Datasheet

www.ti.com
Output Overload Protection
Operating Near Maximum Duty Cycle
OUT DIODE
IN DIODE
V V
V V
+
d =
+
(15)
Dual Supply Operation
TPS54283 , , TPS54286
SLUS749C – JULY 2007 – REVISED OCTOBER 2007
In the event of an overcurrent during soft start on either output (such as starting into an output short),
pulse-by-pulse current limiting and PWM frequency division (see below) are in effect for that output until the
internal soft start timer ends. At the end of the soft start time, a UV condition is declared and a fault is declared.
During this fault condition, both PWM outputs are disabled and the small pulldown MOSFETs (from SWx to
GND) are turned ON. This process ensures that both outputs discharge to GND in the event that overcurrent is
on one output while the other is not loaded. The converter then enters a hiccup mode timeout before attempting
to restart. "Frequency Division" means if an overcurrent pulse is detected, six clock cycles are skipped before a
next PWM pulse is initiated, effectively dividing the operating frequency by six and preventing excessive current
build up in the inductor.
In the event of an overcurrent on either output after the output reaches regulation, pulse-by-pulse current limit is
in effect for that output. In addition, an output undervoltage (UV) comparator monitors the FBx voltage (that
follows the output voltage) to declare a fault if the output drops below 85% of regulation. During this fault
condition, both PWM outputs are disabled and the small pulldown MOSFETs (from SWx to GND) are turned ON.
This design ensures that both outputs discharge to GND, in the event that overcurrent is on one output while the
other is not loaded. The converter then enters a hiccup mode timeout before attempting to restart.
The overcurrent threshold for Output 1 is set nominally at 3.0 A. The overcurrent level of Output 2 is determined
by the state of the ILIM2 pin. The ILIM setting of Output 2 is not latched in place and may be changed during
operation of the converter.
Table 2. Current Limit Threshold Adjustment for
Output 2
ILIM2 Connection OCP Threshold for Output 2
BP or GND 1.5 A nominal setting
(floating) 3.0 A nominal setting
DESIGN HINT
The overcurrent protection threshold refers to the peak current in the internal switch.
Be sure to add one-half of the peak inductor ripple current to the dc load current in
determining how close the actual operating point is to the OCP threshold.
If the TPS5428x operates at maximum duty cycle, and if the input voltage is insufficient to support the output
voltage (at full load or during a load current transient), then there is a possibility that the output voltage will fall
from regulation and trip the output UV comparator. If this should occur, the TPS5428x protection circuitry will
declare a fault and enter a shut down-and-restart cycle.
DESIGN HINT
Ensure that under ALL conditions of line and load regulation, there is sufficient duty
cycle to maintain output voltage regulation.
The operating duty cycle under continuous conduction (neglecting losses) is approximated using Equation 15 .
where
• V
DIODE
is the voltage drop of the rectifier diode
It is possible to operate a TPS5428x from two supply voltages. If this application is desired, then the sequencing
of the supplies must be such that PVDD2 is above the UVLO voltage before PVDD1 begins to rise. This level
requirement ensures that the internal regulator and the control circuitry are in operation before PVDD1 supplies
energy to the output. In addition, Output 1 must be held in the disabled state ( EN1 high) until there is sufficient
voltage on PVDD1 to support Output 1 in regulation. (See the Operating Near Maximum Duty Cycle section.)
26 Submit Documentation Feedback Copyright © 2007, Texas Instruments Incorporated
Product Folder Link(s): TPS54283 TPS54286