NCP1411
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10
pin 1 voltage raised higher than 0.6 V, the IC will be enabled.
The internal circuit will only consume 9.0 mA current
typically from the OUT pin. In order to ensure proper
startup, a timing capacitor C
EN
as shown in Figure 1 is
required to provide the reset pulse during batteries are
plugged in. The product of R
LB1
and C
EN
must be larger than
28 msec.
Low−Battery Detection
A comparator with 30 mV hysteresis is applied to perform
the low−battery detection function. When pin 2 (LBI/EN) is
at a voltage, which can be defined by a resistor divider from
the battery voltage, lower than the internal reference
voltage, 1.190 V, the comparator output will cause a 50 Ohm
low side switch to be turned ON. It will pull down the
voltage at pin 3 (LBO) which has a hundreds kilo−Ohm of
pull−high resistance. If the pin 2 voltage is higher than
1.190 V +30 mV, the comparator output will cause the
50 Ohm low side switch to be turned OFF, pin 3 will become
high impedance, and its voltage will be pulled high.
APPLICATIONS INFORMATION
Output Voltage Setting
The output voltage of the converter is determined by the
external feedback network comprised of R
FB1
and R
FB2
and
the relationship is given by:
V
OUT
+ 1.190 V
ǒ
1 )
R
FB1
R
FB2
Ǔ
where R
FB1
and R
FB2
are the upper and lower feedback
resistors respectively.
Low Battery Detect Level Setting
The Low Battery Detect Voltage of the converter is
determined by the external divider network comprised of
R
LB1
and R
LB2
and the relationship is given by:
V
LB
+ 1.190 V
ǒ
1 )
R
LB1
R
LB2
Ǔ
where R
LB1
and R
LB2
are the upper and lower divider
resistors respectively.
Inductor Selection
The NCP1411 is tested to produce optimum performance
with a 22 mH inductor at V
IN
= 3.0 V, V
OUT
= 3.3 V
supplying output current up to 250 mA. For other
input/output requirements, inductance in the range 10 mH to
47 mH can be used according to end application
specifications. Selecting an inductor is a compromise
between output current capability and tolerable output
voltage ripple. Of course, the first thing we need to obey is
to keep the peak inductor current below its saturation limit
at maximum current and the I
LIM
of the device. In NCP1411,
I
LIM
is set at 1.0 A. As a rule of thumb, low inductance values
supply higher output current, but also increase the ripple at
output and reducing efficiency, on the other hand, high
inductance values can improve output ripple and efficiency,
however it also limit the output current capability at the same
time. One other parameter of the inductor is its DC
resistance, this resistance can introduce unwanted power
loss and hence reduce overall efficiency, the basic rule is
selecting an inductor with lowest DC resistance within the
board space limitation of the end application.
Capacitors Selection
In all switching mode boost converter applications, both
the input and output terminals sees impulsive
voltage/current waveforms. The currents flowing into and
out of the capacitors multiplying with the Equivalent Series
Resistance (ESR) of the capacitor producing ripple voltage
at the terminals. During the syn−rect switch off cycle, the
charges stored in the output capacitor is used to sustain the
output load current. Load current at this period and the ESR
combined and reflected as ripple at the output terminal. For
all cases, the lower the capacitor ESR, the lower the ripple
voltage at output. As a general guide line, low ESR
capacitors should be used. Ceramic capacitors have the
lowest ESR, but low ESR tantalum capacitors can also be
used as a cost effective substitute.
Optional Startup Schottky Diode for Low Battery
Voltage
In general operation, no external schottky diode is
required, however, in case you are intended to operate the
device close to 1.0 V level, a schottky diode connected
between the LX and OUT pins as shown in Figure 27 can
help during startup of the converter. The effect of the
additional schottky was shown in Figure 8.
Figure 27. PCB Layout Recommendations
OUT
L
X
NCP1411
C
OUT
L
MBR0502
V
OUT
PCB Layout Recommendations
Good PCB layout plays an important role in switching
mode power conversion. Careful PCB layout can help to
minimize ground bounce, EMI noise and unwanted
feedback that can affect the performance of the converter.
Hints suggested in below can be used as a guide line in most
situations.
Grounding
Star−ground connection should be used to connect the
output power return ground, the input power return ground
and the device power ground together at one point. All high
current running paths must be thick enough for current
flowing through and producing insignificant voltage drop
