Datasheet
0
0
V
IN
-V
D1
t
ON
t
t
Inductor Current
D = t
ON
/T
SW
V
SW
t
OFF
T
SW
i
L
SW Voltage
'i
L
I
OUT
I
LPK
LM27341, LM27342, LM27341-Q1, LM27342-Q1
www.ti.com
SNVS497E –NOVEMBER 2008–REVISED APRIL 2013
Figure 32. LM27341/LM27342 Waveforms of SW Pin Voltage and Inductor Current
BOOST FUNCTION
Capacitor C
2
in Figure 31, commonly referred to as C
BOOST
, is used to store a voltage V
BOOST
. When the
LM27341/LM27342 starts up, an internal LDO charges C
BOOST
,via an internal diode, to a voltage sufficient to
turn the internal NMOS switch on. The gate drive voltage supplied to the internal NMOS switch is V
BOOST
- V
SW
.
During a normal switching cycle, when the internal NMOS control switch is off (t
OFF
) (refer to Figure 32), V
BOOST
equals V
LDO
minus the forward voltage of the internal diode (V
D2
). At the same time the inductor current (i
L
)
forward biases the catch diode D1 forcing the SW pin to swing below ground by the forward voltage drop of the
catch diode (V
D1
). Therefore, the voltage stored across C
BOOST
is
V
BOOST
- V
SW
= V
LDO
- V
D2
+ V
D1
(1)
Thus,
V
BOOST
= V
SW
+ V
LDO
- V
D2
+ V
D1
(2)
When the NMOS switch turns on (t
ON
), the switch pin rises to
V
SW
= V
IN
– (R
DSON
x I
L
), (3)
reverse biasing D1, and forcing V
BOOST
to rise. The voltage at V
BOOST
is then
V
BOOST
= V
IN
– (R
DSON
x I
L
) + V
LDO
– V
D2
+ V
D1
(4)
which is approximately
V
IN
+ V
LDO
- 0.4V (5)
V
BOOST
has pulled itself up by its "bootstraps", or boosted to 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
BOOST
might not be sufficient for proper operation of the NMOS switch. In this case, C
BOOST
should be charged via an external Schottky diode attached to a 5V voltage rail, see Figure 33. This ensures that
the gate drive voltage is high enough for proper operation of the NMOS switch in the triode region. Maintain
V
BOOST
- V
SW
less than the 6V absolute maximum rating.
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