LM2734
Application Information
(Continued)
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.
shown in
When using a series zener diode from the
input, ensure that the regulation of the input supply doesn’t
create a voltage that falls outside the recommended V
BOOST
voltage.
(V
INMAX
– V
D3
)
<
5.5V
(V
INMIN
– V
D3
)
>
1.6V
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20102309
FIGURE 3. V
OUT
Charges C
BOOST
When the LM2734 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
In the Simplifed Block Diagram of
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
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
When the NMOS switch turns on (T
ON
), the switch pin rises
to
V
SW
= V
IN
– (R
DSON
x I
L
),
forcing V
BOOST
to rise thus reverse biasing D2. The voltage
at V
BOOST
is then
V
BOOST
= 2V
IN
– (R
DSON
x I
L
) – V
FD2
+ V
FD1
which is approximately
2V
IN
- 0.4V
for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately
V
IN
- 0.2V
An alternate method for charging C
BOOST
is to connect D2 to
the output as shown in
The output voltage should
be between 2.5V and 5.5V, so that proper gate voltage will
be applied to the internal switch. In this circuit, C
BOOST
provides a gate drive voltage that is slightly less than V
OUT
.
In applications where both V
IN
and V
OUT
are greater than
5.5V, or less than 3V, C
BOOST
cannot be charged directly
from these voltages. If V
IN
and V
OUT
are greater than 5.5V,
C
BOOST
can be charged from V
IN
or V
OUT
minus a zener
voltage by placing a zener diode D3 in series with D2, as
www.national.com
8
FIGURE 4. Zener Reduces Boost Voltage from V
IN
An alternative method is to place the zener diode D3 in a
shunt configuration as shown in
A small 350mW to
500mW 5.1V zener in a SOT-23 or SOD package can be
used for this purpose. A small ceramic capacitor such as a
6.3V, 0.1µF capacitor (C4) should be placed in parallel with
the zener diode. When the internal NMOS switch turns on, a
pulse of current is drawn to charge the internal NMOS gate
capacitance. The 0.1 µF parallel shunt capacitor ensures
that the V
BOOST
voltage is maintained during this time.
Resistor R3 should be chosen to provide enough RMS cur-
rent to the zener diode (D3) and to the BOOST pin. A
recommended choice for the zener current (I
ZENER
) is 1 mA.
The current I
BOOST
into the BOOST pin supplies the gate
current of the NMOS control switch and varies typically
according to the following formula:
I
BOOST
= 0.56 x (D + 0.54) x (V
ZENER
– V
D2
) mA
where D is the duty cycle, V
ZENER
and V
D2
are in volts, and
I
BOOST
is in milliamps. V
ZENER
is the voltage applied to the
anode of the boost diode (D2), and V
D2
is the average
forward voltage across D2. Note that this formula for I
BOOST
gives typical current. For the worst case I
BOOST
, increase the
current by 40%. In that case, the worst case boost current
will be
I
BOOST-MAX
= 1.4 x I
BOOST
R3 will then be given by
R3 = (V
IN
- V
ZENER
) / (1.4 x I
BOOST
+ I
ZENER
)
For example, let V
IN
= 10V, V
ZENER
= 5V, V
D2
= 0.7V, I
ZENER
= 1mA, and duty cycle D = 50%. Then
I
BOOST
= 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA
R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11kΩ
NSC [ NATIONAL SEMICONDUCTOR ]
