LM26484SQEV National Semiconductor, LM26484SQEV Datasheet - Page 10

• Manufacturer Part Number: LM26484SQEV
• Description: BOARD EVALUATION FOR LM2684
• Manufacturer: National Semiconductor

Specifications of LM26484SQEV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 3, Non-Isolated
• Voltage - Output: 1V, 1.8V, 1V
• Current - Output: 2A, 2A, 1A
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM26484
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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Buck Regulator Functional
Description
The LM26484 incorporates two high efficiency synchronous
switching buck regulators which are 180° out of phase, SW1
and SW2 that deliver voltages from a single DC input voltage.
Using a voltage mode architecture with synchronous rectifi-
cation, both bucks have the ability to deliver up to 2A de-
pending on the input voltage and output voltage (voltage head
room), and the inductor chosen (maximum current capability).
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
battery life. The Standby operating mode turns off the device,
offering the lowest current consumption. Forced PWM is fac-
tory programmed. For Auto PFM-PWM please contact Na-
tional Semiconductor 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 pro-
vides 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 con-
ditions. 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 op-
erates with reduced switching frequency and supply current
to maintain high efficiency. For the PFM mode to be enabled,
please contact National Semiconductor Sales.
q
= 15 µA typ.) and a longer
10
The part will automatically transition into PFM mode when ei-
ther of two conditions occurs for a duration of 32 or more clock
cycles:
or
level
During PFM operation, the converter positions the output volt-
age 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 be-
low the ‘high’ PFM comparator threshold, the PMOS power
switch is turned on. It remains on until the output voltage ex-
ceeds the ‘high’ PFM threshold or the peak current exceeds
the I
PFM mode is:
Once the PMOS power switch is turned off, the NMOS power
switch is turned on until the inductor current ramps to zero.
When the NMOS zero-current condition is detected, the
NMOS power switch is turned off. If the output voltage is be-
low the ‘high’ PFM comparator threshold (see
PMOS switch is again turned on and the cycle is repeated
until the output reaches the desired level. Once the output
reaches the ‘high’ PFM threshold, the NMOS switch is turned
on briefly to ramp the inductor current to zero and then both
output switches are turned off and the part enters an ex-
tremely low power mode. Quiescent supply current during this
‘sleep’ mode is less than 30 µA, which allows the part to
achieve high efficiencies under extremely light load condi-
tions. When the output drops below the ‘low’ PFM threshold,
the cycle repeats to restore the output voltage to ~1.6% above
the nominal PWM output voltage.
If the load current should increase during PFM mode (see
Figure
PFM threshold, the part will automatically transition into fixed-
frequency PWM mode.
During shutdown the PFET switch, reference, control and
bias circuitry of the converters are turned off. The NFET
switch will be on in shutdown to discharge the output. When
the converter is enabled, soft start is activated. It is recom-
mended to disable the converter during the system power up
and under voltage conditions when the supply is less than
3.0V.
A. The inductor current becomes discontinuous
B. The peak PMOS switch current drops below the I
PFM
2) causing the output voltage to fall below the ‘low2’
level set for PFM mode. The typical peak current in
Figure
2), the
MODE