MAX1760 Просмотр технического описания (PDF) - Maxim Integrated

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производитель

MAX1760

0.8A, Low-Noise, 1MHz, Step-Up DC-DC Converter

Maxim Integrated 

0.8A, Low-Noise, 1MHz,

Step-Up DC-DC Converter

VIN = 2.4V

33µF

0.22µF

3.3µH

CLK/SEL

ON

LX

MAX1760

POUT

ISET

REF

OUT

PGND GND FB

Figure 3. Connecting Resistors for External Feedback

VOUT

4.7Ω

100µF

0.68µF

R1

R2

( ) R1 = R2

VOUT

VFB

-

1

VFB = 1.24V

0.22µF

REF

RSS

MAX1760

ISET

ILIM = 1.25A

tSS = RSSCSS

CSS

RSS ≥ 470kΩ

Figure 4. Soft-Start with Maximum Switch Limit Current

0.22µF

RSS2

REF

RSS1

MAX1760

ISET

CSS

( ) ILIM = 1.25A

RSS2

RSS1 + RSS2

tSS = (RSS1 II RSS2) CSS

RSS1 ≥ 470kΩ

Figure 5. Soft-Start with Reduced Switch Current Limit

Reference

The MAX1760 has an internal 1.250V ±1% reference.

Connect a 0.22µF ceramic bypass capacitor from REF

to GND within 0.2in (5mm) of the REF pin. REF can

source up to 50µA of external load current.

Design Procedure

Setting the Output Voltages

For a fixed 3.3V output, connect FB to GND. To set

other output voltages between 2.5V and 5.5V, connect

a resistor voltage-divider to FB from OUT to GND

(Figure 3). The input bias current into FB is <20nA,

allowing large-value divider resistors without sacrificing

accuracy. Connect the resistor voltage-divider as close

to the IC as possible, within 0.2in (5mm) of FB. Choose

R2 of 270kΩ or less, then calculate R1 using:

R1

=

R2





VOUT

VFB

− 1

where VFB, the boost-regulator feedback set point, is

1.24V.

Setting the Switch Current Limit

and Soft-Start

The ISET pin adjusts the inductor current limit and

implements soft-start. With ISET connected to REF, the

inductor current limits at 1.25A. With ISET connected to

a resistive divider set from REF to GND, the current limit

is reduced according to:

ILIM

=

1.25A





VISET

1.25V





Implement soft-start by placing a resistor from ISET to

REF and a capacitor from ISET to GND. In shutdown,

ISET is discharged to GND through an on-chip 100kΩ

resistor. At power-up, ISET is 0V and the LX current is

zero. As the capacitor voltage rises, the current

increases and the output voltage rises. The soft-start

_______________________________________________________________________________________ 9

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