lt3497eddb-trpbf Linear Technology Corporation, lt3497eddb-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt3497eddb-trpbf
• Description: Dual Full Function White Led Driver With Integrated Schottky Diodes
• Manufacturer: Linear Technology Corporation

LT3497
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
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.
10
200mA/DIV
50mA/DIV
50mA/DIV
20V/DIV
20V/DIV
10V/DIV
Figure 4b. Switching Waveforms with Output 1 Open Circuit
V
V
Figure 4a. Transient Response of Switcher 1 with LED1
Disconnected from the Output
V
SW1
SW2
I
CAP
SW
I
I
L1
L2
V
FRONT PAGE
APPLICATION CIRCUIT
V
4 LEDs
LED 2 DISCONNECTED
IN
IN
= 3.6V
= 3.6V
200ms/DIV
500µs/DIV
IN
pin, an inrush current fl ows
LEDs DISCONNECTED
AT THIS INSTANT
3497 F04b
3497 F04a
For low DCR inductors, which are usually the case for this
application, the peak inrush current can be simplifi ed as
follows:
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
PROGRAMMING LED CURRENT
The LED current of each LED string can be set indepen-
dently by the choice of resistors R
respectively. For each LED string, the feedback resistor
(R
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
I
R
V
PK
IN
4.2
4.2
4.2
4.2
SENSE
(V)
2
) and the sense voltage (V
V
r
•
L C
IN
L
L
1
•
•
– .
200
0 6
–
I
0.739
r (Ω)
0.58
LED
1.6
0.8
4
SENSE
mV
r
• exp –
•
2
L
2
selection are shown below.
L (µH)
15
15
15
15
•
2 2
CAP
SENSE1
C
OUT
– V
1
1
1
1
(µF)
LED
and R
) control the
I
0.828
0.682
0.794
0.803
P
SENSE2
(A)
3497f
,