lx1665 Microsemi Corporation, lx1665 Datasheet - Page 11

• Manufacturer Part Number: lx1665
• Description: Dual Output Pwm Controllers With 5-bit Dac
• Manufacturer: Microsemi Corporation

The LX1664/65 devices are very easy to design with, requiring
only a few simple calculations to implement a given design. The
following procedures and considerations should provide effec-
tive operation for virtually all applications. Refer to the Appli-
cation Information section for component reference designa-
tors.
TIMING CAPACITOR SELECTION
The frequency of operation of the LX166x is a function of duty
cycle and OFF-time. The OFF-time is proportional to the timing
capacitor (which is shown as C
this data sheet), and is modulated to minimize frequency
variations with duty cycle. The frequency is constant, during
steady-state operation, due to the modulation of the OFF-time.
following equation:
(recommended to be around 200kHz for optimal operation and
component selection).
The timing capacitor (C
Where I
C
T
Ref
=
DIS
f
S
(1 - V
is fixed at 200µA and f
(1.52 - 0.29 * V
OUT
Description
/V
IN
Ω
) * I
(See Table 6 for values)
T
) should be selected using the
OUT
DIS
8
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
)
in all application schematics in
D
UAL
S
is the switching frequency
U S I N G T H E L X 1 6 6 4 / 6 5 D E V I C E S
O
P
UTPUT
R O D U C T I O N
B I L L O F M A T E R I A L S
PWM C
ONTROLLERS WITH
Part Number / Manufacturer
LX1664/1664A, LX1665/65A
D
can be approximated as follows:
frequency of 183kHz at V
is used, he capacitor value must be changed (the optimal timing
capacitor for 12V input will be in the range of 1000-1500pF).
L
The inductance value chosen determines the ripple current
present at the output of the power supply. Size the inductance
to allow a nominal ±10% swing above and below the nominal DC
load current, using the equation L = V
OFF-time, V
time, and ∆I is peak-to-peak ripple current in the inductor. Be
sure to select a high-frequency core material which can handle
the DC current, such as 3C8, which is sized for the correct power
level. Typical inductance values can range from 2 to 10µH.
Exceeding the ripple current rating of the capacitors could cause
reliability problems.
1
A T A
OUTPUT INDUCTOR SELECTION
When using a 5V input voltage, the switching frequency (f
Choosing a 680pF capacitor will result in an operating
Note that ripple current will increase with a smaller inductor.
C
T
= 0.621 *
S
L
H E E T
is the voltage across the inductor during the OFF-
5-B
IT
I
DIS
f
S
DAC
OUT
= 2.8V. When a 12V power input
L
* ∆T/∆I, where ∆T is the
Qty.
S
)
11