TPS40074 Datasheet, PDF (16/38 Pages) Texas Instruments – MIDRANGE INPUT SYNCHRONOUS BUCK CONTROLLER WITH VOLTAGE FEED-FORWARD

TPS40074
SLUS617 – APRIL 2005
APPLICATION INFORMATION (continued)
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t_BLANKING
HDRV
Clock
V_ILIM
V_VIN−V_SW
SS
7 Current Limit Trips
(HDRV Cycle Terminated by Current Limit Trip)
7
Soft-Start
Cycles
Figure 11. Typical Fault Protection Waveforms
VDG−03174
BOOST AND DBP BYPASS CAPACITANCE
The BOOST capacitance provides a local, low-impedance flying source for the high-side driver. The BOOST
capacitor should be a good quality, high-frequency ceramic capacitor. A minimum value of 100-nF is suggested.
The DBP capacitor has to provide energy storage for switching both the synchronous MOSFET and the high-side
MOSFET (via the BOOST capacitor). The suggested value for this capacitor is 1-µF ceramic, minimum.
INTERNAL REGULATORS
The internal regulators are linear regulators that provide controlled voltages for the drivers and the internal
circuitry to operate from. The low-side driver operates directly from the 8-V regulator supply while the high-side
driver bootstrap capacitor is charged from this supply. The actual voltage delivered to the high-side driver is the
voltage on the DBP pin less any drop from the bootstrap diode. If the internal bootstrap diode is used, the drop
across that diode is nominally 1.4 V at room temperature. This regulator has two modes of operation. At voltages
below 8.5 V on VDD, the regulator is in a low dropout mode of operation and tries to provide as little impedance
as possible from VDD to DBP. When VDD is above 10 V, the regulator regulates DBP to 8 V. Between these two
voltages, the regulator is in whatever state it was in when VDD entered this region. The LVBP pin is connected
to a 4.2-V regulator that supplies power for the internal control circuitry. Small amounts of current can be drawn
from these pins for other external circuit functions, as long as power dissipation in the controller chip remains at
acceptable levels and junction temperature does not exceed 125°C. Any external load connected to LVBP should
be electrically quiet to avoid degrading performance of the TPS40074. Typical output voltages for these two
regulators are shown in Figure 12 and Figure 13.
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