ISL97701

FN6474 Rev 1.00

Page 7 of 11

February 22, 2008

Function Overview

The ISL97701 is a high frequency, high efficiency boost

regulator which operates in constant frequency PWM mode.

The boost converter generates a stable, higher output voltage

from a variable, low voltage input source (e.g. Li-ion battery).

The output voltage level is defined from the feedback resistor

network in Equation 1.

The switching frequency is either generated from the fixed

1MHz internal oscillator or provided externally at the

synchronization pin in the range from 600kHz to 1.4MHz. The

compensation network and soft-start functions are built in with

fixed parameters without any need for further external

components.

To stop battery discharge into the output load when disabled,

the inductor is disconnected from the input supply with a low

ON-resistance power switch.

Built-in fault protection monitors inductor current and output

voltage as well as junction temperature in order to interrupt the

high current circuit path through the inductor and diode in the

event of a load failure.

Low logic input thresholds allow the ISL97701 to interface

directly to microcontrollers with lower supply voltage.

Alternatively, the internal pull-up resistors on all logic inputs

provide level shifting when driven from open collector outputs.

Description of Operation

Enable Pin (Active Low) - NEN

If NEN is high, the ISL97701 shuts down all its internal

functions and deactivates its I/So. Only the internal pull-up

resistor at NEN remains active. If NEN is high, the input

disconnect switch between VDD and VDDOUT interrupts the

circuit path from the input voltage VDD through inductor and

current in VDD is typically less than 0.1µA.

When NEN is driven low the ISL97701 begins with the start-up

sequence.

Start-Up Sequence

After pin NEN is pulled low or a restart is triggered from Fault

Control during operation, the ISL97701 goes through a start-up

sequence with the following six states: Soft Inrush -> VDDOUT

Enable -> Soft Boost 25 -> Soft Boost 50 -> Soft Boost 75 ->

Normal.

If the sequence has completed, the ISL97701 stays in the

“Normal” state until NEN is high again or any fault is detected.

SOFT INRUSH: STATE DURATION ~2.048ms

The switch at VDDOUT is configured as current source and

provides a limited current through the inductor to pre-charge

VDDOUT ENABLE: STATE DURATION ~128Μs

The switch at VDDOUT is fully enabled and connects the

inductor to VDD with a low ON-resistance.

SOFT BOOST 25 -> 50 -> 75: STATE DURATION

3x ~2.048ms

The boost regulator begins to switch at LX.

The LX current limit increases in three steps representing 25%,

50% and 75% of its final value.

NORMAL

If no fault was detected Normal state is entered ~8.256ms after

NEN is pulled low.

The LX current limit steps up to 100%.

In all states Fault Control can force the sequence to restart or

even to shutdown (see Table 1).

Fault Control

monitored and can either restart the start-up sequence or in

Pin Descriptions

PIN

NUMBER

PIN NAME

PIN FUNCTION

1

GND

Ground

2

VDDOUT

Protection Switch Output

3

VDD

Supply Input

4

NSYNC

Synchronization Input (Falling Edge)

5

FB

Feedback Input

6

NC

Do Not Connect

7

GND

Ground

8

NEN

Enable Input (Active Low)

Boost Output Voltage

10

LX

Boost FET

+







=

(EQ. 1)

V(NEN)

I(VDD)

SOFT

INRUSH

75

50

25

NORMAL

FIGURE 14. FAULT CONTROL SEQUENCE