Rainbow-electronics MAX1797 Manual do Utilizador descarregar pdf (Página 10)

MAX1795/MAX1796/MAX1797

pate this energy, minimizing the ringing at LX.

Damping LX ringing does not reduce V

OUT

ripple, but

does reduce EMI (Figures 3, 4, and 5).

Setting the Output Voltage

OUT

can be set to 3.3V or 5.0V by connecting the FB

pin to GND (5V) or OUT (3.3V). To adjust the output

voltage, connect a resistive voltage-divider from OUT to

FB to GND (Figure 6). Choose a value less than 250kΩ

for R2.

Use the following equation to calculate R1:

R1 = R2 [(V

OUT

/ V

) - 1]

where V

= +1.245V, and V

OUT

may range from +2V

to +5.5V.

Low-Battery Detection

The MAX1795/MAX1796/MAX1797 each contain an on-

chip comparator for low-battery detection. If the voltage

at LBI is above 0.85V, LBO (an open-drain output)

sinks current to GND. If the voltage at LBI is below

0.85V, LBO goes high impedance. The LBI/LBO func-

tion remains active even when the part is in shutdown.

Connect a resistive voltage-divider to LBI from BATT to

GND. The low-battery monitor threshold is set by two

resistors, R3 and R4 (Figure 6). Since the LBI bias cur-

rent is typically 2nA, large resistor values (R4 up to

250kΩ) can be used to minimize loading of the input

supply.

Calculate R3 using the following equation:

R3 = R4[(V

TRIP

/ 0.85V) - 1]

Low Supply Current, Step-Up DC-DC Converters

with True-Shutdown

10 ______________________________________________________________________________________

Figure 3. Simplified Diagram of Inductor Damping Switch

MAX1795

MAX1796

MAX1797

DAMPING

SWITCH

BATT

200Ω

OUT

22µH

47µF

OUT

2µs/div

1V/div

Figure 4. LX Ringing for Conventional Step-Up Converter

(without Damping Switch)

2µs/div

1V/div

Figure 5. LX Waveform with Damping Switch

Figure 6. Setting an Adjustable Output

GND

LBI

LBO

BATT

OUT

MAX1795

MAX1796

MAX1797

SHDN

47µF

OUTPUT

2V TO 5.5V

LOW-BATTERY

OUTPUT

47µF

1 2 ... 5 6 7 8 9 10 11 12 13

MAX1795/MAX1796/MAX1797 1

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