Datasheet

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OUT
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LM26480
SNVS543I –JANUARY 2008–REVISED MAY 2013 www.ti.com
Circuit Operation Description
A buck converter contains a control block, a switching PFET connected between input and output, a synchronous
rectifying NFET connected between the output and ground (BCKGND pin) and a feedback path. During the first
portion of each switching cycle, the control block turns on the internal PFET switch. This allows current to flow
from the input through the inductor to the output filter capacitor and load. The inductor limits the current to a
ramp with a slope of
(1)
by storing energy in a magnetic field. During the second portion of each cycle, the control block turns the PFET
switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor
draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor
current down with a slope of
(2)
The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage
across the load.
Sync Function
The LM26480SQ-BF is the only version of the part that has the ability to use an external oscillator. The source
must be 13 MHz nominal and operate within a range of 15.6 MHz and 10.4 MHz, proportionally the same limits
as the 2.0 MHz internal oscillator. The LM26480SQ-BF has an internal divider which will divide the speed down
by 6.5 to the nominal 2MHz and use it for the regulators. This SYNC function replaces the internal oscillator and
works in forced PWM only. The buck regulators no longer have the PFM function enabled. When the
LM26480SQ-BF is sold with this feature enabled, the part will not function without the external oscillator present.
Please contact Texas Instruments Sales Office/Distributors for availability of LM26480SQ-BF.
PWM Operation
During PWM operation the converter operates as a voltage-mode controller with input voltage feed forward. This
allows the converter to achieve excellent load and line regulation. The DC gain of the power stage is proportional
to the input voltage. To eliminate this dependence, feed forward voltage inversely proportional to the input
voltage is introduced.
Internal Synchronous Rectification
While in PWM mode, the buck uses an internal NFET as a synchronous rectifier to reduce rectifier forward
voltage drop and associated power loss. Synchronous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier
diode.
Current Limiting
A current limit feature allows the converter to protect itself and external components during overload conditions.
PWM mode implements current limiting using an internal comparator that trips at 2.0A for both bucks (typ). If the
output is shorted to ground the device enters a timed current limit mode where the NFET is turned on for a
longer duration until the inductor current falls below a low threshold, ensuring inductor current has more time to
decay, thereby preventing runaway.
PFM Operation
At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply
current to maintain high efficiency.
The part will automatically transition into PFM mode when either of two conditions occurs for a duration of 32 or
more clock cycles:
A. The inductor current becomes discontinuous
or
B. The peak PMOS switch current drops below the I
MODE
level
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