TPS40052

SLUS563C − AUGUST 2003 − REVISED FEBRUARY 2005

9

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APPLICATION INFORMATION

BP5 AND BP10 INTERNAL VOLTAGE REGULATORS

Start-up characteristics of the BP5 and BP10 regulators are shown in Figure 2. Slight variations in the BP5

occurs dependent upon the switching frequency. Variation in the BP10 regulation characteristics is also based

on the load presented by switching the external MOSFETs.

SELECTING THE INDUCTOR VALUE

The inductor value determines the magnitude of ripple current in the output capacitors as well as the load current

at which the converter enters discontinuous mode. Too large an inductance results in lower ripple current but

is physically larger for the same load current. Too small an inductance results in larger ripple currents and a

greater number of (or more expensive output capacitors for) the same output ripple voltage requirement. A good

compromise is to select the inductance value such that the converter doesn’t enter discontinuous mode until

the load approximated somewhere between 10% and 30% of the rated output. The inductance value is

described in equation (2).

L

+

V

V

O

V

IN

DI

f

SW

(Henries)

where:.

CALCULATING THE OUTPUT CAPACITANCE

The output capacitance depends on the output ripple voltage requirement, output ripple current, as well as any

output voltage deviation requirement during a load transient.

The output ripple voltage is a function of both the output capacitance and capacitor ESR. The worst case output

ripple is described in equation (3).

DV + DI ESR )

1

8

C

O

f

SW

V

P

*P

The output ripple voltage is typically between 90% and 95% due to the ESR component.

The output capacitance requirement typically increases in the presence of a load transient requirement. During

a step load, the output capacitance must provide energy to the load (light to heavy load step) or absorb excess

inductor energy (heavy to light load step) while maintaining the output voltage within acceptable limits. The

amount of capacitance depends on the magnitude of the load step, the speed of the loop and the size of the

inductor.

(2)

(3)