NCP1410

http://onsemi.com

10

Low−Battery Detection

A comparator with 30 mV hysteresis is applied to perform

the low−battery detection function. When pin 2 (LBI) 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 by the

external resistor.

APPLICATIONS INFORMATION

Output Voltage Setting

The output voltage of the converter is determined by the

the relationship is given by:

VOUT + 1.190 V

1

)

RFB1

RFB2

resistors respectively.

Low Battery Detect Level Setting

The Low Battery Detect Voltage of the converter is

determined by the external divider network comprised of

VLB + 1.190 V

1

)

RLB1

RLB2

resistors respectively.

Inductor Selection

The NCP1410 is tested to produce optimum performance

with a 22

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

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 pulsating 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 terminals. 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 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. Schottky Device Between LX and

OUT Pins

NCP1410

OUT

LX

MBR0502

L

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 in the following paragraphs, can be used as guidelines

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

along the path. Feedback signal path must be separated with

the main current path and sensing directly at the anode of the

output capacitor.