NX2139A datasheet(14/21 Pages) MICROSEMI

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NX2139A

Rev. 2.3

03/19/09

at 0.75V. The divider consists of two ratioed resistors

so that the output voltage applied at the Fb pin is 0.75V

when the output voltage is at the desired value.

The following equation applies to figure 11, which

shows the relationship between

OUT

REF

and volt-

age divider.

Vout

Vref

Figure 11 - Voltage Divider

REF

OUT

REF

-V

...(18)

where R

2 is part of the compensator, and the value

of R1 value can be set by voltage divider.

Mode Selection

NX2139A can be operated in PFM mode, ultra-

sonic PFM mode, CCM mode and shutdown mode by

applying different voltage on ENSW/MODE pin.

When VCC applied to ENSW /MODE pin,

NX2139A is In PFM mode. The low side MOSFET emu-

lates the function of diode when discontinuous con-

tinuous mode happens, often in light load condition.

During that time, the inductor current crosses the zero

ampere border and becomes negative current. When

the inductor current reaches negative territory, the low

side MOSFET is turned off and it takes longer time for

the output voltage to drop, the high side MOSFET waits

longer to be turned on. At the same time, no matter

light load and heavy load, the on time of high side

MOSFET keeps the same. Therefore the lightier load,

the lower the switching frequency will be. In ultrosonic

PFM mode, the lowest frequency is set to be 25kHz to

avoid audio frequency modulation. This kind of reduc-

tion of frequency keeps the system running at light light

with high efficiency.

In CCM mode, inductor current zero-crossing

sensing is disabled, low side MOSFET keeps on even

when inductor current becomes negative. In this way

the efficiency is lower compared with PFM mode at

light load, but frequency will be kept constant.

Over Current Protection

Over current protection for NX2139A is achieved

by sensing current through the low side MOSFET. An

typical internal current source of 24uA flows through

an external resistor connected from OCSET pin to SW

node sets the over current protection threshold. When

synchronous FET is on, the voltage at node SW is given

DSON

=-IR

The voltage at pin OCSET is given as

OCP

When the voltage is below zero, the over current

occurs as shown in figure below.

OCP

comparator

OCP

24uA

OCP

vbus

Figure 12 - Over Voltage Protection

The over current limit can be set by the following

equation.

=×

SET

OCP

DSON

If the low side MOSFET R

DSON=10mΩ at the OCP

occuring moment, and the current limit is set at 12A,

then

SET

DSON

OCP

12A 10m

24uA

×Ω

=Ω

Choose R

OCP=5kΩ

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