Datasheet

V
IN
80:
I
PFM
= 66 mA
+
V
IN
100:
I
MODE
< 66 mA
)
(Typically +
LM26484
www.ti.com SNVS573F –SEPTEMBER 2008–REVISED MARCH 2013
There are three modes of operation depending on the current required - PWM, PFM, and shutdown. PWM mode
handles current loads of approximately 70 mA or higher, delivering voltage precision with high efficiency. Lighter
output current loads cause the device to automatically switch into PFM for reduced current consumption (I
q
= 15
µA typ.) and a longer battery life. The Standby operating mode turns off the device, offering the lowest current
consumption. Forced PWM is factory programmed. For Auto PFM-PWM please contact TI Sales.
Both SW1 and SW2 can operate up to a 100% duty cycle (PMOS switch always on) for low drop out control of
the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage.
Additional features include soft-start, under-voltage lockout, current overload protection, and thermal overload
protection.
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 the LM26484 and any external components during overload
conditions. An internal comparator senses the voltage across an internal sense resistor and will turn on the NFET
when the output current is sensed at 2.5A (min.) with 0.5 µH inductors. 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. For the PFM mode to be enabled, please contact TI Sales.
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
(1)
During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage
during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy
load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output
FETs such that the output voltage ramps between 0.8% and 1.6% (typ.) above the nominal PWM output voltage.
If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on. It
remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the I
PFM
level
set for PFM mode. The typical peak current in PFM mode is:
(2)
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