EV0034 データシートの表示（PDF） - Monolithic Power Systems

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EV0034

2A, 1.3MHz Step-Up Converter

Monolithic Power Systems 

Monolithic Power Systems

MP1527

2A, 1.3MHz

Step-Up Converter

The fault time-out period is determined by the

equation:

tFAULT = 6*105 * CSS

If multiple MP1527 regulators are used in the

same circuit, the FAULT input/outputs can be

connected together. Should any one regulator

indicate a fault, it pulls all FAULT input/outputs

low, disabling all regulators. This insures that

all outputs are disabled should any one output

detect a fault. Pull-up FAULT to the input

voltage (IN) through a 100KΩ resistor. The

leakage current at FAULT is less than 250nA,

so up to 20 FAULT input/outputs can be

connected together through a single 100KΩ

pull-up resistor. To reduce current draw when

FAULT is active, a higher value pull-up resistor

may be used. Calculate the pull-up resistor

value by the equation:

100kΩ ≤ RPULL-UP ≤ 2MΩ / N

Where N is the number of FAULT input/outputs

connected together.

Setting the Output Voltage

Set the output voltage by selecting the

resistive voltage divider ratio. The voltage

divider drops the output voltage to the 1.22V

feedback threshold voltage. Use 10KΩ for the

low-side resistor of the voltage divider.

Determine the high side resistor by the

equation:

RH = (VOUT - VFB) / (VFB / RL)

where RH is the high-side resistor, RL is the

low-side resistor, VOUT is the output voltage

and VFB is the feedback regulation threshold.

For RL = 10KΩ and VFB = 1.22V, then

RH (KΩ) = 8.20* (VOUT – 1.22V)

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor, while limiting

noise at the input source. A low ESR capacitor

is required to keep the noise at the IC to a

minimum. Ceramic capacitors are preferred,

but tantalum or low-ESR electrolytic capacitors

may also suffice.

Use an input capacitor value greater than

4.7µF. The capacitor can be electrolytic,

tantalum or ceramic. However since it absorbs

the input switching current it requires an

adequate ripple current rating. Use a capacitor

with RMS current rating greater than the

inductor ripple current (see Selecting The

Inductor to determine the inductor ripple

current).

To insure stable operation place the input

capacitor as close to the IC as possible.

Alternately a smaller high quality ceramic

0.1µF capacitor may be placed closer to the IC

with the larger capacitor placed further away. If

using this technique, it is recommended that

the larger capacitor be a tantalum or

electrolytic type. All ceramic capacitors should

be placed close to the MP1527.

Selecting the Output Capacitor

The output capacitor is required to maintain

the DC output voltage. Low ESR capacitors

are preferred to keep the output voltage ripple

to a minimum. The characteristic of the output

capacitor also affects the stability of the

regulation control system. Ceramic, tantalum,

or low ESR electrolytic capacitors are

recommended. In the case of ceramic

capacitors, the impedance of the capacitor at

the switching frequency is dominated by the

capacitance, and so the output voltage ripple is

mostly independent of the ESR. The output

voltage ripple is estimated to be:

VRIPPLE

≈

⎜⎜⎝⎛1-

VIN

VOUT

⎟⎟⎠⎞

× ILOAD

C2 × fSW

Where VRIPPLE is the output ripple voltage, VIN

and VOUT are the DC input and output voltages

respectively, ILOAD is the load current, fSW is the

switching frequency, and C2 is the capacitance

of the output capacitor.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance

MP1527 Rev 1.8_8/31/05

Monolithic Power Systems, Inc.

9

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