LM2734YMKX National Semiconductor, LM2734YMKX Datasheet - Page 7

• Manufacturer Part Number: LM2734YMKX
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,BICMOS,TSOP,6PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2734YMKX

• Rohs Compliant: NO

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 (Figure 1) and to the wave-
forms in Figure 2. The LM2734 supplies a regulated output
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
is measured by the current-sense amplifier, which
, and the inductor current (I
SW
L
) increases with a linear
) swings up to approxi-
D
) 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 Figure 3 are used to gen-
Inductor Current
BOOST
. V
BOOST
SW
. Although the LM2734 will oper-
be greater than 2.5V above V
- V
SW
is the gate drive voltage
BOOST
needs to be at
20102306
www.national.com
20102307
SW