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ISL8540

DC/DC Power Switching Regulator

Intersil 

ISL8540

Applications Information

Product Description

The ISL8540 is a non-synchronous, integrated FET 2A step

down regulator which operates from an unregulated input of

9V to 40V. The output voltage is user-adjustable with a pair

of external resistors. Frequency of operation is adjustable

from 100kHz to 600kHz set by RTCT. An external signal with

higher frequency can be injected to SYNCH to synchronize

the controller.

The buck controller drives an internal N-Channel MOSFET

and requires an external diode to deliver load current up to

2A. A Schottky diode is recommended for improved

efficiency and performance over a standard diode. The

converter output is regulated down to 1.21V from an input

source. These features make the ISL8540 ideally suited for

telecommunication power applications.

The PWM control loop uses a single output voltage loop with

input voltage feed forward which simplifies feedback loop

compensation and rejects input voltage variation. External

feedback loop compensation allows flexibility in output filter

component selection.

The buck regulator is equipped with a lossless current limit

scheme. The current limit in the buck regulator is achieved

by monitoring the drain-to-source current of the internal

switching power MOSFET. The current limit threshold is

internally set at 4.5A peak. Additional features include

programmable soft-start to support proper start-up of wide

output capacitance range.

Start-Up and Shut-Down

When the EN pin is connected to a logic high, the ISL8540 is

in the shut-down mode. All the control circuitry and both

MOSFETs are off, and VOUT falls to zero. In this mode, the

total input current is less than 110µA.

When the EN pin is tied to GND, and VIN reaches

approximately 9V, the regulator begins to switch. The output

voltage is gradually increased to ensure proper soft-start

operation.

When the EN reaches logic LOW, the regulator repeats the

start-up procedure, including the soft-start function. Connect

a capacitor from SS pin to ground. This capacitor, along with

an internal 10µA current source sets the soft-start interval of

the converter, tSS.

CSS[μF] ≈ 8.3 ⋅ tSS[s]

(EQ. 1)

Operating Frequency

The ISL8540 can operate at switching frequencies from

100kHz to 600kHz. A resistor tied from the RTCT pin to VIN

and a capacitor from RTCT to GND are used to program the

switching frequency through Equation 2.

C1[nF]

=

----6---.--2---5-----

R[kΩ]

⎛

⎝

---------1----0---0---0-----------

fOSC[kHz]

–

0.3⎠⎞

(EQ. 2)

Caution: When the ISL8540 is in disabled state, the voltage

across RTCT pin will reach VIN voltage. Make sure that the

voltage rating of the RTCT capacitor is rated as high as the

input voltage.

Output Voltage Selection

The output voltage of the regulator can be programmed via

an external resistor divider that is used to scale the output

voltage relative to the internal reference voltage and feed it

back to the inverting input of the error amplifier. Refer to

Figure 28.

The output voltage programming resistor, R3, will depend on

the value chosen for the feedback resistor and the desired

output voltage of the regulator. The value for the feedback

resistor is typically between 1kΩ and 10kΩ.

R3

=

-----R----2-----×----1----.-2----1----V------

VOUT – 1.21V

(EQ. 3)

If the output voltage desired is 1.21V, then R3 is left

unpopulated.

Fault Protection

The ISL8540 monitors the output of the regulator for

overcurrent and undervoltage events. The ISL8540 also

provides protection from excessive junction temperatures.

Overcurrent Protection

The overcurrent function protects the switching converter from

a shorted output by monitoring the current flowing through the

upper MOSFETs.

Upon detection of overcurrent condition, the upper MOSFET

will be immediately turned off and will not be turned on again

until the next switching cycle. Upon detection of the initial

overcurrent condition, the overcurrent fault counter is set to

1 and the overcurrent condition flag is set from LOW to

HIGH. If, on the subsequent cycle, another overcurrent

condition is detected, the OC fault counter will be

incremented. If there are 16 sequential OC fault detections,

the regulator will be shut down under an overcurrent fault

condition. An overcurrent fault condition will result with the

regulator attempting to restart in a hiccup mode with the

delay between restarts being 4 soft-start periods. At the end

of the fourth soft-start wait period, the fault counters are

reset and soft-start is attempted again. If the overcurrent

condition goes away prior to the OC fault counter reaching a

count of four, the overcurrent condition flag will set back to

LOW.

If the overcurrent condition flag is HIGH and the overcurrent

fault counter is less than four and an undervoltage event is

detected, the regulator will be shut down immediately.

11

FN6495.5

September 9, 2008

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