lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 10

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

www.national.com
Block Diagram
Application Information
THEORY OF OPERATION
The LM27341/LM27342 is a constant-frequency, peak cur-
rent-mode PWM buck regulator IC that delivers a 1.5 or 2A
load current. The regulator has a preset switching frequency
of 2 MHz. This high frequency allows the LM27341/LM27342
to operate with small surface mount capacitors and inductors,
resulting in a DC-DC converter that requires a minimum
amount of board space. The LM27341/LM27342 is internally
compensated, which reduces design time, and requires few
external components.
The following operating description of the LM27341/LM27342
will refer to the Block Diagram (Figure 1) and to the waveforms
in Figure 2. The LM27341/LM27342 supplies a regulated out-
put voltage by switching the internal NMOS switch at a con-
stant frequency and varying the 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 switch. During this on-
time, the SW pin voltage (V
V
The current-sense amplifier measures i
output proportional to the switch current typically called the
sense signal. The sense signal is summed with the regulator’s
corrective ramp and compared to the error amplifier’s output,
which is proportional to the difference between the feedback
IN
, and the inductor current (i
SW
L
) increases with a linear slope.
) swings up to approximately
L
, which generates an
FIGURE 1.
10
voltage (V
parator goes high, the switch turns off until the next switching
cycle begins. During the switch off-time (t
rent discharges through the catch diode D1, which forces the
SW pin (V
(V
cycle (D) to maintain a constant output voltage.
D1
FIGURE 2. LM27341/LM27342 Waveforms of SW Pin
) of the catch diode. The regulator loop adjusts the duty
FB
SW
) and V
) to swing below ground by the forward voltage
Voltage and Inductor Current
REF
. When the output of the PWM com-
OFF
), inductor cur-
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