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

with 500mA Linear Regulator

_______________________________________________________________________________________

9

PIN

NAME

FUNCTION

1

FBL

Low-Dropout Linear Regulator Dual-Mode Feedback Input. Connect FBL to ground for 2.85V nominal

output voltage. Connect FBL to a resistor-divider from OUTL to ground for an adjustable output voltage.

FBL regulates to 1.25V.

2

ISET

Set N-Channel Current Limit. For maximum current limit, connect ISET to REF. To reduce current limit,

use a resistor-divider from REF to GND. If soft-start is desired, a capacitor can be added from ISET to

Ω switchable

resistor discharges ISET to GND.

3

REF

1.25V Reference Output. Connect a 0.22µF bypass capacitor to GND; 50µA of external load current is

allowed. The reference is enabled if ONA = HI, ONB = LO, or ONL = HI.

4

GND

Ground. Connect to PGND with short trace.

5FB

Boost Converter Feedback Input. Connect a resistor-divider between OUT and GND to set the output

voltage in the range of 2.5V to 5V. In track mode, FB is disabled after OUTL is in regulation.

6

OUT

Boost Converter IC power is derived from OUT. Connect OUT to POUT through a 4.7

Ω resistor and

bypass to GND with a 0.68µF capacitor.

7

ONA

ON Input. When high, the DC-DC is operational (Table 2).

8

CLK/SEL

CLOCK Input for the DC-DC Converter. Also serves to program operating mode of switch as follows:

CLK/SEL = LOW: Normal mode. Operates at a fixed frequency, automatically switching to low-power

(SKIP) mode when the load is minimized.

CLK/SEL = HI: Forced PWM mode. Operates in low-noise, constant-frequency mode at all loads.

CLK/SEL = Clocked: Synchronized forced PWM mode. The internal oscillator is synchronized to an

external clock in the 500kHz to 1200kHz frequency range.

Pin Description

0

0.4

0.2

0.8

0.6

1.2

1.0

1.4

0

0.4

0.6

0.2

0.8

1.0

1.2

1.4

Typical Operating Characteristics (continued)

LIGHT-LOAD SWITCHING WAVEFORMS

MAX1765 toc17

A: LX NODE, 5V/div

B: INDUCTOR CURRENT, 200mA/div, AC-COUPLED

C: OUTPUT RIPPLE, 20mV/div, AC-COUPLED

A

B

C

500ns/div

450

0.1

1

10

LINEAR-REGULATOR OUTPUT NOISE

50

0

FREQUENCY (MHz)

150

250

350