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NCP1411

Sync-Rect PFM Step-Up DC-DC Converter with Low-Battery Detector and Ring-Killer

ON Semiconductor 

NCP1411

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 CEN as shown in Figure 1 is

required to provide the reset pulse during batteries are

plugged in. The product of RLB1 and CEN 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 RFB1 and RFB2 and

the relationship is given by:

ǒ Ǔ VOUT + 1.190 V

1

)

RFB1

RFB2

where RFB1 and RFB2 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

RLB1 and RLB2 and the relationship is given by:

ǒ Ǔ VLB + 1.190 V

1

)

RLB1

RLB2

where RLB1 and RLB2 are the upper and lower divider

resistors respectively.

Inductor Selection

The NCP1411 is tested to produce optimum performance

with a 22 mH inductor at VIN = 3.0 V, VOUT = 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 ILIM of the device. In NCP1411,

ILIM 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.

MBR0502

L

VOUT

OUT

NCP1411

LX

COUT

Figure 27. PCB Layout Recommendations

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

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