LM2734ZEVAL National Semiconductor, LM2734ZEVAL Datasheet - Page 6

• Manufacturer Part Number: LM2734ZEVAL
• Description: BOARD EVALUATION LM2734Z
• Manufacturer: National Semiconductor

Specifications of LM2734ZEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 1A
• Voltage - Input: 3 ~ 20V
• Regulator Topology: Buck
• Frequency - Switching: 3MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2734
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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BOOST FUNCTION
Capacitor C
erate a voltage V
to the internal NMOS control switch. To properly drive the in-
ternal NMOS switch during its on-time, V
least 1.6V greater than V
erate with this minimum voltage, it may not have sufficient
gate drive to supply large values of output current. Therefore,
it is recommended that V
V
maximum operating limit of 5.5V.
5.5V > V
When the LM2734Z 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
SW
BOOST
BOOST
IN
IN
BOOST
. During a normal switching cycle, when the internal NMOS
minus the forward voltage of D2 (V
From the input voltage (V
From the output voltage (V
From an external distributed voltage rail (V
From a shunt or series zener diode
for best efficiency. V
FD1
is then
until the voltage at the feedback pin is greater than
and diode D2 supply the gate-drive current for the
BOOST
BOOST
V
). Therefore the voltage stored across C
BOOST
BOOST
FIGURE 3. V
V
– V
to rise thus reverse biasing D2. The voltage at
BOOST
= 2V
BOOST
V
and diode D2 in Figure 3 are used to gen-
SW
BOOST
SW
IN
- V
> 2.5V for best performance.
OFF
. V
= V
– (R
SW
2V
BOOST
BOOST
) (refer to Figure 2), V
V
SW
BOOST
to a voltage sufficient to turn the
IN
OUT
IN
BOOST
IN
= V
DSON
. Although the LM2734Z will op-
– (R
- 0.2V
- 0.4V
IN
Charges C
- V
OUT
IN
– V
)
be greater than 2.5V above
DSON
x I
is charged via diode D2 by
- V
SW
SW
)
BOOST
L
ON
FD2
) – V
is the gate drive voltage
x I
should not exceed the
BOOST
FD2
), the switch pin rises
+ V
BOOST
L
),
FD2
BOOST
is to connect D2 to
), during which the
FD1
:
+ V
needs to be at
EXT
BOOST
FD1
BOOST
BOOST
)
equals
20130308
. This
is
6
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
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
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:
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 25%. In that case, the worst case boost current will be
R3 will then be given by
For example, let V
= 1mA, and duty cycle D = 50%. Then
BOOST
BOOST
BOOST
FIGURE 4. Zener Reduces Boost Voltage from V
R3 = (10V - 5V) / (1.25 x 4.3mA + 1mA) = 787Ω
is in milliamps. V
voltage is maintained during this time.
can be charged from V
R3 = (V
BOOST
I
BOOST
I
BOOST
into the BOOST pin supplies the gate current
IN
= (0.5 + 0.5) x (5 - 0.7) mA = 4.3mA
I
= (D + 0.5) x (V
BOOST-MAX
- V
IN
(V
(V
ZENER
= 10V, V
IN
INMAX
INMIN
BOOST
and V
ZENER
) / (1.25 x I
– V
– V
= 1.25 x I
ZENER
cannot be charged directly from
ZENER
IN
D3
IN
D3
OUT
is the voltage applied to the
and V
) > 1.6V
or V
) < 5.5V
BOOST
ZENER
and V
= 5V, V
D2
are greater than 5.5V,
OUT
BOOST
BOOST
is the average forward
, increase the current
OUT
– V
ZENER
D2
minus a zener volt-
D2
are greater than
+ I
D2
are in volts, and
BOOST
) mA
) is 1 mA. The
BOOST
OUT
ZENER
BOOST
= 0.7V, I
.
20130309
gives typ-
)
provides
voltage.
ZENER
IN