lm2738xsdx National Semiconductor Corporation, lm2738xsdx Datasheet - Page 8

• Manufacturer Part Number: lm2738xsdx
• Description: Lm2738 550khz/1.6mhz 1.5a Step-down Dc-dc Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Block Diagram
Application Information
THEORY OF OPERATION
The LM2738 is a constant frequency PWM buck regulator IC
that delivers a 1.5A load current. The regulator has a preset
switching frequency of either 550kHz (LM2738Y) or 1.6MHz
(LM2738X). These high frequencies allow the LM2738 to op-
erate with small surface mount capacitors and inductors,
resulting in DC/DC converters that require a minimum amount
of board space. The LM2738 is internally compensated, so it
is simple to use, and requires few external components. The
LM2738 uses current-mode control to regulate the output
voltage.
The following operating description of the LM2738 will refer
to the Simplified Block Diagram (Figure 1) and to the wave-
forms in Figure 2. The LM2738 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
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-
FIGURE 1. Simplified Internal Block Diagram
8
below ground by the forward voltage (V
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 2.5V greater than V
be greater than 2.5V above V
V
FIGURE 2. LM2738 Waveforms of SW Pin Voltage and
SW
should not exceed the maximum operating limit of 5.5V.
BOOST
BOOST
and diode D2 in Figure 3 are used to gen-
Inductor Current
. V
BOOST
SW
. It is recommended that V
SW
- V
for best efficiency. V
SW
is the gate drive voltage
D
BOOST
) of the catch diode.
needs to be at
30049106
30049107
BOOST
BOOST
–