ISL6118

FN9008 Rev 2.00

Page 9 of 10

March 2004

Implementing Autoreset on the ISL6118H

Hot Swap Controllers

Abstract

In applications where the cost, complexity or requirement for a

system controller is avoided and an autonomous power control

function is desired, a device that can monitor and protect

against excessive current failures is needed. This shows how

to implement such an autonomous controller using the

ISL6118HIB. This application works only with the ‘H’ version of

these devices. The ‘H’ version refers to the enable function

being asserted upon a high input.

Introduction

The ISL6118, ISL6119 and ISL6121 are all 2.5V to 5V power

supply controllers, each having a different level of current

regulation (CR). The ISL6118 and ISL6119 have 2 independent

controllers with CR levels of 0.6A and 1.0A respectively

whereas the ISL6121 is a single supply controller with a 2A CR

level. Each of these devices features integrated power

switch(es) for power control. Each switch is driven by a

constant current source giving a controlled ramp up of the

output voltage. This provides a soft start turn-on eliminating

bus voltage drooping caused by in-rush current while

charging heavy load capacitances. The independent enabling

inputs and fault reporting outputs for each channel are

available and necessary for the autonomous autoreset

application.

The undervoltage (UV) feature prevents turn-on of the outputs

unless the ENABLE pin and VIN are >2.5V. During initial turn-

on the ISL6118 prevents fault reporting by blanking the fault

signal. Rising and falling outputs are current-limited voltage

ramps so that both the inrush current and voltage slew rate are

limited, independent of load. This reduces supply droop due to

surge and eliminates the need for external EMI filters. During

operation, once an OC condition is detected the appropriate

output is current limited to the appropriate level for 10ms to

allow transient conditions to pass. If still in current limit after the

current limit period has elapsed, the output is latched off and

the fault is reported by pulling the corresponding FAULT low.

The FAULT signal is latched low until reset by the ENABLE

signal being de-asserted at which time the FAULT signal will

clear.

It is this described sequence of events that allows for the

autoreset function to be implemented in a cost efficient manner

requiring the addition of only an RC network per channel to the

typical application.

Figure 15 illustrates the RC network needed with suggested

component values and the configuration of the relevant pins for

each autoreset channel.

Description of Operation

Initially as voltage is applied to VIN, the pull up resistor (Rpu)

provides for pull up to VIN on both the ENABLE pin asserting the

output once VIN > 2.5V and on the FLTn pin. Once turned on and

an overcurrent (OC) condition occurs the IC provides CR

protection for 10ms and then the FLTn pin pulls low through Rpu

and also pulling the ENABLE low thus resetting the device fault

condition. At this time the Rpu charges the cap and the voltage on

the ENABLE/FLTn node rises until the ENABLE > 2.0 and the

output is asserted on once again. This automatic reset cycle will

continue until the OC fault no longer exists on the output. After

several seconds in this mode of operation the IC thermal

protection invokes adjusting the timing of the on-off cycle to

prevent excessive thermal dissipation in the power switch

protecting itself and surrounding circuitry. See Figure 16 for

operation waveform.

.

Applications

•USB

• 2.5V to 5V up to 10W power port protection

FIGURE 15.

VIN

ENABLE

FLTn

GND

ISL6118H

Rpu = 2K

C = 0.1µF

FIGURE 16. AUTO RESET OPERATION

IOUT 1A/DIV

VOUT 2V/DIV

VIN/FLTn 5V/DIV

0A

0V

4ms/DIV