8

Power Good

This output is asserted high when the PWM is enabled, and

Vout is within 8.0% typical of its final value, and is active low

outside this range. When disabled, the output turns active

low. It is recommended to leave the PG pin unconnected

when not used.

PWM Overvoltage and Overcurrent Protection

The PWM output current is sampled at the end of each PWM

cycle, exceeding the overcurrent limit, causes a 4 bit

up/down counter to increment by one LSB. A normal current

state causes the counter to decrement by one LSB (the

counter will not however “rollover” or count below 0000).

When the PWM goes into overcurrent, the counter rapidly

reaches count 1111 and the PWM output is shut down and

the soft-start counter is reset. After 16 clocks the PWM

output is enabled and the soft-start cycle is started.

If Vout exceeds the overvoltage limit for 32 consecutive clock

cycles the PWM output is shut off and the soft-start cycle is

initiated.

No Load Operation

If there is no load connected to the output, the converter will

regulate the output voltage by allowing the inductor current

to reverse for a short period of time.

Output Capacitor Selection

For best performance, a low ESR output capacitor is

needed. Output voltages below 1.8V require a larger output

capacitor and ESR value to improve the performance and

stability of the converter. For 1.8V output applications, a

ceramic capacitor of 10µF or higher value with ESR

≤50mΩ

is recommended.

The RMS ripple current is calculated as:

L = the inductor value

f = the switching frequency

The overall output ripple voltage is the sum of the voltage spike

caused by the output capacitor ESR and the voltage ripple

caused by charge and discharge of the output capacitor:

Where the highest output voltage ripple occurs at the highest

input voltage VIN.

Input Capacitor Selection

The input current to the buck converter is pulsed, and

therefore a low ESR input capacitor is required. This results

in good input voltage filtering and minimizes the interference

it causes to other circuits. The input capacitor should have a

minimum value of 10

µF and a higher value can be selected

for improving input voltage filtering. The input capacitor

should be rated for the maximum input ripple current

calculated as:

The worst case RMS ripple current occurs at D = 0.5 and is

calculated as: Irms = Io/2.

D = Duty Cycle

Ceramic capacitors are preferred because of their low ESR

value. They are also less sensitive to voltage transients

when compared to tantalum capacitors. It is good practice to

place the input capacitor as close as possible to the input pin

of the IC for optimum performance.

Inductor Selection

The ISL6410 is an internally compensated device and hence

a minimum of 8.2

minimum of 12

must have a low DC resistance and a saturation current

greater than the maximum inductor current value can be

calculated from the equations below

where

dIL = the peak to peak inductor current

L = the inductor value

f = the switching frequency

ILmax = the max inductor current

TABLE 2. RECOMMENDED OUTPUT CAPACITORS

CAPACITOR

VALUE

ESR

(m

Ω)

COMPONENT

SUPPLIER

COMMENTS

10

µF

<50 AVX 08056D106KAT2A

Ceramic

()

Vo

1

Vo

Vin

---------

–

Lf

×

-------------------

1

23

×

-----------------

×

×

=

Vo

∆

Vo

1

Vo

Vin

---------

–

Lf

×

-------------------

1

8Cof

×

×

------------------------- ESR

+





×

×

=

TABLE 3. RECOMMENDED INDUCTORS

INDUCTOR VALUE

DCR (m

Ω)

COMPONENT

SUPPLIER

8.2

µH75

Coilcraft

MSS6122-822MX

12

µH

100

Coilcraft

MSS6122-123MX

Io max

()

Vo

Vin

---------

1

Vo

Vin

---------

–





×

×

=

dIL

Vo

1

Vo

Vin

---------

–

Lf

×

-------------------

×

=

IL max

Io max dIL

2

---------

+

=

ISL6410, ISL6410A