LX1745 Ver la hoja de datos (PDF) - Microsemi Corporation
Número de pieza
componentes Descripción
Fabricante
LX1745 Datasheet PDF : 16 Pages
| |||
INTEGRATED PRODUCTS
LX1745
Triple Output Boost – LED Driver / LCD Bias
PRODUCTION DATASHEET
APPLICATION NOTE
negative voltage transition that is greater than the output
voltage.
POWER MOSFET SELECTION
The LX1745 can source up to 100mA of gate current.
An logic level N-channel MOSFET with a low turn on
threshold voltage, low gate charge and low RDS(ON) is
required to optimize overall circuit performance.
can be chosen from the following equation:
RCS
≅
IPK − 0.185
30 ⋅10−6
eq. 11
which is taken from the following graph (Figure 7).
OVER VOLTAGE PROTECTION PROGRAMMING
Since the output of the LED Driver is a current mode
configuration, it may be desirable to protect the output from
an over-voltage condition in the event the load is removed
or not present.
The LX1745 includes an over voltage monitor that is
easily programmed with two external resistors (Figure 6).
This feature eliminates the need for a Zener Diode clamp on
the output.
Programming is accomplished by first selecting ROVP_2
and then calculating ROVP_1 using the following equation.
ROVP _1
=
ROVP _ 2
VOVP - VREF
VREF
eq. 9
1000
800
600
400
200
0
0
5
10
15
20
RCS (kΩ )
Figure 7 – Peak Current Programming Resistor
where VOVP is the desired maximum voltage on the output.
This voltage should be selected to accommodate the
maximum forward voltage of all the LEDs, over
temperature, plus the maximum feedback voltage.
Conversely, it may also be selected according to the
maximum VDS voltage of the output MOSFET.
INDUCTOR CURRENT LIMIT PROGRAMMING
Setting of the peak inductor current limit is an important
aspect of the PFM constant off-time architecture; it
determines the maximum output power capability and has a
marked effect on efficiency.
It is recommended that the peak inductor current be set
to approximately two times the expected maximum DC
input current. This setting will minimize the inductor size,
the input ripple current, and the output ripple voltage. Care
should be taken to use inductors that will not saturate at the
peak inductor current level. The desired peak inductor
current can be estimated by the following equation:
This graph characterizes the relationship between peak
inductor current, the inductance value, and the RCS
programming resistor.
INDUCTOR SELECTION
An inductor value of 47µH has been show to yield very
good results. Choosing a lower value emphasizes peak
current overshoot, effectively raises the switching
frequency, and increases the dissipative losses due to
increased currents.
OUTPUT CAPACITOR SELECTION
Output voltage ripple is a function of the several
parameters: inductor value, output capacitance value, peak
switch current, load current, input voltage, and the output
voltage. All of these factors can be summarized by the
following equation:
VRIPPLE
≅
L
⋅
IPK ⋅IOUT
COUT
VIN
1
− (VSW
+
VL )
+
IPK ⋅ IOUT
VOUT + VF −
VIN
eq.
12
IPK
=
2 ⋅ POUT
η ⋅ VIN
eq. 10
where POUT is the total output power, η is the expected
conversion efficiency, and VIN is the input voltage.
From the calculated desired IPK an RCS resistance value
where VL is the voltage drop across the inductor, VF is the
forward voltage of the output catch diode, and VSW is the
voltage drop across the power switch. VL+VSW can be
approximated at 0.4V and VF can be approximated at 0.4V.
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 9
Share Link: 