Datasheet

117 mA
Imax = 227 mA + = 285 mA
2
117 mA
Imin = 227 m A = 169 mA
2
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Given the values above, an inductor with a current rating greater than 290mA is needed. Plenty of margin should
be kept to the rating in the inductor vendors data sheets as the maximum current is typically specified at a
inductance drop of 20% or even 30%. A list of tested inductors is given in Table 6 with the test conditions as
mentioned below.
Test conditions:
• Vin = 2.8V
• Vf = 3.2V (per LED)
• Vf = 0.5V (Schottky diode)
• Iout = 25mA per string; no dimming
Table 6. Tested Inductors
LED CONFIGURATION INDUCTOR TYPE INDUCTOR VALUE SUPPLIER
1 × 6LEDs LPS3015 18 μH Coilcraft
2 × 6LEDs LPS4018 47 μH Coilcraft
1 × 10LEDs LPS4018 47 μH Coilcraft
Other inductors, with lower or higher inductance values can be used. A higher inductance will cause a lower
inductor current ripple and therefore will provide higher efficiency. The boost converter will also stay in
continuous conduction mode over a wider load current range. The energy stored in an inductor is given by
E=1/2L × I
2
where I is the peak inductor current. The maximum current in the inductor is limited by the internal
current limit of the device, so the maximum power is given by the minimum peak current limit (see electrical
specifications) times the inductance value. For highest output power, a large inductance value is needed. The
minimum inductor value possible is limited by the energy needed to supply the load. The limit for the minimum
inductor value is given during the on-time of the switch such that the current limit is not reached.
Example for the minimum inductor value:
Vin = 4.2 V, Vout = 19.7 V, Iout = 5 mA, Vsw =0.1 V
→ D = 79%
→ ton = 703 ns
During the on-time, the inductor current should not reach the current limit of 1.4 A.
With V… voltage across the inductor (V = Vin–Vsw)
→ L = V × dt/di = (4.2 V–0.1 V) × 703ns/1.4A = 2μH
Diode Selection
Due to the non-synchronous design of the boost converter, an external diode is needed. For best performance, a
Schottky diode with a voltage rating of 40V or above should be used. A diode such as the MBR0540 with an
average current rating of 0.5A is sufficient.
Output Capacitor Selection
A ceramic capacitor such as X5R or X7R type is required at the output. See Table 7 for reference.
Table 7. Tested Capacitor
TYPICAL VOLTAGE
LED CAPACITOR CAPACITOR
ACROSS OUTPUT CAPACITOR TYPE MANUFACTURER
CONFIGURATION VALUE SIZE
CAPACITOR
2x6LEDs or 1x6LEDs 21 V 4.7 μF / 50 V 1206 UMK316BJ475KL Taiyo Yuden
1x10LEDs 35 V 4.7 μF / 50 V 1210 GRM32ER71H475KA Murata
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