NX2116/2116A/2116B/2117/2117A

13

Rev. 3.0

03/14/06

2

HCON

OUT

DS(ON)

2

LCON

OUT

DS(ON)

TOTAL

HCON

LCON

P

=I

DR

P

=I

(1 D) RK

P

=PP

× ××

× −

××

+

K

...(20)

the worst case, in which K approximately equals to 1.4

tion 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.

SW

IN

OUT

SWS

1

P

V

I

TF

2

= ×

×××

R

and T

S

dependent.

Also MOSFET gate driver loss should be consid-

ered when choosing the proper power MOSFET.

MOSFET gate driver loss is the loss generated by dis-

charg

circuits.It is proportional to frequency and is defined as:

gate

HGATE

HGS

LGATE

LGSS

P

(Q

V

Q

V

)F

=

×

+

××

...(22)

is the high side gate source voltage, and V

side gate source voltage.

This power dissipation should not exceed maxi-

mum power dissipation of the driver device.

Soft Start and Enable

NX2116 has digital soft start for switching control-

ler and has one enable pin for this start up. When the

Power Ready (POR) signal is high and the voltage at

enable pin is above 1.25V the internal digital counter

starts to operate and the voltage at positive input of Error

amplifier starts to increase, the feedback network will

force the output voltage follows the reference and starts

the output slowly. After 2048 cycles, the soft start is

complete and the output voltage is regulated to the de-

sired voltage decided by the feedback resistor divider.

POR

Digital

start

up

1.25V/

1.15V

Vbus

R1

R2

EN

10k

ON

OFF

Figure 9 - Enable and Shut down the NX2116

with Enable pin.

The start up of NX2116 can be programmed through

resistor divider at Enable pin. For example, if the input

bus voltage is12V and we want NX2116 starts when Vbus

is above 9V. We can select using the following equa-

tion.

−×

=

2

1

(9V 1.25V)R

R

1.25V

The NX2116 can be turned off by pulling down the

Enable pin by extra signal MOSFET as shown in the

above Figure. When Enable pin is below 1.25V

tal soft start is reset to zero. In addition, all the high side

and low side driver is off and no negative spike will be

generated during the turn off.

Over Current Protection

Over current protection is achieved by sensing cur-

rent through the low side MOSFET. An internal current

source of 40uA flows through an external resistor con-

nected from OCP pin to SW node sets the over current

protection threshold. When synchronous FET is on, the

voltage at node SW is given as

SW

L

DSON

V

=-IR

×

The voltage at pin OCP is given as

OCP

OCP

SW

I

R

+V

×

When the voltage is below zero, the over current

occurss as shown in figure 10.