Datasheet

R
z
2
V
(z)
–V
ref
V
ref
–V
O
R1
V
ref
R2
TPS5103
MULTIPLE MODE SYNCHRONOUS DC/DC CONTROLLER
SLVS240A – SEPTEMBER 1999 – REVISED MAY 2001
21
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
output voltage setpoint calculation
The output voltage is set by the reference voltage and the voltage divider. In the TPS5102, the reference voltage
is 1.185 V, and the divider is composed of two resistors in the EVM design that are R4 and R5, or R14 and R15.
The equation for the setpoint is shown below.
R2
R1 V
ref
V
O
V
ref
Where R1 (> 10 kΩ) is the top resistor R2 is the bottom resistor (kΩ), V
O
is the required output voltage, and V
ref
is the reference voltage (1.185 V in TPS5103).
Example: R1 = 1 kΩ; V
ref
= 1.185 V; V
O
= 1.8 V, then R2 = 1.9 kΩ.
Some of the most popular output voltage setpoints are calculated in Table 2.
Table 2. Output Voltage Setpoints
V
O
1.3 V 1.5 V 1.8 V 2.5 V 3.3 V 5.0 V
R1 (kΩ) 10 10 10 10 10 10
R2 (kΩ) 100 37 19 9 5.6 3.1
If higher precision resistor is used, the output voltage setpoint can be more accurate.
In some applications, the output voltage is required to be lower than the reference voltage. With a few extra
components, this lower voltage can be easily achieved. Figure 40 shows the method for accomplishing this.
TPS5103
V
CC
V
O
R
1
R
2
R
z1
Zener
INV
R
z2
Figure 40. Application With Extra Components for Lower Output Voltage
In the schematic, R
z1
, R
z2
, and the Zener are the extra components. R
z1
is used to give the Zener enough
current to build up the Zener voltage. The Zener voltage is added to INV through R
z2
. Therefore, the voltage
on INV is still equal to the IC internal voltage (1.185 V), even if the output voltage is regulated at a lower setpoint.
The equation for setting up the output voltage is shown below:
Where R
z2
is the adjusting resistor for low-output voltage, V
(z)
is the Zener voltage, V
ref
is the internal reference
voltage, R1 is the top resistor of the voltage sensing network, R2 is the bottom resistor of the sensing network,
and V
O
is the required output voltage setpoint.
Example: Assuming the required output voltage setpoint is V
O
= 0.8 V, V
(z)
= 5 V, R1 = 1 kΩ; R2 = 1 kΩ, then
the R
z2
= 2.43 kΩ.