Rz2

+

(Vz–Vr)

Vr–Vo

Rtop )

Vr

Rbtm

TPS5103

MULTIPLE MODE SYNCHRONOUS DC/DC CONTROLLER

SLVS240 – SEPTEMBER 1999

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

output voltage setpoint calculation

The output voltage is set by the reference voltage and the voltage divider. In 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:

R2

+ R1

Vr

Vo

* Vr

Where R1 is the top resistor (k

Ω) like R4 or R15; R2 is the bottom resistor (kΩ) such as R5 or R14; Vo is the

required output voltage (V); Vr is the reference voltage (1.185 V in TPS5103).

Example: R1 = 1 k

Ω; Vr = 1.185 V; Vo = 1.8 V, then R2 = 1.9 kΩ.

For your convenience, some of the most popular output voltage setpoints are calculated in the table below:

Vo

1.3 V

1.5 V

1.8 V

2.5 V

3.3 V

5.0 V

R1 (top) (k

Ω)

1

1

1

1

1

1

R2 (bottom) (k

Ω)

10

3.7

1.9

0.9

0.56

0.31

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 few extra

components, the lower voltage can be easily achieved. The drawing below shows the method.

TPS5103

VCC

VO

R(top)

R(bottom)

Rz1

Zener

INV

Rz2

In the schematic, the Rz1, Rz1, and the zener are the extra components. Rz1 is used to give zener enough

current to build up the zener voltage. The zener voltage is added to INV through Rz2. Therefore, the voltage

on INV is still equal to the IC internal voltage (1.185 V) even if the output voltage is regulated at lower setpoint.

The equation for setting up the output voltage is shown below:

Where Rz2 is the adjusting resistor for low output voltage; Vz is the zener voltage; Vr is the internal reference

voltage; Rtop is the top resistor of voltage sensing network; Rbtm is the bottom resistor of the sensing network;

Vo is the required output voltage setpoint.

Example: Assuming the required output voltage setpoint is Vo = 0.8 V, Vz = 5 V; Rtop = 1 k

Ω; Rbottom = 1 kΩ,

then the Rz2 = 2.43 k

Ω.