SFTB NSC [National Semiconductor], SFTB Datasheet - Page 5

• Manufacturer Part Number: SFTB
• Description: Thin SOT23 1A Load Step-Down DC-DC Regulator
• Manufacturer: NSC [National Semiconductor]

Block Diagram
Application Information
THEORY OF OPERATION
The LM2734Z is a constant frequency PWM buck regulator
IC that delivers a 1A load current. The regulator has a preset
switching frequency of 3MHz. This high frequency allows the
LM2734Z to operate with small surface mount capacitors
and inductors, resulting in a DC/DC converter that requires a
minimum amount of board space. The LM2734Z is internally
compensated, so it is simple to use, and requires few exter-
nal components. The LM2734Z uses current-mode control to
regulate the output voltage.
The following operating description of the LM2734Z will refer
to the Simplified Block Diagram (Figure 1) and to the wave-
forms in Figure 2. The LM2734Z supplies a regulated output
voltage by switching the internal NMOS control switch at
constant 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 control logic turns on the internal NMOS control
switch. During this on-time, the SW pin voltage (V
up to approximately V
creases with a linear slope. I
sense amplifier, which generates an output proportional to
the switch current. The sense signal is summed with the
regulator’s corrective ramp and compared to the error am-
plifier’s output, which is proportional to the difference be-
tween the feedback voltage and V
comparator output goes high, the output 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
IN
, and the inductor current (I
L
is measured by the current-
REF
. When the PWM
SW
) swings
L
) in-
FIGURE 1.
5
forward voltage (V
adjusts the duty cycle (D) to maintain a constant output
voltage.
BOOST FUNCTION
Capacitor C
generate a voltage V
voltage to the internal NMOS control switch. To properly
drive the internal NMOS switch during its on-time, V
needs to be at least 1.6V greater than V
LM2734Z will operate with this minimum voltage, it may not
have sufficient gate drive to supply large values of output
FIGURE 2. LM2734Z Waveforms of SW Pin Voltage and
BOOST
D
and diode D2 in Figure 3 are used to
) of the catch diode. The regulator loop
Inductor Current
BOOST
. V
BOOST
- V
SW
SW
is the gate drive
. Although the
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BOOST