LM2734Y EVAL National Semiconductor, LM2734Y EVAL Datasheet - Page 7

• Manufacturer Part Number: LM2734Y EVAL
• Description: BOARD EVALUATION LM2734Y
• Manufacturer: National Semiconductor

Specifications of LM2734Y EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 1A
• Voltage - Input: 3 ~ 20V
• Regulator Topology: Buck
• Frequency - Switching: 550kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2734
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Block Diagram
Application Information
THEORY OF OPERATION
The LM2734 is a constant frequency PWM buck regulator IC
that delivers a 1A load current. The regulator has a preset
switching frequency of either 550kHz (LM2734Y) or 1.6MHz
(LM2734X). These high frequencies allow the LM2734 to op-
erate with small surface mount capacitors and inductors,
resulting in DC/DC converters that require a minimum amount
of board space. The LM2734 is internally compensated, so it
is simple to use, and requires few external components. The
LM2734 uses current-mode control to regulate the output
voltage.
The following operating description of the LM2734 will refer
to the Simplified Block Diagram
forms in
voltage by switching the internal NMOS control switch at con-
stant frequency and variable duty cycle. A switching cycle
begins at the falling edge of the reset pulse generated by the
internal oscillator. When this pulse goes low, the output con-
trol logic turns on the internal NMOS control switch. During
this on-time, the SW pin voltage (V
mately V
slope. I
generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp
and compared to the error amplifier’s output, which is propor-
tional to the difference between the feedback voltage and
V
put switch turns off until the next switching cycle begins.
During the switch off-time, inductor current discharges
through Schottky diode D1, which forces the SW pin to swing
below ground by the forward voltage (V
REF
. When the PWM comparator output goes high, the out-
L
IN
Figure
is measured by the current-sense amplifier, which
, and the inductor current (I
2. The LM2734 supplies a regulated output
(Figure
SW
L
) increases with a linear
) swings up to approxi-
D
1) and to the wave-
) of the catch diode.
FIGURE 1.
7
The regulator loop adjusts the duty cycle (D) to maintain a
constant output voltage.
BOOST FUNCTION
Capacitor C
erate a voltage V
to the internal NMOS control switch. To properly drive the in-
ternal NMOS switch during its on-time, V
least 1.6V greater than V
ate with this minimum voltage, it may not have sufficient gate
drive to supply large values of output current. Therefore, it is
recommended that V
FIGURE 2. LM2734 Waveforms of SW Pin Voltage and
BOOST
BOOST
and diode D2 in
Inductor Current
BOOST
. V
BOOST
SW
. Although the LM2734 will oper-
be greater than 2.5V above V
- V
SW
Figure 3
is the gate drive voltage
BOOST
are used to gen-
needs to be at
20102306
www.national.com
20102307
SW