nx2138 Microsemi Corporation, nx2138 Datasheet - Page 11

• Manufacturer Part Number: nx2138
• Description: Single Channel Mobile Pwm Controller
• Manufacturer: Microsemi Corporation

APPLICATION INFORMATION
Symbol Used In Application Information:
Design Example
the schematic is figure 1.
On_Time and Frequency Calculation
NX2138 delivers high efficiency, excellent transient dy-
namic response, make it a good candidate for step down
notebook applications.
driver with an on time which is proportional to the input
supply
voltage
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.
to be 1Mohm, when VIN=22V, the TON is 372nS and
Rev. 1.6
12/09/09
V
V
I
DV
DI
V
V
I
DV
F
F
DV
OUT
OUT
OUT
IN
S
IN
OUT
S
RIPPLE
=220kHz
DROOP
RIPPLE
RIPPLE
= 7 to 22V
The following is typical application for NX2138,
=7A
=1.8V
The constant on time control technique used in
The equation setting the On Time is as follows:
In this application example, the RTON is chosen
F
TON
An internal one shot timer turns on the high side
S
V
V
<=60mV @ 3A step
IN
<=60mV
- Working frequency
- Inductor current ripple
- Output current
- Input voltage
- Output voltage
- Output voltage ripple
OUT
V
as well inversely proportional to the output
IN
V
. During this time, the output inductor
4.45 10
OUT
TON
V
IN
12
0.5V
R
TON
V
OUT
...(1)
...(2)
F
Output Inductor Selection
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:
DO5010H-152 with L=1.5uH is chosen.
Output Capacitor Selection
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
condition is determined by equation(5).
series resistance,C
tors.
NX2139's applications. The amount of the output volt-
age ripple is dominated by the first term in equation(5)
S
is around 220kHz.
The value of inductor is decided by inductor ripple
where k is percentage of output current.
In this example, inductor from COILCRAFT
Current Ripple is recalculated as below:
Output capacitor is basically decided by the
The amount of voltage ripple during the DC load
Where ESR is the output capacitors' equivalent
Typically POSCAP is recommended to use in
I
RIPPLE
L
I
RIPPLE
V
OUT
RIPPLE
=5A
=
=
=
=k I
(22V-1.8V) 372nS
(V -V
V -V
IN
IN
ESR
OUTPUT
I
RIPPLE
OUT
L
OUT
1.5uH
OUT
OUT
) T
is the value of output capaci-
I
RIPPLE
T
ON
ON
8 F
...(4)
I
S
...(3)
RIPPLE
C
NX2138
OUT
...(5)
11