nx2139a Microsemi Corporation, nx2139a Datasheet - Page 14

• Manufacturer Part Number: nx2139a
• Description: Single Channel Mobile Pwm And Ldo Controller
• Manufacturer: Microsemi Corporation

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.
shows the relationship between
age divider.
of R
Mode Selection
sonic PFM mode, CCM mode and shutdown mode by
applying different voltage on 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-
Rev. 2.3
03/19/09
1
where R
NX2139A can be operated in PFM mode, ultra-
When VCC applied to ENSW /MODE pin,
value can be set by voltage divider.
R =
The following equation applies to figure 11, which
1
Figure 11 - Voltage Divider
R2
R1
Vout
V
R
O U T
2
2
is part of the compensator, and the value
Vref
-V
V
Fb
R E F
R E F
V
...(18)
OUT
,
V
REF
and volt-
tion of frequency keeps the system running at light light
with high efficiency.
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
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
as
occurs as shown in figure below.
equation.
occuring moment, and the current limit is set at 12A,
then
In CCM mode, inductor current zero-crossing
Over current protection for NX2139A is achieved
The voltage at pin OCSET is given as
When the voltage is below zero, the over current
The over current limit can be set by the following
If the low side MOSFET R
Choose R
I
I
R
V =-I
OCP
SET
OCP
comparator
SW
OCP
Figure 12 - Over Voltage Protection
R
I
OCP
L
I
OCP
SET
R
OCP
+V
I
R
OCP
DSON
R
OCP
=5k
SW
DSON
/R
24uA
I
OCP
DSON
12A 10m
24uA
OCP
DSON
R
OCP
=10m at the OCP
NX2139A
vbus
5k
SW
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