ZXLD1350EV4
ZXLD1350EV4 User Guide Iss. 1
7/8
10-04-07
Changing the LED current
1. Switch off the power supply.
2. Remove Rs.
3. Calculate and fit a new sense resistor, Rs, the value of which is based on the required LED current without
dimming. Rs can be calculated using following equation :
Rs = 0.1V/I
OUT
where I
OUT
= the LED current.
Rs = the sense resistor value in ohms.
Do not use a resistor value lower than 0.27
Ω.
0.1V is the nominal sense voltage with ADJ open circuit or set to 1.25V.
4. Follow the ‘ZXLD1350EV4 Power Up’ sequence.
Using external LEDs or loads
1. Switch off the power supply.
2. Connect external LEDs across test pins ‘LED a’ and ‘LED k’. ‘LED a’ is the LEDs' anode connection point
and ‘LED k’ is the LEDs' cathode connection point. The number of external LEDs that can be connected
depends on their operating power and forward voltage drop. For an external load other than LEDs, the
positive terminal of the load should be connected to test pin ‘LED a’ and the negative terminal of the load
should be connected to test pin ‘LED k’.
3. Follow the ‘ZXLD1350EV4 Power Up’ sequence.
PERFORMANCE
The system efficiency depends on the sense resistor, supply voltage, switching inductor and the number of 1W LEDs
connected in series.
The graph below shows the efficiency with a 0.33
Ω
sense resistor R
S,
and a 100uH inductor, for 1 to 3 series
connected 1W LEDs.
With a 24V supply, the switching frequency is typically 545kHz for three series-connected 1-watt LEDs and 300kHz
for a single 1-watt LED.
With a 12V supply, the switching frequency is typically 160kHz for three series-connected 1-watt LEDs and 280kHz
for a single 1-watt LED
The detailed performance information for the device can be found in the datasheets.
Figure 5: Efficiency vs supply voltage
Efficiency vs Vin
Rsense=0.33 Ohms, L=100uH (CoilCraft MSS6132)
75
80
85
90
95
0
5
10
15
20
25
30
35
Vin (V)
Eff (%)
1 LED
2 LED
3 LED
