ISL6121

FN9004 Rev 2.00

Page 10 of 11

November 12, 2004

Implementing Autoreset on the ISL6121

Hot Swap Controller.

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

tech brief shows how to implement such an autonomous

controller using the ISL6121HIB. This application works only

with the ‘H’ version of this device. 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 ISL6121 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 18 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 19

for operation waveform.

.

Applications

•USB

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

FIGURE 18.

VIN

ENABLE

FLTn

GND

ISL6121H

Rpu = 2k

C = 0.1

F

FIGURE 19. AUTO RESET OPERATION

IOUT 1A / DIV

VOUT 2V / DIV

VIN / FLTn 5V / DIV

0A

0V

4ms/DIV