lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 11

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

BOOST FUNCTION
Capacitor C
used to store a voltage V
starts up, an internal LDO charges C
diode, to a voltage sufficient to turn the internal NMOS switch
on. The gate drive voltage supplied to the internal NMOS
switch is V
During a normal switching cycle, when the internal NMOS
control switch is off (t
V
the same time the inductor current (i
catch diode D1 forcing the SW pin to swing below ground by
the forward voltage drop of the catch diode (V
the voltage stored across C
Thus,
When the NMOS switch turns on (t
reverse biasing D1, and forcing V
at V
which is approximately
V
a higher voltage.
LOW INPUT VOLTAGE CONSIDERATIONS
When the input voltage is below 5V and the duty cycle is
greater than 75 percent, the gate drive voltage developed
across C
the NMOS switch. In this case, C
an external Schottky diode attached to a 5V voltage rail, see
Figure 3. This ensures that the gate drive voltage is high
enough for proper operation of the NMOS switch in the triode
region. Maintain V
imum rating.
HIGH OUTPUT VOLTAGE CONSIDERATIONS
When the output voltage is greater than 3.3V, a minimum load
current is needed to charge C
mum load current forward biases the catch diode D1 forcing
the SW pin to swing below ground. This allows C
charge, ensuring that the gate drive voltage is high enough
for proper operation. The minimum load current depends on
many factors including the inductor value.
LDO
BOOST
BOOST
minus the forward voltage of the internal diode (V
V
FIGURE 3. External Diode Charges C
has pulled itself up by its "bootstraps", or boosted to
BOOST
BOOST
is then
BOOST
2
in Figure 1, commonly referred to as C
V
V
= V
BOOST
might not be sufficient for proper operation of
BOOST
- V
V
IN
BOOST
SW
SW
– (R
= V
OFF
- V
V
.
= V
IN
BOOST
- V
SW
DSON
) (refer to Figure 2), V
SW
+ V
IN
SW
BOOST
= V
+ V
– (R
LDO
. When the LM27341/LM27342
x I
BOOST
less than the 6V absolute max-
LDO
LDO
L
- 0.4V
BOOST
DSON
) + V
is
BOOST
ON
- V
- V
, see Figure 4. The mini-
), the switch pin rises to
x I
LDO
D2
BOOST
L
D2
should be charged via
) forward biases the
L
to rise. The voltage
+ V
+ V
),
– V
D1
D1
,via an internal
D2
D1
BOOST
BOOST
). Therefore,
+ V
D1
BOOST
BOOST
30005626
equals
D2
). At
, is
to
11
ENABLE PIN / SHUTDOWN MODE
Connect the EN pin to a voltage source greater than 1.8V to
enable operation of the LM27341/LM27342. Apply a voltage
less than 0.4V to put the part into shutdown mode. In shut-
down mode the quiescent current drops to typically 70 nA.
Switch leakage adds another 40 nA from the input supply. For
proper operation, the LM27341/LM27342 EN pin should nev-
er be left floating, and the voltage should never exceed V
0.3V.
The simplest way to enable the operation of the LM27341/
LM27342 is to connect the EN pin to AVIN which allows self
start-up of the LM27341/LM27342 when the input voltage is
applied.
When the rise time of V
the LM27341/LM27342 this method may result in an over-
shoot in output voltage. In such applications, the EN pin
voltage can be controlled by a separate logic signal, or tied to
a resistor divider, which reaches 1.8V after V
lished (see Figure 5). This will minimize the potential for
output voltage overshoot during a slow V
Use the lowest value of V
calculating the resistor network, to ensure that the 1.8V min-
imum EN threshold is reached.
FIGURE 4. Minimum Load Current for L = 1.5 µH
FIGURE 5. Resistor Divider on EN
IN
is longer than the soft-start time of
IN
, seen in your application when
IN
IN
ramp condition.
30005636
is fully estab-
www.national.com
30005608
IN
+