lm2735xsdx National Semiconductor Corporation, lm2735xsdx Datasheet - Page 8

• Manufacturer Part Number: lm2735xsdx
• Description: 520khz/1.6mhz - Space-efficient Boost And Sepic Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Simplified Internal Block Diagram
Application Information
THEORY OF OPERATION
The LM2735 is a constant frequency PWM boost regulator IC
that delivers a minimum of 2.1A peak switch current. The reg-
ulator has a preset switching frequency of either 520 kHz or
1.60 MHz. This high frequency allows the LM2735 to operate
with small surface mount capacitors and inductors, resulting
in a DC/DC converter that requires a minimum amount of
board space. The LM2735 is internally compensated, so it is
simple to use, and requires few external components. The
LM2735 uses current-mode control to regulate the output
voltage. The following operating description of the LM2735
will refer to the Simplified Block Diagram (Figure 1) the sim-
plified schematic (Figure 2), and its associated waveforms
(Figure 3). The LM2735 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
GND, and the inductor current (I
slope. I
generates an output proportional to the switch current. The
sensed 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
REF
. When the PWM comparator output goes high, the out-
L
is measured by the current sense amplifier, which
SW
) decreases to approximately
L
) increases with a linear
FIGURE 1. Simplified Block Diagram
8
put switch turns off until the next switching cycle begins.
During the switch off-time, inductor current discharges
through diode D1, which forces the SW pin to swing to the
output voltage plus the forward voltage (V
regulator loop adjusts the duty cycle (D) to maintain a con-
stant output voltage .
FIGURE 2. Simplified Schematic
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D
) of the diode. The
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