Datasheet

Tss(ms) Iss( A)
Css(nF)
Vref(V)
´ m
=
( )
ENFALLING
STOP ENFALL ING p h
R1 V
R2 =
V V + R1 I + I
´
-
TPS5432
SLVSB89A –MARCH 2012–REVISED OCTOBER 2012 www.ti.com
spacer
(3)
Where I
h
= 3.4 µA, I
P
= 1.2 µA, V
ENRISING
= 1.23V, V
ENFALLING
= 1.19 V
V
START
is the target VIN UVLO rising threshold set by EN resistors.
V
STOP
is the target VIN UVLO falling threshold set by EN resistors.
SLOW START PIN
The TPS5432 regulates to the lower of the SS pin and the internal reference voltage. A capacitor on the SS pin
to ground implements a slow start time. The TPS5432 has an internal pull-up current source of 2 μA which
charges the external slow start capacitor. Equation 4 calculates the required slow start capacitor value where
TSS is the desired slow start time in ms, ISS is the internal slow start charging current of 2 μA, and Vref is the
internal voltage reference of 0.808 V.
If during normal operation, the VIN goes below the UVLO, EN pin pulled below 1.19 V, or a thermal shutdown
event occurs, the TPS5432 stops switching and the SS is discharged to 0 volts before reinitiating a powering up
sequence.
(4)
OVERCURRENT PROTECTION
The TPS5432 implements a cycle by cycle current limit. During each switching cycle the high side switch current
is compared to the voltage on the COMP pin. When the instantaneous switch current intersects the COMP
voltage, the high side switch is turned off. During overcurrent conditions that pull the output voltage low, the error
amplifier responds by driving the COMP pin high, increasing the switch current. The error amplifier output is
clamped internally. This clamp functions as a switch current limit.
FREQUENCY SHIFT
To operate at high switching frequencies and provide protection during overcurrent conditions, the TPS5432
implements a frequency shift. If frequency shift was not implemented, during an overcurrent condition the low
side MOSFET may not be turned off long enough to reduce the current in the inductor, causing a current
runaway. With frequency shift, during an overcurrent condition the switching frequency is reduced from 100%,
then 50%, then 25% as the voltage decreases from 0.808V to 0V on VSENSE pin to allow the low side MOSFET
to be off long enough to decrease the current in the inductor. During start-up, the switching frequency increases
as the voltage on VSENSE increases from 0V to 0.808V. See Figure 5 for details.
REVERSE OVERCURRENT PROTECTION
The TPS5432 implements low side current protection by detecting the voltage across the low side MOSFET.
When the converter sinks current through its low side FET, the control circuit turns off the low side MOSFET if
the reverse current is more than 1.8 A. By implementing this additional protection scheme, the converter is able
to protect itself from excessive current during power cycling and start-up into pre-biased outputs.
OVERVOLTAGE TRANSIENT PROTECTION
The TPS5432 incorporates an overvoltage transient protection (OVTP) circuit to minimize voltage overshoot
when recovering from output fault conditions or strong unload transients. The OVTP feature minimizes the output
overshoot by implementing a circuit to compare the VSENSE pin voltage to the OVTP threshold which is 107%
of the internal voltage reference. If the VSENSE pin voltage is greater than the OVTP threshold, the high side
MOSFET is disabled preventing current from flowing to the output and minimizing output overshoot. When the
VSENSE voltage drops lower than the OVTP threshold, which is 105% of the internal voltage reference, the high
side MOSFET is allowed to turn on the next clock cycle.
12 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated
Product Folder Links: TPS5432