ISL6410AIU datasheet(21/26 Pages) RENESAS

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ISL6410, ISL6410A

FN9149 Rev 3.00

Page 8 of 13

September 17, 2004

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

50m 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

F 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

H must be used for the ISL6410 and a

minimum of 12

H for the ISL6410A. The selected inductor

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



COMPONENT

SUPPLIER

COMMENTS

F

<50 AVX 08056D106KAT2A

Ceramic

IRMS Co



Vin

---------

–



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



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





Vin

---------

–



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

8Cof



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







TABLE 3. RECOMMENDED INDUCTORS

INDUCTOR VALUE

DCR (m



COMPONENT

SUPPLIER

8.2

H75

Coilcraft

MSS6122-822MX

H

100

Coilcraft

MSS6122-123MX

IRMS

Io max



Vin

---------

Vin

---------

–







dIL

Vin

---------

–



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



IL max

Io max

dIL

---------

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