LT3492
10
3492fa
is similar to your application and tune the compensation
network to optimize the performance. The stability, PWM
dimming waveforms and the start-up time should be
checked across all operating conditions.
Open-LED Protection
The LT3492 has open-LED protection for all the three
converters. As shown in Figure 1, the OVP1 pin receives
the output voltage (the voltage across the output capacitor)
feedback signal from an external resistor divider. OVP1
voltage is compared with a 1V internal voltage reference by
comparator A6. In the event the LED string is disconnected
or fails open, converter 1 output voltage will increase, caus-
ing OVP1 voltage to increase. When OVP1 voltage exceeds
1V, the power switch Q1 will turn off, and cause the output
voltage to decrease. Eventually, OVP1 will be regulated to
1V and the output voltage will be limited. In the event one
of the converters has an open-LED protection, the other
converters will continue functioning properly.
Switching Frequency and Soft-Start
The LT3492 switching frequency is controlled by FADJ
pin voltage. Setting FADJ voltage to be less than 1V will
reduce switching frequency.
If FADJ voltage is higher than 1V, the default switch-
ing frequency is 2.1MHz. In general, a lower switching
frequency should be used where either very high or very
low switch duty cycle is required or higher effi ciency is
desired. Selection of a higher switching frequency will
allow use of low value external components and yield a
smaller solution size and profi le.
As a cautionary note, operation of the LT3492 at a com-
bination of high switching frequency with high output
voltage and high switch current may cause excessive
internal power dissipation. Consideration should be given
to selecting a switching frequency less than 1MHz if these
conditions exist.
Connecting FADJ pin to a lowpass fi lter (R5 and C4 in
Figure 1) from the REF pin provides a soft-start function.
During start-up, FADJ voltage increases slowly from 0V
to the setting voltage. As a result, the switching frequency
increases slowly to the setting frequency. This function
limits the inrush current during start-up.
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
IN
pin of the LT3492. A
1µF or greater capacitor with low ESR should be used. A
ceramic capacitor is usually the best choice.
In the buck mode confi guration, the capacitor at PV
IN
has
large pulsed currents due to the current returned though
the Schottky diode when the switch is off. For the best
reliability, this capacitor should have low ESR and ESL
and have an adequate ripple current rating. The RMS
input current is:
I
IN(RMS)
=I
LED
•1–D
()
•D
where D is the switch duty cycle. A 1µF ceramic type ca-
pacitor placed close to the Schottky diode and the ground
plane is usually suffi cient for each channel.
Output Capacitor Selection
The selection of output fi lter capacitor depends on the load
and converter confi guration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output fi lter capacitor should be
large enough to attenuate the current ripple.
To achieve the same LED ripple current, the required fi lter
capacitor value is larger in the boost and buck-boost mode
applications than that in the buck mode applications. For
the LED buck mode applications at 1.3MHz, a 0.22µF ce-
ramic capacitor is usually suffi cient for each channel. For
the LED boost and buck-boost applications at 1.3MHz, a
1µF ceramic capacitor is usually suffi cient for each chan-
nel. Lower switching frequency requires proportionately
higher capacitor values. If higher LED current ripple can
be tolerated, a lower output capacitance can be selected
to reduce the capacitor’s cost and size.
Use only ceramic capacitors with X7R or X5R dielectric,
as they are good for temperature and DC bias stability of
the capacitor value. All ceramic capacitors exhibit loss of
capacitance value with increasing DC voltage bias, so it
may be necessary to choose a higher value capacitor to get
the required capacitance at the operation voltage. Always
check that the voltage rating of the capacitor is suffi cient.
Table 1 shows some recommended capacitor vendors.
APPLICATIONS INFORMATION
