NX2113 datasheet(15/19 Pages) MICROSEMI

15 / 19 page

NX2113/2113A

Rev. 2.0

11/18/05

There are three factors causing the MOSFET power

loss: conduction loss, switching loss and gate driver loss.

Gate driver loss is the loss generated by discharg-

ing the gate capacitor and is dissipated in driver circuits.

It is proportional to frequency and is defined as:

gate

HGATE

HGS

LGATE

LGSS

××

...(20)

where QHGATE is the high side MOSFETs gate

charge,QLGATEis the low side MOSFETs gate charge,VHGS

is the high side gate source voltage, and V

LGS is

the low side gate source voltage.

According to equation (20), PGATE =0.14W. This

power dissipation should not exceed maximum power

dissipation of the driver device.

Conduction loss is simply defined as:

HCON

OUT

DS(ON)

LCON

OUT

DS(ON)

TOTAL

HCON

LCON

(1 D) RK

=PP

× ××

× −

××

...(21)

where the RDS(ON) will increases as MOSFET junc-

tion temperature increases, K is RDS(ON) temperature

dependency. As a result, RDS(ON) should be selected for

the worst case, in which K equals to 1.4 at 125oC ac-

cording to IRFR3706 datasheet

. Using equation (21),

the result of PTOTAL is 0.54W. Conduction loss should

not exceed package rating or overall system thermal

budget.

Switching loss is mainly caused by crossover con-

duction at the switching transition. The total switching

loss can be approximated.

OUT

SWS

= ×

×××

...(22)

where IOUT is output current, TSW is the sum of T

R and TF

which can be found in mosfet datasheet, and FS is switch-

ing frequency. The result of PSW is 3W. Swithing loss

PSW is frequency dependent.

Soft Start, Enable and shut Down

The NX2113 has a digital start up. It is based on

digital counter with 1024 cycles. For NX2113 with 300kHz

operation, the start up time is about 3.5ms. For NX2113A

with 600kHz operation, the start up time is about half of

NX2113, 1.75mS.

POR

Digital

start

1.25/1.15

Vbus

10k

OFF

Figure 14 - Enable and Shut down NX2113 by

pulling down EN pin.

The start up of NX2113/2113A can be programmed

through resistor divider at Enable pin. For example, if

the input bus voltage is 12V and we want NX2113 starts

when Vbus is above 8V. We can select

R2=1.24k

(8V

1.25V)R

6.8k

1.25V

−×

=Ω

The NX2113 can be turned off by pulling down the

ENable pin by extra signal MOSFET or NPN transistor

such as 2N3904 as shown in the above Figure. When

Enable pin is below 1.15V, the digital soft start is reset

to zero. In addition, all the high side is off and output

voltage is turned off.

A resistor should be added as preload to prevent

leakage current from FB pin charging the output capaci-

tors.

Feedback Under Voltage Shut Down

NX2113 relies on the Feedback Under Voltage Lock

Out (FB UVLO ) to provide short circuit protection. Ba-

sically, NX2113 has a comparator compares the feed-

back voltage with the FB UVLO threshold 0.4V.

During the normal operation, if the output is short,

the feedback voltage will be lower than 0.4V and com-

parator will change the state. After certain internal delay,

both high side and low side driver will be turned off. The

output will be latched. The normal operation should be

achieved by removing the short and recycle the VCC.

During the start up, the output voltage is dis-

charged to zero by the synchronous FET. FB voltage

starts increase from zero when digital start block

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