Datasheet

LT3497
10
3497f
APPLICATIONS INFORMATION
In the event one of the converters has an output open
circuit, its output voltage will be clamped at 32V. However,
the other converter will continue functioning properly.
The photo in Figure 4b shows circuit operation with
converter 2 output open circuit and converter 1 driving
4 LEDs at 20mA. Converter 2 starts switching at a lower
peak inductor current and begins skipping pulses, thereby
reducing its input current.
INRUSH CURRENT
The LT3497 has built-in Schottky diodes. When supply
voltage is applied to the V
IN
pin, an inrush current fl ows
through the inductor and the Schottky diode and charges
up the CAP voltage. Both the Schottky diodes in the LT3497
can sustain a maximum current of 1A. The selection of
inductor and capacitor value should ensure the peak of
the inrush current to be below 1A.
For low DCR inductors, which are usually the case for this
application, the peak inrush current can be simplifi ed as
follows:
α
ω
ω
α
ω
π
=
=
=
r
L
LC
r
L
I
V
L
PK
IN
2
1
4
06
2
2
•
•
–
•
–.
•
• exp – •
22
⎛
⎝
⎜
⎞
⎠
⎟
where L is the inductance, r is the DCR of the inductor
and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3: Inrush Peak Currents
V
IN
(V) r (Ω)L (µH)C
OUT
(µF) I
P
(A)
4.2 0.58 15 1 0.828
4.2 1.6 15 1 0.682
4.2 0.8 15 1 0.794
4.2 0.739 15 1 0.803
PROGRAMMING LED CURRENT
The LED current of each LED string can be set indepen-
dently by the choice of resistors R
SENSE1
and R
SENSE2
,
respectively. For each LED string, the feedback resistor
(R
SENSE
) and the sense voltage (V
CAP
– V
LED
) control the
LED current.
For each independent LED string, the CTRL pin controls
the sense reference voltage as shown in the Typical
Performance Characteristics. For CTRL higher than 1.5V,
the sense reference is 200mV, which results in full LED
current. In order to have accurate LED current, precision
resistors are preferred (1% is recommended). The formula
and Table 4 for R
SENSE
selection are shown below.
R
mV
I
SENSE
LED
=
200
Figure 4a. Transient Response of Switcher 1 with LED1
Disconnected from the Output
Figure 4b. Switching Waveforms with Output 1 Open Circuit
I
SW
200mA/DIV
V
CAP
10V/DIV
V
IN
= 3.6V
FRONT PAGE
APPLICATION CIRCUIT
500µs/DIV
3497 F04a
LEDs DISCONNECTED
AT THIS INSTANT
I
L1
50mA/DIV
I
L2
50mA/DIV
V
SW1
20V/DIV
V
SW2
20V/DIV
V
IN
= 3.6V
4 LEDs
LED 2 DISCONNECTED
200ms/DIV
3497 F04b