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
LM2734
www.ti.com SNVS288I –SEPTEMBER 2004–REVISED APRIIL 2013
APPLICATION INFORMATION
THEORY OF OPERATION
The LM2734 is a constant frequency PWM buck regulator IC that delivers a 1A load current. The regulator has a
preset switching frequency of either 550kHz (LM2734Y) or 1.6MHz (LM2734X). These high frequencies allow the
LM2734 to operate with small surface mount capacitors and inductors, resulting in DC/DC converters that require
a minimum amount of board space. The LM2734 is internally compensated, so it is simple to use, and requires
few external components. The LM2734 uses current-mode control to regulate the output voltage.
The following operating description of the LM2734 will refer to the Simplified Block Diagram (Figure 22) and to
the waveforms in Figure 23. The LM2734 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 23. LM2734 Waveforms of SW Pin Voltage and Inductor Current
BOOST FUNCTION
Capacitor C
BOOST
and diode D2 in Figure 24 are used to generate a voltage V
BOOST
. V
BOOST
- V
SW
is the gate
drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time,
V
BOOST
needs to be at least 1.6V greater than V
SW
. Although the LM2734 will operate with this minimum voltage,
it may not have sufficient gate drive to supply large values of output current. Therefore, it is recommended that
V
BOOST
be greater than 2.5V above V
SW
for best efficiency. V
BOOST
– V
SW
should not exceed the maximum
operating limit of 5.5V.
5.5V > V
BOOST
– V
SW
> 2.5V for best performance.
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