MAX1642 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1642

High-Efficiency, Step-Up DC-DC Converters for 1V Inputs

Maxim Integrated 

High-Efficiency, Step-Up

DC-DC Converters for 1V Inputs

BATTLO (MAX1643)

The MAX1643 contains an on-chip comparator for low-

battery detection. If the voltage at BATT drops below

1V, BATTLO sinks current. BATTLO is an open-drain

output. In combination with PFI/PFO, this allows moni-

toring of both the input and output voltages.

Reverse-Battery Protection

The MAX1642/MAX1643 can sustain/survive single-cell

battery reversal up to the package power-dissipation

limit. An internal 5Ω resistor in series with a diode limits

reverse current to less than 220mA, which prevents dam-

age to the MAX1642/MAX1643. Prolonged operation

above 220mA reverse-battery current can degrade the

devices’ performance.

________________Design Information

Output Voltage Selection

The MAX1642/MAX1643 operate with a 3.3V ±4% or

adjustable output. To select fixed-voltage operation, con-

nect FB to GND. For an adjustable output between 2V

and 5.2V, connect FB to a resistor voltage divider

between OUT and GND (Figure 4). FB regulates to 1.23V.

Since FB leakage is 10nA max, select feedback resistor

R2 in the 100kΩ to 1MΩ range. R1 is given by:

R1

=

R2





VOUT

VREF

- 1

where VREF = 1.23V.

Power-Fail Detection

The MAX1642/MAX1643 have an on-chip comparator for

power-fail detection. This comparator can detect loss of

power at the input or output. If the voltage at PFI falls

below 614mV, the PFO output sinks current to GND.

Hysteresis at the power-fail input is 1%. The power-fail

monitor’s threshold is set by two resistors: R3 and R4

(Figure 5). Set the threshold using the following equation:

R3

=

R4





VTH

VPFI

- 1

where VTH is the desired threshold of the power-fail

detector, and VPFI is the 614mV reference of the power-

fail comparator. Since PFI leakage is 10nA max, select

feedback resistor R4 in the 100kΩ to 1MΩ range.

Low-Battery Start-Up

The MAX1642/MAX1643 are bootstrapped circuits with

a low-voltage start-up oscillator. They can start under

low-load conditions at lower battery voltages than at full

load. Once started, the output can maintain the load as

0.88V to 1.65V INPUT

22µF

0.1µF

100µH, 350mA

BATT

LX

PFI

OUT

OUT

MAX1642

0.1µF

PF0

SHDN

GND

FB

3.3VOUT

22µF

Figure 2. MAX1642 3.3V Standard Application Circuit

0.88V to 1.65V INPUT 100µH, 350mA

22µF

0.1µF

BATT

LX

OUT

OUT

MAX1643

PFI

BATTLO

PFO

GND

FB

3.3VOUT

0.1µF 22µF

Figure 3. MAX1643 3.3V Standard Application Circuit

the battery voltage decreases below the start-up volt-

age (see Typical Operating Characteristics).

Inductor Selection

A 100µH inductor is recommended for most appli-

cations. The use of lower inductor values (down to

68µH) increases maximum output current. Higher val-

ues (up to 220µH) reduce peak inductor current and

consequent ripple and noise. The inductor’s saturation-

current rating must exceed the peak current limit syn-

thesized by the MAX1642/MAX1643’s timing

algorithms:

IPEAK

=

KMAX

LMIN

where KMAX = 35V-µs. The maximum recommended

IPEAK is 350mA. For best efficiency, inductor series

resistance should be less than 1Ω.

8 _______________________________________________________________________________________

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