MAX1678EUA 查看數據表（PDF） - Maxim Integrated

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MAX1678EUA

1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter

Maxim Integrated 

1-Cell to 2-Cell, Low-Noise,

High-Efficiency, Step-Up DC-DC Converter

Applications Information

Output Voltage Selection

The MAX1678 operates with a fixed 3.3V or adjustable

output. To select fixed-voltage operation, connect FB to

GND (Figure 7). For an adjustable output between 2V

and 5.5V, connect FB to a resistor voltage-divider

between OUT and GND (Figure 8). 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

x





VOUT

VREF

−



1

where VREF = 1.23V.

Maximum Output Current

and Inductor Selection

The MAX1678 is designed to work well with a 47µH

inductor in most low-power applications. 47µH is a suf-

ficiently low value to allow the use of a small surface-

mount coil, but large enough to maintain low ripple. The

Typical Operating Characteristics section shows perfor-

mance curves with several 47µH and 22µH coils. Low

inductance values supply higher output current but

also increase ripple and reduce efficiency. Note that

values below 22µH are not recommended due to

MAX1678 switch limitations. Higher inductor values

reduce peak inductor current (and consequent ripple

and noise) and improve efficiency, but also limit output

current.

The relationship between current and inductor value is

approximately:

IOUT(MAX)

=

Mx

1

2

x

K

L

x

VBATT

VOUT

where M is an empirical factor that takes into account

losses in the MAX1678 internal switches and in the

inductor resistance. K is the V-µs factor that governs

the inductor charge time. Nominally, M = 0.9 and

K = 8V-µs. M should be further reduced by 0.1 for each

ohm of inductor resistance.

The inductor’s saturation-current rating must exceed

the worst-case peak current limit set by the MAX1678’s

timing algorithm:

IPEAK

=

KMAX

L

where KMAX = 11.2V-µs. It is usually acceptable to

exceed most coil saturation-current ratings by 20% with

no ill effects; however, the maximum recommended IPEAK

for the MAX1678 internal switches is 550mA, so inductor

values below 22µH are not recommended. For optimum

efficiency, inductor series resistance should be less than

150mV/IPEAK. Table 1 lists suggested inductors and sup-

pliers.

Table 1. Suggested Inductors and Suppliers

PIN

INDUCTOR

PHONE

Coilcraft

DS1608C-223,

DS1608C-473

(847) 639-6400

Murata

LQH4N470K,

LQH3C470K

(814) 237-1431

Sumida

CD43-220,

CD43-470

(847) 956-0666

TDK

NLC453232T-220K,

NLC453232T-470K

(847) 390-4373

INPUT

0.87V TO VOUT

C1

R3

10µF

R4

L1

47µH, 200mA

BATT

LX

PFI

OUT

OUT

MAX1678

R5

PFO

SHDN

GND

FB

3.3VOUT

C2

10µF

INPUT

0.87V TO VOUT

C1

10µF

R3

R4

L1

47µH

BATT

LX

PFI

OUT

OUT

MAX1678

R5

PFO

FB

SHDN

GND

VOUT = 2V

TO 5.5V

R1

C2

R2

Figure 7. 3.3V Standard Application Circuit

Figure 8. Adjustable Output Circuit

10 ______________________________________________________________________________________

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