lx1996 Microsemi Corporation, lx1996 Datasheet - Page 17

• Manufacturer Part Number: lx1996
• Description: Multi-string Led Back-light Controller
• Manufacturer: Microsemi Corporation

Copyright © 2007
Rev. 1.0, 2007-12-14
B
The LX1996 boost converter operates always in PFM mode,
and in conjunction with the current sink circuit to support digital
dimming mode with a minimum pulse width of 4μs. Therefore,
a boost inductor must be selected to satisfied digital dimming
mode minimum pulse width requirements, with enough energy
to supply the output during this period.
The LX1996 controls the off time and peak current for
hysteretic PFM converter topology.
vary proportionally with input voltage per the following
formula:
And PFM frequency:
For any instant, the stored energy must be large enough to
supply the output load and output capacitor. Therefore:
From equation 1, 2, and 3, the minimum boost inductor value
can be calculated.
With a specified minimum pulse-width of 4μsec, the maximum
boost inductor value must allow the inductor to reach the
maximum peak current in less than 4μsec. Otherwise, the LED
current can not get to the maximum.
To calculate the maximum inductor value, V
minimum, di will be at maximum, and dt is set at 2us which is
½ of the minimum pulse width.
For example:
With 10 white LED in series for 6 strings with I-LED = 20mA,
total I
Minimum input voltage is 6V.
Per Nichia V
case for the peak LED voltage is 31.7V. Adding 0.8V for the
output diode forward drop, the boost voltage will be:
The maximum peak inductor current is 1.55A with 75mΩ
current sense resistor.
OOST
OUT
C
L
V
ONVERTER
F
½ L
= 120mA.
MAX
TM
OUT
PFM
MIN
F (AVG)
= 31.7V + 0.8V = 32.5V
= V
= V
*I²F
T
IN
IN
OFF
/(V
= 3.15V, and 3σ = 0.15V, then the worst
PWM
dt/di
I
NDUCTOR
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
= 150 + (22 x V
OUT
≥ V
x T
OUT
OFF
S
x I
) (Hz)
ELECTION
OUT
The off time (T
IN
) (nsec)
A P P L I C A T I O N I N F O R M A T I O N
(L
BOOST
Analog Mixed Signal Group
IN
)
will be at
(1)
(2)
(3)
(4)
Microsemi
®
OFF
) will
Substitute values into equations 1, 2, and 3:
From equation 4, the maximum value of the boost inductor is:
The inductor value will be in the range of 4.96uH to 7.7uH.
Lower inductance provides faster transient response; higher
inductance reduces ripple current and improves efficiency.
When selecting the inductor, peak current rating, series
resistance, and physical size should be all considered. The right
candidate for this part is 6.8uH, 1.6A (6x6mm2) from TDK or
Coil craft.
OVP S
The worst case for white LED forward voltage V
Therefore, the boost voltage that is set for 10 LEDs will be:
To minimize power loss through the OVP resistor network,
select a top resistor R
selected:
Select the closest resistor value: R
Multi-string LED Back-light Controller
ETTING
T
F
L
L
V
R
R
PWM
OFF
MIN
MAX
BOTTOM
BOTTOM
LED
P
= 10xV
RODUCTION
=
=
=
=
= V
= (2 x 1MΩ)/(37-2) = 57.1kΩ
F
OVP
+2V = 37V
TOP
x R
282nsec
654.7kHz
4.96uH
7.7uH
= 1MΩ.
D
TOP
ATASHEET
/( V
BOTTOM
LED
The bottom resistor is
– V
= 56.2kΩ.
OVP
);
LX1996
F
= 3.5V.
Page 17