LM3679TL-1.8EV National Semiconductor, LM3679TL-1.8EV Datasheet - Page 8

• Manufacturer Part Number: LM3679TL-1.8EV
• Description: BOARD EVALUATION LM3679TL-1.8
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3679TL-1.8EV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 350mA
• Voltage - Input: 2.5 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 3MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3679
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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Operation Description
DEVICE INFORMATION
The LM3679, a high efficiency step down DC-DC switching
buck converter, delivers a constant voltage from a single Li-
Ion battery and input voltage rails from 2.5V to 5.5V such as
cell phones and PDAs. Using a voltage mode architecture
with synchronous rectification, the LM3679 has the ability to
deliver up to 350mA depending on the input voltage and out-
put voltage, ambient temperature, and the inductor chosen.
There are three modes of operation depending on the current
required - PWM (Pulse Width Modulation), PFM (Pulse Fre-
quency Modulation), and shutdown. The device operates in
PWM mode at load current of approximately 80 mA or higher,
having a voltage precision of ±2.5% with 90% efficiency or
better. Lighter load current causes the device to automatically
switch into PFM mode for reduced current consumption (I
16 µA typ) and a longer battery life. Shutdown mode turns off
the device, offering the lowest current consumption
(I
Additional features include soft-start, under voltage protec-
tion, current overload protection, and thermal shutdown pro-
tection. As shown in Figure 1, only three external power
components are required for implementation.
Using the UR package allows for a low profile solution size
(0.55mm max height, including external components). The
recommended external components are stated within the ap-
plication information. The max output current is 300mA when
these specific low profile external components are used.
The part uses an internal reference voltage of 0.5V. It is rec-
ommended to keep the part in shutdown until the input voltage
exceeds 2.5V.
CIRCUIT OPERATION
The LM3679 operates as follows. During the first portion of
each switching cycle, the control block in the LM3679 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
(V
During the second portion of each cycle, the controller 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 - V
The output filter stores charge when the inductor current is
high, and releases it when inductor current is low, smoothing
the voltage across the load.
The output voltage is regulated by modulating the PFET
switch on time to control the average current sent to the load.
The effect is identical to sending a duty-cycle modulated rect-
angular wave formed by the switch and synchronous rectifier
at the SW pin to a low-pass filter formed by the inductor and
output filter capacitor. The output voltage is equal to the av-
erage voltage at the SW pin.
PWM OPERATION
During PWM operation, the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve good load and line regulation. The
DC gain of the power stage is proportional to the input voltage.
To eliminate this dependence, feed forward inversely propor-
tional to the input voltage is introduced.
SHUTDOWN
IN
–V
OUT
)/L, by storing energy in a magnetic field.
= 0.01 µA typ).
OUT
/L.
Q
=
8
While in PWM mode, the output voltage is regulated by
switching at a constant frequency and then modulating the
energy per cycle to control power to the load. At the beginning
of each clock cycle the PFET switch is turned on and the in-
ductor current ramps up until the comparator trips and the
control logic turns off the switch. The current limit comparator
can also turn off the switch in case the current limit of the
PFET is exceeded. Then the NFET switch is turned on and
the inductor current ramps down. The next cycle is initiated
by the clock turning off the NFET and turning on the PFET.
Internal Synchronous Rectification
While in PWM mode, the LM3679 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 LM3679 to protect itself and
external components during overload conditions. PWM mode
implements current limiting using an internal comparator that
trips at 920 mA (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 op-
erates with reduced switching frequency and supply current
to maintain high efficiency.
The part will automatically transition into PFM mode when ei-
ther of the following conditions occurs for a duration of 32 or
more clock cycles:
level, (Typically I
A.
B.
The NFET current reaches zero.
The peak PMOS switch current drops below the I
FIGURE 4. Typical PWM Operation
MODE
< 75mA + V
IN
/55 Ω ).
30016380
MODE