Rainbow-electronics MAX1765 Bedienungsanleitung PDF herunterladen (Seite 14)

MAX1765

that V

FBL

= 0.5V (above the Dual Mode™ threshold)

when OUTL is regulated, to ensure that the linear regu-

lator is saturated. Another method to configure the

MAX1765 for true shutdown is shown in Figure 6. This

shutdown function is active high and connects to the

gate of a low-impedance PFET and ONB. The PFET

acts like a switch in this situation and disconnects the

input from the load.

Reference

The MAX1765 has an internal 1.25V, 1% reference.

Connect a 0.22µF ceramic bypass capacitor to GND

within 0.2in (5mm) of the REF pin. REF can source up

to 50µA of external load current. Typically connect ISET

to REF to give the MAX1765 full inductor current limit.

Design Procedure

Setting DC-DC Converter Voltage

Set the output voltage between +2.5V and +5.5V by

connecting a resistor voltage-divider from OUT to FB to

GND (Figure 7). Connect the resistor voltage-divider as

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

Choose R2 of 40kΩ or less, then calculate R1 using:

where V

, the boost-regulator feedback set point, is

+1.25V.

For output voltages above 4V, connect a Schottky

diode between LX and POUT to prevent voltage transi-

tion from exceeding the LX voltage rating.

Setting the Linear Regulator Voltage

The LDO regulation voltage can also be set similarly to

the DC-DC converter. Connecting FBL to GND sets the

LDO output to 2.85V. To set other output voltages

between 1.25V and POUT, connect a resistor-divider

from OUTL to FBL to GND (Figure 7). Connect the

resistor voltage-divider as close to the IC as possible,

within 0.2in (5mm) of FBL. The maximum input bias cur-

rent for the FBL input is 50nA. Choose R4 of 40kΩ or

less, then calculate R3 using:

where V

FBL

, the linear regulator feedback set point, is

+1.25V.

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:

Implement soft-start by placing a resistor from ISET to

REF and a capacitor from ISET to GND (Figure 8). In

shutdown, ISET is discharged to GND through an on-

chip 100kΩ resistor. At power-up, ISET is 0V and the

current limit is zero. As the capacitor voltage rises, the

I =

LIM

125

SS SS













OUTL

FBL

34 =













- 1

OUT

12 =













- 1

800mA, Low-Noise, Step-Up DC-DC Converter

with 500mA Linear Regulator

14 ______________________________________________________________________________________

Table 2. Operating Mode Truth Table

OPERATING

MODE

TRACK ONA ONB ONL

LINEAR

REGULATOR

DC-DC

CONVERTER

REF

Shutdown X L H L OFF OFF OFF

HHX X

Track

HXLX

ON ON ON

LHXH

Independent Regulation

LXLH

ON ON ON

LHX L

DC-DC Only

LXLL

OFF ON ON

LDO Only X L H H ON OFF ON

Dual Mode is a trademark of Maxim Integrated Products

1 2 ... 9 10 11 12 13 14 15 16 17 18 19

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