lx1993 Microsemi Corporation, lx1993 Datasheet - Page 5

• Manufacturer Part Number: lx1993
• Description: High Efficiency Led Driver
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.0b, 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 A1 activates the control logic. The control
logic activates the DRV output circuit that connects to the
gate of the internal FET. The output (i.e., SW) 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
comparator A2.
output capacitor during the inductor charging cycle. Once
the peak inductor current value is achieved, the 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 6, channel 2). This causes the output
voltage to continue to rise across R
feedback circuit. The LX1993 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
LX1993 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 LX1993 is achieved by driving the
SHDN pin with a low-level logic signal thus reducing the
device power consumption to approximately 0.5µA (typ).
SET
PERATING
The LED load is powered from energy stored in the
. The voltage at the FB pin is the product of I
The LX1993 is a PFM boost converter that is
The application of an external voltage source at the
The value of R
I N T E G R A T E D
CS
T
R
HEORY
resistor and monitored through the CS input by
SET
=
⎡
⎢ ⎣
V
PWM
ADJmax
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
SET
≥ 100KHz). The second option is
I
LEDmax
SET
is established by dividing the
(i.e., V
⎤
⎥ ⎦
P R O D U C T S
SET
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
at the input to the
SET
.
Integrated Products Division
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 428mA.
O
0.5V value in the denominator is an estimate of the voltage
drop across the inductor and the FET RDS_ON:
(max. = 500mA rms). The minimum I
when R
R
(I
internal operation of comparator A. A typical value at 25
is 850ns. A typical value of I
All of these parameters have an effect on the final I
value.
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
ROGRAMMING
UTPUT
CS
MIN
ESIGN
The initial droop can be estimated as follows where the
Setting the level of peak inductor current to approximately
I
PEAK
is zero. A typical value for the minimum peak current
The maximum I
Determine I
Output voltage ripple is a function of the inductor value
V
) at 25
I
RIPPLE
CS
PEAK
R
E
=
197mA
IPPLE AND
High Efficiency LED Driver
XAMPLE
S
is zero. The minimum I
IN
ELECTION AND
= ∆V
o
) and the output voltage (V
C is 197mA. The parameter t
=
+
PEAK
I
⎛
⎜
⎝
:
DROOP
PEAK
MIN
3.0V
C
APACITOR
PEAK
P
where V
), the load current (I
47µ7
+
RODUCTION
+ ∆V
O
⎛
⎜
⎝
UTPUT
value is limited by the I
V
⎞
× ⎟
⎠
OVERSHOOT
SCALE
5 8
IN
IN
S
ELECTION
0ns
PEAK
C
L
equals 3.0V and R
URRENT
PEAK
+
at 25
⎞
⎟
⎠
(
44mA
t
equation is:
OUT
D
value is defined when
+ 10mV
PEAK
o
OUT
), the peak switch
+
C is 44mA per KΩ.
PEAK
L
kΩ
IMIT
) for a this boost
(
value is defined
I
OUT
)
D
×
), the feedback
SCALE
4.02K Ω
is related to
), the input
LX1993
CS
)
SW
PEAK
R
equals
value
CS
PEAK
o
of
Page 5
C