Datasheet

LM2734
BOOST
SW
GND
L
D1
D2
C
OUT
C
BOOST
V
OUT
C
IN
V
IN
V
IN
V
BOOST
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
LM2734Z
www.ti.com SNVS334E –JANUARY 2005–REVISED APRIL 2013
Figure 9. LM2734Z Waveforms of SW Pin Voltage and Inductor Current
BOOST FUNCTION
Capacitor C
BOOST
and diode D2 in Figure 10 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 LM2734Z 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.
Figure 10. V
OUT
Charges C
BOOST
When the LM2734Z starts up, internal circuitry from the BOOST pin supplies a maximum of 20mA to C
BOOST
.
This current charges C
BOOST
to a voltage sufficient to turn the switch on. The BOOST pin will continue to source
current to C
BOOST
until the voltage at the feedback pin is greater than 0.76V.
There are various methods to derive V
BOOST
:
1. From the input voltage (V
IN
)
2. From the output voltage (V
OUT
)
3. From an external distributed voltage rail (V
EXT
)
4. From a shunt or series zener diode
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