TPS5210

PROGRAMMABLE SYNCHRONOUS BUCK REGULATOR CONTROLLER

SLVS171A – SEPTEMBER 1998 – REVISED MAY 1999

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

frequency calculation

A detailed derivation of frequency calculation is shown in the application report, “

Designing Fast Response

Synchronous Buck Regulators Using the TPS5210”, TI Literature number SLVA044. When less accurate results

are acceptable, the simplified equation shown below can be used:

V

O

V

ESR

V

I

L

Hysteresis Window

Control Section

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

derivations of the equations used in this section are available in the application report “

Designing Fast Response

Synchronous Buck Regulators Using the TPS5210”, TI Literature number SLVA044.

output voltage selection

Of course 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

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