Datasheet

0
0
V
IN
V
D
T
ON
t
t
Inductor
Current
D = T
ON
/T
SW
V
SW
T
OFF
T
SW
I
L
I
PK
SW
Voltage
LM2738
SNVS556B –APRIL 2008–REVISED APRIL 2013 www.ti.com
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 operate 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 27) and to
the waveforms in Figure 28. The LM2738 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
SW
) swings up to approximately V
IN
, and
the inductor current (I
L
) increases with a linear slope. I
L
is measured by the current-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 amplifier’s output, which is proportional to the difference between the
feedback voltage and V
REF
. When the PWM 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 forward voltage (V
D
) of the catch diode. The regulator
loop adjusts the duty cycle (D) to maintain a constant output voltage.
Figure 28. LM2738 Waveforms of SW Pin Voltage and Inductor Current
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