TPS5211

HIGH FREQUENCY PROGRAMMABLE HYSTERETIC

REGULATOR CONTROLLER

SLVS243 – SEPTEMBER 1999

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

Control Section

Below are the equations needed to select the various components within the control section.

output voltage selection

The most important function of the power supply is to regulate the output voltage to a specific value. Values

between 1.3 V and 3.5 V can be easily set by shorting the correct VID inputs to ground. Values above the

maximum reference voltage (3.5 V) can be set by setting the reference voltage to any convenient voltage within

its range and selecting values for R2 and R3 to give the correct output. Select R3:

Then, calculate R2 using:

V

V

R2

R3

R2

+

R3

V

V

REF

V

REF

or

These equations are accurate if R2<<R11. If this condition is not fullfilled, the following equation must be used:

V

V

R2 x R11

R3 x (R2

) R11)

Another soultion is to use 0.1-

µF DC decoupling capacitor in series with R11. In such a case, R11 does not

influence the output voltage value.

R2 and R3 can also be used to make small adjusts to the output voltage within the reference-voltage range

and/or to adjust for load-current active droop compensation. If there is no need to adjust the output voltage, R3

can be eliminated. R2, R3 (if used), and C7 are used as a noise filter; calculate using:

C7

+ 150 ns

R2

ø R3

slowstart timing

Slowstart reduces the startup stresses on the power-stage components and reduces the input current surge.

Slowstart timing is a function of the reference-voltage current (determined by R6) and is independent of the

reference voltage. The first step in setting slowstart timing will be to determine R6:

R6 should be between 7 k

Ω and 300 kΩ, a recommended value is 20 kΩ.

Set the slowstart timing using the formula:

C5

+

t

SS

5

R

VREFB

≅

t

SS

(5

R6)

Where C5 = Slowstart capacitance in

µF