lm2738xsdx National Semiconductor Corporation, lm2738xsdx Datasheet - Page 9

• Manufacturer Part Number: lm2738xsdx
• Description: Lm2738 550khz/1.6mhz 1.5a Step-down Dc-dc Switching Regulator
• Manufacturer: National Semiconductor Corporation

5.5V > V
When the LM2738 starts up, internal circuitry from the
BOOST pin supplies a maximum of 20mA to C
current charges C
switch on. The BOOST pin will continue to source current to
C
0.76V.
There are various methods to derive V
1.
2.
3.
4.
In the Simplifed Block Diagram of Figure 1, capacitor
C
NMOS switch. Capacitor C
V
control switch is off (T
V
current in the inductor (L) forward biases the Schottky diode
D1 (V
When the NMOS switch turns on (T
to
forcing V
V
which is approximately
for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately
An alternate method for charging C
the output as shown in Figure 3. 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
a gate drive voltage that is slightly less than V
In applications where both V
5.5V, or less than 3V, C
these voltages. If V
C
age by placing a zener diode D3 in series with D2, as shown
in Figure 4. 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
(V
IN
IN
BOOST
BOOST
BOOST
BOOST
INMAX
. During a normal switching cycle, when the internal NMOS
minus the forward voltage of D2 (V
From the input voltage (3.0V < V
From the output voltage (2.5V < V
From an external distributed voltage rail (2.5V < V
5.5V)
From a shunt or series zener diode
FD1
is then
can be charged from V
– V
until the voltage at the feedback pin is greater than
and diode D2 supply the gate-drive voltage for the
BOOST
). Therefore the voltage stored across C
BOOST
V
BOOST
D3
FIGURE 3. V
) < 5.5V
V
– V
to rise thus reverse biasing D2. The voltage at
BOOST
= 2V
V
SW
BOOST
SW
IN
IN
- V
> 2.5V for best performance.
OFF
= V
BOOST
– (R
SW
2V
and V
V
) (refer to Figure 2), V
to a voltage sufficient to turn the
IN
OUT
IN
BOOST
IN
= V
DSON
– (R
- 0.2V
cannot be charged directly from
- 0.4V
IN
Charges C
IN
IN
OUT
DSON
and V
or V
x I
is charged via diode D2 by
- V
BOOST
L
are greater than 5.5V,
FD2
ON
) – V
IN
OUT
x I
BOOST
OUT
FD2
), the switch pin rises
OUT
< 5.5V)
+ V
L
),
FD2
minus a zener volt-
is to connect D2 to
BOOST
), during which the
< 5.5V)
FD1
are greater than
:
+ V
BOOST
OUT
BOOST
BOOST
FD1
BOOST
BOOST
.
provides
voltage.
equals
EXT
30049108
. This
is
<
9
(V
An alternative method is to place the zener diode D3 in a
shunt configuration as shown in Figure 5. 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 capac-
itance. The 0.1 µF parallel shunt capacitor ensures that the
V
FIGURE 5. Boost Voltage Supplied from the Shunt Zener
Resistor R3 should be chosen to provide enough RMS current
to the zener diode (D3) and to the BOOST pin. A recom-
mended choice for the zener current (I
current I
of the NMOS control switch and varies typically according to
the following formula for the X version:
I
where D is the duty cycle, V
I
anode of the boost diode (D2), and V
voltage across D2. Note that this formula for I
ical current. For the worst case I
by 40%. In that case, the worst case boost current will be
R3 will then be given by
BOOST
BOOST
BOOST
INMIN
FIGURE 4. Zener Reduces Boost Voltage from V
I
can be calculated for the Y version using the following:
BOOST
I
is in milliamps. V
– V
voltage is maintained during this time.
BOOST
BOOST
R3 = (V
D3
) > 2.5V
= 0.56 x (D + 0.54) x (V
= 0.22 x (D + 0.54) x (V
into the BOOST pin supplies the gate current
IN
I
BOOST-MAX
- V
ZENER
ZENER
on V
) / (1.4 x I
ZENER
= 1.4 x I
is the voltage applied to the
IN
BOOST
and V
D2
BOOST
BOOST
ZENER
ZENER
is the average forward
, increase the current
ZENER
D2
+ I
– V
- V
are in volts, and
BOOST
) is 1 mA. The
ZENER
D2
D2
30049109
www.national.com
) mA
) µA
gives typ-
)
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IN