Datasheet

TPS40055-EP

SGLS310D –JULY 2005–REVISED FEBRUARY 2012

www.ti.com

Stepping the load from a heavy load to a light load results in an output overshoot. Excess energy stored in the

inductor must be absorbed by the output capacitance. The energy stored in the inductor is described in

Equation 6.

(6)

where:

(7)

is the output current under heavy load conditions

is the output current under light load conditions

Some applications may require an additional circuit to prevent false restarts at the UVLO voltage level. This

applies to applications which have high impedance on the input voltage line or which have excessive ringing on

the V

line. The input voltage impedance can cause the input voltage to sag enough at start-up to cause a

UVLO shutdown and subsequent restart. Excessive ringing can also affect the voltage seen by the device and

cause a UVLO shutdown and restart. A simple external circuit provides a selectable amount of hysteresis to

prevent the nuisance UVLO shutdown.

Assuming a hysteresis current of 10% I

KFF

and the peak detector charges to 8 V and V

IN(min)

= 10 V, the value of

is calculated by Equation 8 using a R

KFF

= 71.5 kΩ.

(8)

is chosen to maintain the peak voltage between switching cycles. To keep the capacitor charge from drooping

0.1 V, or from 8 V to 7.9 V.

(9)

The value of C

may calculate to less than 10 pF, but some standard value up to 47 pF works adequately. The

diode can be a small signal switching diode or Schottky rated for more then 20 V. Figure 10 illustrates a typical

implementation using a small switching diode.

The tolerance on the UVLO set point also affects the maximum duty cycle achievable. If the UVLO starts the

device at 10% below the nominal start up voltage, the maximum duty cycle is reduced approximately 10% at the

nominal start up voltage.

Product Folder Link(s): TPS40055-EP