
TPS54672
SLVS397C JULY 2001 REVISED FEBRUARY 2005
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If the current limit comparator trips for longer than 100 ns,
the PWM latch resets before the PWM ramp exceeds the
error amplifier output. The high-side FET turns off and
low-side FET on to decrease the output current. This
process is repeated each cycle in which the current limit
comparator is tripped.
Dead-Time Control and MOSFET Drivers
Adaptive
dead-time
control
prevents
shoot-through
current from flowing in both N-channel power MOSFETs
during the switching transitions by actively controlling the
turnon times of the MOSFET drivers. The high-side driver
does not turn on until the gate drive voltage to the low-side
FET is below 2 V, while the low-side driver does not turn
on until the voltage at the junction of the power MOSFETs
(PH pin) is below 2 V.
The high-side and low-side drivers are designed with
300-mA source and sink capability to quickly drive the
power MOSFETs gates. The low-side driver is supplied
from VIN, while the high-side drive is supplied from the
BOOT pin. A bootstrap circuit uses an external BOOT
capacitor and internal 2.5-
bootstrap switch connected
between the VIN and BOOT pins. The bootstrap switch is
turned on when the low-side FET is on to charge the BOOT
capacitor. The low resistance of the bootstrap switch
improves drive efficiency and reduces external component
count.
Overcurrent Protection
The cycle by cycle current limiting is achieved using a
sense-FET on the high-side MOSFET and differential
amplifier with preset overcurrent threshold. The high-side
MOSFET is turned off within 200 ns of the voltage on the
sense-FET exceeding the current limit threshold. A 100-ns
leading edge blanking circuit prevents false tripping of the
current limit when the high-side switch is turning on.
Current limit detection occurs only when current flows from
VIN to PH when current is being sourced to the output filter.
Load protection during current sink operation is provided
by thermal shutdown.
Thermal Shutdown
Thermal shutdown turns off the power MOSFETs and
disables the control circuits if the junction temperature
exceeds the 150
°C. The device is released from shutdown
automatically when the junction temperature decreases to
10
°C, and starts up under control of the slow-start circuit.
Status (STATUS)
The status pin is an open drain output that indicates when
internal conditions are sufficient for proper operation.
STATUS can be coupled back to a system controller or
monitor circuit to indicate that the termination or tracking
regulator is ready for start up. STATUS is high impedance
when the TPS54672 is operating or ready to be enabled.
STATUS is active low if any of the following occur:
D VIN < UVLO threshold
D VBIAS or internal reference have not settled.
D Thermal shutdown is active.