Datasheet
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
LM2736
SNVS316F –SEPTEMBER 2004–REVISED APRIL 2013
www.ti.com
Figure 20. LM2736 Waveforms of SW Pin Voltage and Inductor Current
BOOST FUNCTION
Capacitor C
BOOST
and diode D2 in Figure 21 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 LM2736 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 21. V
OUT
Charges C
BOOST
When the LM2736 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 1.18V.
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
In the Simplifed Block Diagram of Figure 19, capacitor C
BOOST
and diode D2 supply the gate-drive current for the
NMOS switch. Capacitor C
BOOST
is charged via diode D2 by V
IN
. During a normal switching cycle, when the
internal NMOS control switch is off (T
OFF
) (refer to Figure 20), V
BOOST
equals V
IN
minus the forward voltage of D2
(V
FD2
), during which the current in the inductor (L) forward biases the Schottky diode D1 (V
FD1
). Therefore the
voltage stored across C
BOOST
is
V
BOOST
- V
SW
= V
IN
- V
FD2
+ V
FD1
(1)
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