Rainbow-electronics MAX669 Manual do Utilizador descarregar pdf (Página 16)

MAX668/MAX669

larger or smaller than the output voltage, such as when

converting four NiMH, NiCd, or Alkaline cells to a 5V

output. The SEPIC configuration is often a good choice

for combined step-up/step-down applications.

The N-channel MOSFET (Q1) must be selected to with-

stand a drain-to-source voltage (V

) greater than the

sum of the input and output voltages. The coupling

capacitor (C2) must be a low-ESR type to achieve max-

imum efficiency. C2 must also be able to handle high

ripple currents; ordinary tantalum capacitors should not

be used for high-current designs.

The circuit in Figure 6 provides greater than 1A output

current at 5V when operating with an input voltage from

3V to 25V. Efficiency will typically be between 70% and

85%, depending upon the input voltage and output cur-

rent.

Isolated +5V to +5V Power Supply

The circuit of Figure 7 provides a 5V isolated output at

400mA from a 5V input power supply. Transformer T1

provides electrical isolation for the forward path of the

converter, while the TLV431 shunt regulator and

MOC211 opto-isolator provide an isolated feedback

error voltage for the converter. The output voltage is set

by resistors R2 and R3 such that the mid-point of the

divider is 1.24V (threshold of TLV431). Output voltage

can be adjusted from 1.24V to 6V by selecting the

proper ratio for R2 and R3. For output voltages greater

than 6V, substitute the TL431 for the TLV431, and use

2.5V as the voltage at the midpoint of the voltage-

divider.

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

16 ______________________________________________________________________________________

100k

0.02Ω

75k

25k

520pF

3V to 25V

30V

FDS6680

CTX5-4

MAX668

SHDNV

910

LDO

FREQ

D1: MBR5340T3, 3A, 40V SCHOTTKY DIODE

R4: WSL-2512-R020F, 0.02Ω

C3: AVX TPSZ686M020R0150, 68µF, 150mΩ ESR

REF

EXT

CS+

PGNDGND

1µF

22µF x 3

@ 35V

68µF x 3

OUT

5V @ 1A

4.9µH

10µF @ 35V

40V

0.22µF

Figure 6. MAX668 in SEPIC Configuration

1 2 ... 11 12 13 14 15 16 17 18

MAX668/MAX669 1
Controllers in µMAX 1

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