltage than the internal reference volt-
age needs additional circuitry. The backside components shown in the upper
section of figure 1 along with the deletion of R4 will implement this option. The
backside components function as a second (but fixed) voltage source to artifi-
cially increase the voltage at the feedback terminal, thus requiring less output
voltage to satisfy the error amplifier. The two voltage sources are summed at
the error amplifier input node and compared to the internal reference voltage
of 1.224 V. The contribution of the TL431 reference is:
V
A
+
R3
R3
)
R6
V
REF
+
R3
R3
)
R6
2.495 V
The contribution of the output voltage is:
V
B
+
R6
R3
)
R6
V
O
The total feedback voltage at V
FB
then is the sum of the two previous equa-
tions:
V
FB
+
V
A
)
V
B
+
ǒ
R3
R3
)
R6
2.495
Ǔ
)
ǒ
R6
R3
)
R6
V
O
Ǔ
Selecting R6 = 130 k
Ω
and solving for R3 yields:
R3
+
ǒ
V
FB
*
V
O
Ǔ
ǒ
V
REF
*
V
FB
Ǔ
R6
+
125.19
*
ǒ
102.28 V
O
Ǔ
k
Ω
This circuit can only work down to 0 V due to the lack of a negative supply volt-
age. The accuracy of the output voltage is determined by the accuracy of both
regulators, but since the load for the TL431 portion of the circuit is constant,
only its line regulation and drift are important to the output voltage accuracy.
Additional features include a delayed enable by placing a capacitor for C2 and
calculating R1 and C2 for an RC circuit. If no delayed enable is needed, then
place a zero ohm resistor for R2 since the enable pin is active low. (note that
C2 and R2 share the same component location).