TPS23750
TPS23770
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SLVS590A–JULY 2005–REVISED AUGUST 2005
CONVERTER STARTUP
An imbalance between converter output capacitance, converter current limit, input bulk capacitance, and softstart
time causes the converter to hiccup when attempting to start. The converter has a hard input current limit
enforced by either the internal hotswap MOSFET or the PSE. If this current is exceeded, the converter meets its
energy demands by drawing down the voltage on the bulk capacitor. As the capacitor voltage falls, the voltage
across the internal MOSFET increases. If the voltage across the TPS23750’s MOSFET reaches 12 V, it turns the
converter off, falls back into inrush, and tries to restart.
To successfully start up, the design should balance the output capacitance, converter current limit, input bulk
capacitance, and softstart time. Minimize the output capacitance and converter current limit. Use a long softstart
period to control the output capacitor charge current. Finally, use a larger input capacitor that provides an energy
store to get over this peak demand. The input bulk capacitor voltage droop should generally not exceed 5 V.
TMR OPERATION
TMR provides both a softstart and fault protection by means of a hiccup mode. Each cycle of hiccup operation
consists of time-limited overload, followed by an enforced quiescent period, and an automatic restart. The
benefits of hiccup operation include reduction of average thermal stress during faults and an automatic restart if a
transient condition shuts the converter down.
During softstart, the converter is enabled and CTMR charges at 50 µA from a low voltage towards 3 V. If VCOMP is
less than 4.2 V when VTMR reaches 3 V, CTMR continues to charge towards the 3.5 V clamp level and the
converter remains enabled. Internal scaling assures that a VCOMP less than 4 V will yield the maximum peak
current limit. A VCOMP of less than 4.2 V means that the voltage loop is in regulation. A high VCOMP is an
indication that there is a problem, with the most likely problem being an output overload. If VCOMP is above 4.2 V
when VTMR reaches 3.0 V, the converter disables and CTMR discharges at 5 µA. A new softstart cycle begins
when TMR reaches 0.3 V.
If the converter is operating normally, and VCOMP exceeds 4.2 V, CTMR begins to discharge at 5 µA towards 3 V. If
TMR reaches 3 V, the converter shuts off and a hiccup cycle begins. If VCOMP falls below 4.2 V before TMR
reaches 3 V, CTMR recharges at 50 µA and converter operation continues uninterrupted. Brief transients does not
cause a hiccup due to the inherent filtering set by the value of CTMR
.
Softstart behaves differently when the internal error amplifier is used or disabled. The error amplifier, if used,
regulates the voltage on FB to equal the voltage on TMR minus 0.5 V during softstart. The output voltage rises
slowly and is in regulation when FB equals 1.5 V. If the error amplifier is disabled, the PWM comparator trip point
ramps from 0 V to 0.5 V as VTMR transitions from 0.54 V to 1.54 V.
TMR is discharged with a 1 kΩ pull down resistance when the converter is disabled.
Several other conditions interact with TMR:
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TMR is held low when the PoE control disables the converter.
TMR is held low if converter UVLOs are not satisfied.
TMR is held low in thermal shutdown.
TMR hiccups after four consecutive fault comparator trips. Switching is suspended immediately while a
hiccup cycle occurs.
Figure 36 illustrates the operation of the TMR pin. These waveforms were obtained using the circuits of
Figure 38 and Figure 40. The buck converter shows softstart with the internal error amplifier and the flyback
example shows operation when an optocoupler circuit is used. The flyback example shows COMP voltage
droops due to the secondary-side softstart rather than from the internal error amplifier.
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