Datasheet

TPS40055-EP
SGLS310D –JULY 2005–REVISED FEBRUARY 2012 www.ti.com
Figure 5. Undervoltage Lockout Operation
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.
The impedance of the input voltage can cause the input voltage, at the controller, to sag when the converter
starts to operate and draw current from the input source. Therefore, there is voltage hysteresis that prevents
nuisance shutdowns at the UVLO point. With R
T
chosen to select the operating frequency and R
KFF
chosen to
select the start-up voltage, the approximate amount of hysteresis voltage is shown in Figure 7.
Figure 6. UNDERVOLTAGE LOCKOUT THRESHOLD
vs
HYSTERESIS
Figure 7.
BP5 AND BP10 INTERNAL VOLTAGE REGULATORS
Start-up characteristics of the BP5 and BP10 regulators over different temperature ranges are shown in Figure 8
and Figure 9. 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.
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