LT3496
10
3496ff
dimming waveforms and the start-up time should be
checked across all operating conditions.
Open-LED Protection
The LT3496 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 LT3496 switching frequency is controlled by f
ADJ
pin
voltage. Setting f
ADJ
voltage to be less than 1V will reduce
switching frequency.
If f
ADJ
voltage is higher than 1V, the default switching
frequency is 2.1MHz. In general, a lower switching fre-
quency should be used where either very high or very
low switch duty cycle is required or higher efficiency is
desired. Selection of a higher switching frequency will
allow use of low value external components and yield a
smaller solution size and profile.
Connecting f
ADJ
pin to a lowpass filter (R5 and C4 in
Figure 1) from the REF pin provides a soft-start function.
During start-up, f
ADJ
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.
Undervoltage Lockout
The LT3496 has an undervoltage lockout circuit that
shuts down all the three converters when the input volt-
age drops below 2.4V. This prevents the converter from
switching in an erratic mode when powered from a low
supply voltage.
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
IN
pin of the LT3496. A
1µF or greater capacitor with low ESR should be used. A
ceramic capacitor is usually the best choice.
In the buck mode configuration, 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 sufficient for each channel.
Output Capacitor Selection
The selection of output filter capacitor depends on the load
and converter configuration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output filter capacitor should be
large enough to attenuate the current ripple.
To achieve the same LED ripple current, the required filter
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, a 0.22µF ceramic capacitor
is usually sufficient for each channel. For the LED boost
and buck-boost applications, a 1µF ceramic capacitor is
usually sufficient for each channel. 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 sufficient.
Table 1 shows some recommended capacitor vendors.
applications inForMation
