Datasheet

PVIN
AVIN
AGND
PGND
FB
SW
LM2854
SS
R
T1
R
T2
V
IN
C
IN
Master Power
Supply
V
OUT1
V
OUT2
L
O
C
O
V
SS
LM2854
SNVS560C –MARCH 2008–REVISED APRIL 2013 www.ti.com
APPLICATIONS INFORMATION
GENERAL
The LM2854 PowerWise synchronous DC-DC buck regulator belongs to the Texas Instruments SIMPLE
SWITCHER family of switching regulators. Integration of the power MOSFETs and associated drivers,
compensation component network and the PWM controller reduces the number of external components
necessary for a complete power supply design, without sacrificing performance.
Operation Description
SWITCHING FREQUENCY
The LM2854 is available in two switching frequency options, 500 kHz and 1 MHz. Generally, a higher switching
frequency allows for faster transient response and a reduction in the footprint area and volume of the external
power stage components, while a lower switching frequency affords better efficiency. These factors should be
considered when selecting the appropriate switching frequency for a given application.
ENABLE
The LM2854 features a enable (EN) pin and associated comparator to allow the user to easily sequence the
LM2854 from an external voltage rail, or to manually set the input UVLO threshold. The turn-on or rising
threshold and hysteresis for this comparator are typically 1.23V and 0.15V respectively. The precise reference for
the enable comparator allows the user to ensure that the LM2854 will be disabled when the system demands it
to be.
SOFT-START
The LM2854 begins to operate when both the AVIN and EN voltages exceed the rising UVLO and enable
thresholds, respectively. A controlled soft-start eliminates inrush currents during start-up and allows the user
more control and flexibility when sequencing the LM2854 with other power supplies. An external soft-start
capacitor is used to control the LM2854 start-up time. During soft-start, the voltage on the feedback pin is
connected internally to the non-inverting input of the error amplifier. The soft-start period lasts until the voltage on
the soft-start pin exceeds the LM2854 reference voltage of 0.8V. At this point, the reference voltage takes over at
the non-inverting amplifier input.
In the event of either AVIN or EN decreasing below the falling UVLO or enable threshold respectively, the
voltage on the soft-start pin is collapsed by discharging the soft-start capacitor through a 5 kΩ transistor to
ground.
TRACKING
The LM2854 can track the output of a master power supply during soft-start by connecting a resistor divider to
the SS pin. In this way, the output voltage slew rate of the LM2854 will be controlled by a master supply for loads
that require precise sequencing. When the tracking function is used, a small value soft-start capacitor can be
connected to the SS pin to alleviate output voltage overshoot when recovering from a current limit fault.
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