LM3678SD-1.2EV National Semiconductor, LM3678SD-1.2EV Datasheet - Page 9

• Manufacturer Part Number: LM3678SD-1.2EV
• Description: BOARD EVAL FOR LM3678SD-1.2
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3678SD-1.2EV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 0.8V or 1.2V
• Current - Output: 1.5A
• Voltage - Input: 2.5 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 3.3MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3678
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

Operation Description
DEVICE INFORMATION
The LM3678, 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 to
portable devices such as cell phones and PDAs. Using a volt-
age mode architecture with synchronous rectification, the
LM3678 has the ability to deliver up to 1.5A depending on the
input voltage, output voltage, ambient temperature and the
inductor chosen.
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.
The part uses an internal reference voltage of 0.5V. It is rec-
ommended to keep the part in shutdown until the input voltage
is 2.5V or higher.
CIRCUIT OPERATION
During the first portion of each switching cycle, the control
block in the LM3678 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
ergy in a magnetic field.
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 device 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.
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 inductor 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
OUT
/L.
IN
–V
OUT
)/L, by storing en-
9
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
The LM3678 uses an internal NFET as a synchronous rectifier
to reduce rectifier forward voltage drop and associated power
loss. Synchronous rectification provides a significant im-
provement in efficiency whenever the output voltage is rela-
tively low compared to the voltage drop across an ordinary
rectifier diode.
CURRENT LIMITING
A current limit feature allows the LM3678 to protect itself and
external components during overload conditions by imple-
menting current limiting with an internal comparator that trips
at 2.15A (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. This allows the inductor current more time to
decay, thereby preventing runaway.
SHUTDOWN MODE
Setting the EN input pin low (<0.4V) places the LM3678 in
shutdown mode. During shutdown the PFET switch, NFET
switch, reference, control and bias circuitry of the LM3678 are
turned off. Setting EN high (>1.0V) enables normal operation.
It is recommended to set EN pin low to turn off the LM3678
during system power up and undervoltage conditions when
the supply is less than 2.5V. Do not leave the EN pin floating.
SOFT START
The LM3678 has a soft-start circuit that limits in-rush current
during start-up. During start-up the switch current limit is in-
creased in steps. Soft start is activated only if EN goes from
logic low to logic high after V
plemented by increasing switch current limit in steps of
250mA, 500mA, 1A and 2A (typical switch current limit). The
start-up time thereby depends on the output capacitor and
load current demanded at start-up.
FIGURE 3. Typical PWM Operation
IN
reaches 2.5V. Soft start is im-
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