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ISL8540

DC/DC Power Switching Regulator

Renesas Electronics 

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F  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

=

R----6---.-k-2---5-----





f--O-----S--1--C-0----0-k--0--H-----z----

–

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 1k and 10k.

R3 = -V----RO----U2----T-----–1----.-12---.-1-2---V1----V--

(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.

FN6495 Rev 5.00

September 9, 2008

Page 11 of 17

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