NX2139A datasheet(11/21 Pages) MICROSEMI

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NX2139A

Rev. 2.3

03/19/09

APPLICATION INFORMATION

Symbol Used In Application Information:

VIN

- Input voltage

VOUT

- Output voltage

IOUT

- Output current

DVRIPPLE - Output voltage ripple

- Working frequency

DIRIPPLE - Inductor current ripple

Design Example

The following is typical application for NX2139A,

the schematic is figure 1.

VIN = 7 to 22V

VOUT=1.8V

FS=220kHz

IOUT=7A

DVRIPPLE <=60mV

DVDROOP<=60mV @ 3A step

On_Time and Frequency Calculation

The constant on time control technique used in

NX2139A delivers high efficiency, excellent transient

dynamic response, make it a good candidate for step

down notebook applications.

An internal one shot timer turns on the high side

driver with an on time which is proportional to the input

supply

VIN as well inversely proportional to the output

voltage

VOUT. During this time, the output inductor

charges the output cap increasing the output voltage

by the amount equal to the output ripple. Once the

timer turns off, the Hdrv turns off and cause the output

voltage to decrease until reaching the internal FB volt-

age of 0.75V on the PFM comparator. At this point the

comparator trips causing the cycle to repeat itself. A

minimum off time of 400nS is internally set.

The equation setting the On Time is as follows:

TON

OUT

4.45 10

TON

0.5V

−

×××

−

...(1)

OUT

TON

...(2)

In this application example, the RTON is chosen

to be 1Mohm, when VIN=22V, the TON is 372nS and

S is around 220kHz.

Output Inductor Selection

The value of inductor is decided by inductor ripple

current and working frequency. Larger inductor value

normally means smaller ripple current. However if the

inductance is chosen too large, it brings slow response

and lower efficiency. The ripple current is a design free-

dom which can be decided by design engineer accord-

ing to various application requirements. The inductor

value can be calculated by using the following equa-

tions:

(

)

OUT

RIPPLE

OUTPUT

V -VT

=kI

...(3)

where k is percentage of output current.

In this example, inductor from COILCRAFT

DO5010H-152 with L=1.5uH is chosen.

Current Ripple is recalculated as below:

OUT

RIPPLE

OUT

(V -V

(22V-1.8V) 372nS

1.5uH

=5A

...(4)

Output Capacitor Selection

Output capacitor is basically decided by the

amount of the output voltage ripple allowed during

steady state(DC) load condition as well as specifica-

tion for the load transient. The optimum design may

require a couple of iterations to satisfy both conditions.

Based on DC Load Condition

The amount of voltage ripple during the DC load

condition is determined by equation(5).

∆

× ∆+

××

RIPPLE

OUT

ESRI

8 FC

...(5)

Where ESR is the output capacitors' equivalent

series resistance,C

OUT is the value of output capaci-

tors.

Typically POSCAP is recommended to use in

NX2139's applications. The amount of the output volt-

age ripple is dominated by the first term in equation(5)

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