TPS23750
TPS23770
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SLVS590A–JULY 2005–REVISED AUGUST 2005
The COMP output should not be over-driven when the internal error amplifier is active. The amplifier can source
and sink significant currents which will greatly increase power dissipation. TMR may be pulled low to turn the
converter off when the internal error amplifier is used. The error amplifier will source current when COMP is
pulled below its saturated low voltage due to the nature of the class AB amplifier stage.
DET – Connect a 24.9 kΩ, ±1% resistor (RDET), between DET and VDD. RDET is connected across the input line
when VDD lies between 1.4 V and 10.1 V, and is disconnected when the line voltage exceeds 12 V to conserve
power. RDET may be adjusted to compensate for input diode characteristics.
FB – This is the internal dc/dc converter error amplifier’s inverting input. FB is used for output voltage feedback
and loop compensation. FB equals 1.5 V when the feedback loop is in regulation. FB should be tied to RTN
when the error amplifier is disabled using MODE. The internal level translator drives this pin with a source
impedance of about 15 kΩ when it is enabled using SEN.
FREQ – A resistor connected from FREQ to RTN programs the converter switching frequency. This feature
allows an existing design to be easily upgraded to use the TPS23750 without requiring redesign of the magnetics
and filtering. While the oscillator is characterized between 100 kHz and 500 kHz, it operates properly down to a
frequency of a few kilohertz.
15000
R
(kW) +
FREQ
Switching_Frequency (kHz)
(1)
Although this expression is reasonably accurate, the frequency will be slightly lower than predicted at higher
frequencies.
FREQ must not be shorted to ground or have voltage applied.
GATE – DC/DC converter’s switching MOSFET driver output. This pin has an internal pull-down to keep the
external switching MOSFET off when the converter is inactive.
MODE –This pin disables the converter error amplifier, allowing an optocoupler to drive the PWM comparator
directly from COMP. Connecting MODE to RTN enables the error amplifier, and to VBIAS disables it. MODE
should not be left floating.
RSN – This pin is the current-mode controller’s quiet "ground" reference for current sensing and other low-level
signals. RTN, RSN, and COM should be tied together.
RSP – This pin is the current-mode controller’s current-sense input. Current-mode control monitors the switching
MOSFET peak current, which is sensed as voltage between RSP and RSN, to set the PWM duty cycle. The
peak current limit is established by limiting the maximum sense voltage to about 0.5 V.
MOSFET current may rise to high levels during the blanking period when there is a short in the power circuit. If
the RSP peak voltage exceeds 0.75 V on four successive switching cycles, the converter is turned off and a
hiccup cycle is started.
If the blanking is sufficient to eliminate the need for an input RC filter, this pin may be directly connected to the
sense resistor.
RTN – An internal MOSFET connects this pin to VSS. This MOSFET is controlled by the PoE section UVLO,
inrush limit, current limit, thermal limit, and fault voltage limiting.
Most applications connect RSN, COM, and RTN together through a ground plane.
SEN – SEN is the negative input for the level translator. It can be used in buck converters as demonstrated in
Figure 40. The translator is enabled by connecting SEN above 1 V with respect to VSS. The level translator
applies VSENP-SEN to the FB pin through an internal 15 kΩ resistor. This feature simplifies feedback voltage
sensing above RTN. Connect SEN to RSN if the level translator is not used.
SENP – SENP is the positive input for the level translator. It is used in conjunction with SEN as demonstrated in
Figure 40. The presence of this pin allows a filter inductor to be placed in the positive power rail between VDD
and the output. Connect SENP to VDD when the level translator is disabled. The voltage on SENP should always
be greater than the voltage on SEN.
TMR – Connect a capacitor from TMR to RTN to program the softstart and hiccup timer functions. Pull this pin to
RTN to disable the converter.
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