Datasheet
R
SENSE
PWM Logic
Driver
+
-
Switch
0.15:
FB
Error Amplifier
SYNC
Freq. Foldback Amplifier
-
+
V
REF
1.0V
+
-
1.13V
-
+
OVP Comparator
+
-
-
+
0.53V
+
-
Thermal
Shutdown
Current
Limit
LDO
EN
PVIN
AVIN
BOOST
SW
L
C3
C2
V
OUT
GND
Under
Voltage
Lockout
Corrective
Ramp
+
+
I
SENSE
Reset
Pulse
PWM
Comparator
Error
Signal
Oscillator
Internal
Compensation
Soft Start
+
-
+
Current Sense
Amplifier
D2
D1
i
L
R1
R2
LM27341, LM27342, LM27341-Q1, LM27342-Q1
SNVS497E –NOVEMBER 2008–REVISED APRIL 2013
www.ti.com
Block Diagram
Figure 31.
APPLICATION INFORMATION
THEORY OF OPERATION
The LM27341/LM27342 is a constant-frequency, peak current-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 31) and to
the waveforms in Figure 32. The LM27341/LM27342 supplies a regulated output voltage by switching the internal
NMOS switch at a constant 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 control logic turns on
the internal NMOS switch. During this on-time, the SW pin voltage (V
SW
) swings up to approximately V
IN
, and the
inductor current (i
L
) increases with a linear slope. The current-sense amplifier measures i
L
, which generates an
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 voltage (V
FB
) and V
REF
. When the output of the PWM comparator goes high, the switch
turns off until the next switching cycle begins. During the switch off-time (t
OFF
), inductor current discharges
through the catch diode D1, which forces the SW pin (V
SW
) to swing below ground by the forward voltage (V
D1
)
of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage.
10 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM27341 LM27342 LM27341-Q1 LM27342-Q1