LX1992-02W EVAL KIT MICROSEMI, LX1992-02W EVAL KIT Datasheet - Page 5

• Manufacturer Part Number: LX1992-02W EVAL KIT
• Manufacturer: MICROSEMI

Specifications of LX1992-02W EVAL KIT

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Copyright © 2000
Rev. 1.1b, 2005-03-03
O
optimized for driving a string of series connected LEDs. It
operates in a pseudo-hysteretic mode with a fixed switch
“off time” of 300ns. Converter switching is enabled as
LED current decreases causing the voltage across R
decrease to a value less than the voltage at the VADJ pin.
When the voltage across R
comparator A activates the control logic. The control logic
activates the DRV output circuit that connects to the gate
of the external FET. The DRV output is switched “on”
(and remains “on”) until the inductor current ramps up to
the peak current level. This current level is set via the
external R
SRC inputs by comparator B.
output capacitor during the inductor charging cycle. Once
the peak inductor current value is achieved, the NDRV
output is turned off (off-time is typically 300ns) allowing a
portion of the energy stored in the inductor to be delivered
to the load (e.g., see Figure 5, channel 2). This causes the
output voltage to continue to rise across R
the feedback circuit. The LX1992 continues to switch until
the voltage at the FB pin exceeds the control voltage at the
ADJ pin.
maximum adjust voltage by the maximum series LED
current. A minimum value of 15Ω is recommended for
R
the current through the LED chain) and R
ADJ pin provides for output current adjustment over the
entire dimming range and the designer can select one of
two possible methods. The first option is to connect a
PWM logic signal to the ADJ pin (e.g., see Figure 1). The
LX1992 includes an internal 50pF capacitor to ground that
works with an external resistor to create a low-pass filter
(i.e., filter out the AC component of a pulse width
modulated input of f
to adjust the reference voltage directly at the ADJ pin by
applying a DC voltage from 0.0 to 0.3V (e.g., see Figure
2). The adjustment voltage level is selectable (with limited
accuracy) by implementing the voltage divider created
between the external series resistor and the internal 2.5MΩ
resistor. Disabling the LX1992 is achieved by driving the
SHDN pin with a low-level logic signal thus reducing the
device power consumption to less than 0.5µA (typ).
PERATING
SET
The LED load is powered from energy stored in the
The LX1992 is a PFM boost converter that is
The value of R
. The voltage at the FB pin is the product of I
The application of an external voltage source at the
TM
CS
T
HEORY
resistor and monitored through the CS and
R
PWM
SET
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
SET
=
≥ 100KHz). The second option is
⎡
⎢ ⎣
V
is established by dividing the
SET
ADJmax
(i.e., V
I
LEDmax
FB
) is less than VADJ,
⎤
⎥ ⎦
A P P L I C A T I O N I N F O R M A T I O N
SET
SET
.
Linfinity Microelectronics Division
at the input to
OUT
®
Microsemi
SET
(i.e.,
to
I
P
2X the expected maximum DC input current will minimize
the inductor size, the input ripple current, and the output
ripple voltage. The designer is encouraged to use inductors
that will not saturate at the peak inductor current level. An
inductor value of 47µH is recommended. Choosing a lower
value emphasizes peak current overshoot while choosing a
higher value emphasizes output ripple voltage. The peak
switch current is defined using a resistor placed between the
CS terminal and ground and the I
The result of this example yields a nominal I
approximately 225mA.
O
0.5 value in the denominator is an estimate of the voltage
drop across the inductor and the FET’s R
(max. = 0.8A
when R
I
section of this data sheet. The parameter t
internal operation of the device. A typical value at 25
800ns. R
the SRC pin. A typical value at 25
parameters have an effect on the final I
D
4.02KΩ using nominal values for all other parameters.
(L), the output capacitor value (C
current setting (I
voltage (V
converter regulation scheme. When the switch is first turned
on, the peak-to-peak voltage ripple is a function of the output
droop (as the inductor current charges to I
transition error (i.e., typically 10mV), and the output
overshoot (when the stored energy in the inductor is
delivered to the load at the end of the charging cycle).
Therefore the total ripple voltage is
NDUCTOR
CS
ROGRAMMING
UTPUT
ESIGN
Setting the level of peak inductor current to approximately
The initial droop can be estimated as follows where the
I
PEAK
The maximum I
are provided in the E
Determine I
Output voltage ripple is a function of the inductor value
V
I
RIPPLE
PEAK
R
CS
E
=
IPPLE AND
ICS
XAMPLE
S
73
is zero. The value range for parameters I
IN
ELECTION AND
High Efficiency LED Driver
mA
=
) and the output voltage (V
= ∆V
is the internal current sense resistor connected to
P
I
RMS
RODUCTION
MIN
+
PEAK
⎛
⎜
⎝
:
DROOP
). The minimum I
PEAK
3.0V
C
+
APACITOR
⎛
⎜
⎝
PEAK
where V
), the load current (I
47
V
+ ∆V
IN
µ
O
H
value is limited by the I
UTPUT
L
⎞
× ⎟
⎠
⎞
⎟
⎠
D
OVERSHOOT
800ns
t
LECTRICAL
IN
S
D
ATA
ELECTION
PEAK
C
+
equals 3.0V and R
URRENT
⎛
⎜
⎝
o
+
C is 200mΩ. All of these
S
I
⎛
⎜
⎝
CS
5
OUT
equation is:
HEET
0 .
PEAK
PEAK
+ 10mV
R
µ
OUT
), the peak switch
A
ICS
PEAK
L
C
200m
value.
) for a this boost
IMIT
value is defined
⎞
⎟
⎠
HARACTERISTICS
OUT
R
), the feedback
D
CS
Ω
is related to
), the input
⎞
× ⎟
⎠
DS_ON
LX1992
SRC
. 4
CS
PEAK
02
MIN
: The
equals
K
value
o
C is
Ω
and
of
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