LT3593
APPLICATIONS INFORMATION
Overvoltage Protection
The LT3593 has an internal open-circuit protection circuit.
In the case of an output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail open, V CAP
is clamped at 38V. The LT3593 will then switch at a very
low frequency to minimize input current. V CAP and input
current during output open circuit are shown in the Typical
Performance Characteristics. Figure 5 shows the transient
response when the LEDs are disconnected.
Low Input Voltage Applications
The LT3593 can be used in low input voltage applications.
The input supply voltage to the LT3593 must be 2.7V or
higher, but the inductor can be run off a lower battery
voltage. This technique allows the LEDs to be powered off
two alkaline cells. Most portable devices have a 3.3V logic
supply voltage which can be used to power the LT3593.
The LEDs can be driven straight from the battery, resulting
in higher ef?ciency.
Figure 6 shows three LEDs powered by two AA cells.
The battery is connected to the inductors and the chip is
powered off a 3.3V logic supply voltage.
L1
2 AA CELLS
2V TO 3.2V
C1
10μH
D1
C2
1μF
SW
1μF
I L
3.3V
C3
1μF
V IN
LT3593
CAP
200mA/DIV
V CAP
SHUTDOWN AND
CURRENT CONTROL
CTRL
GND
LED
20V/DIV
V SW
3593 F06
20V/DIV
C1: TAIYO YUDEN EMK107B5105MA
V IN = 3.6V
400μs/DIV
3593 F05
FRONT PAGE LEDs DISCONNECTED
APPLICATION CIRCUIT AT THIS INSTANT
Figure 5. Output Open-Circuit Waveform
C2: MURATA GRM31MR71H105KA88
L1: MURATA LQH43CN100
D1: CENTRAL CMDSH05-4
Figure 6. Two AA Cells to Three White LEDs
3593f
9
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