11

FN6830.0

December 19, 2008

This resistor sets the threshold for the Circuit Breaker

This resistor sets up the current in the internal current

voltage at the OCREF pin is the same as the internal

resistor is simply:

internal current source as shown in Figure 2, because of the

4:1 current mirror. This equates to the resistor value of

14.7k.

Selection of Rs1 and Rs2

These resistors set the UV detect point. The UV comparator

detects the undervoltage condition when it sees the voltage

at UV pin drop below 0.633V. The resistor divider values

should be selected accordingly.

charge pump, which operates at 450kHz set internally. The

output resistance of the charge pump, which affects the

charge-pump switch resistance, and the frequency and ESR

(minimum) to 0.1µF (maximum) range. Only ceramic

capacitors are recommended. Use 0.1µF cap if CPVDD

output is expected to power an external circuit, in which case

the current draw from CPVDD must be kept below 10mA.

may be used if low ripple performance is desired.

This capacitor determines the current regulation delay

period. As shown in Figure 2, when the voltage across this

capacitor exceeds 1.178V, the time-out comparator detects it

and the gate voltage is pulled to 0V thus shutting down the

channel. An internal 10µA current source charges this

capacitor. Hence, the value of this capacitor is determined by

Equation 2.

Where,

The rate of change of voltage (dv/dt) on this capacitor, which

is determined by the internal 10µA current source, is the

same as that on the output load capacitance. Hence, the

value of this capacitor directly controls the inrush current

amplitude during hot swap operation.

Where,

the load capacitance charging current. If the DC steady-state

load remains disabled until after the soft-start period expires

(PGx could be used as a load enable signal, for example),

then only the capacitor charging current should be used as

of that of Ciss of the MOSFET to ensure proper soft-start

operation. This is because the Ciss is charged from 24µA

current source, whereas the Css gets charged from a 10µA

necessary.

ISL6174 Evaluation Platform

The ISL617XEVAL1Z is the primary evaluation board for this

IC. For the BOM, schematic and photograph, see the “BOM

for ISL617XEVAL1Z Board and Schematic” on page 15.

The evaluation board has been designed with a typical

application in mind and with accessibility to all the featured

pins to enable a user to understand and verify these features

of the IC. The two circuit breaker levels are programmed to

2.2A for each input rail but they can easily be scaled up or

down by adjusting some component values.

There are two input voltages, one for each channel that are

switched by a dual N-Channel MOSFET (Q1) to the output

connectors.

LOAD CURRENT CARRYING

TRACES

CURRENT

SENSE

TRACES

FIGURE 17. RECOMMENDED CURRENT SENSE RESISTOR

PCB LAYOUT

C

T

10

μAT

OUT

•

() 1.178

⁄

=

(EQ. 2)

C

SS

C

O

10

μAI

INRUSH

⁄

()

•

=

(EQ. 3)

ISL6174