Datasheet LT3595 (Analog Devices) - 9

LT3595
OPERATION Discontinuous Current Mode
The LT3595 can drive a 10-LED string at 15mA LED cur- VSW 20V/DIV rent operating in continuous conduction mode using the recommended external components shown in the ap- plication circuit on page 1 of this data sheet. As current IL 20mA/DIV is further reduced, the regulator enters discontinuous conduction mode. The photo in Figure 1 details circuit operation driving ten LEDs at 10mA load. The inductor 3595 F01 400ns/DIV current reaches zero during the discharge phase and the VIN = 45V SW pin exhibits ringing. The ringing is due to the LC tank 10 LEDS AT 10mA circuit formed by the inductor in combination with the
Figure 1. Switching Waveforms
switch and diode capacitance. This ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions.
TYPICAL APPLICATIONS Inductor Selection Table 1. Inductor Manufacturers INDUCTANCE
A 100μH inductor is recommended for most LT3595
RANGE (μH)
applications. Although small size and high effi ciency are
PART RELEVANT TO DIMENSIONS
major concerns, the inductor should have low core losses
VENDOR SERIES LT3595 (mm)
at 2MHz and low DCR (copper wire resistance). Some Coilcraft DO1605 100 to 680 5.4 × 4.2 × 1.8 www.coilcraft.com LPS4012 100 to 680 4 × 4 × 1.2 inductors that meet these criteria are listed in Table 1. An 1812FS 100 to 680 5.8 × 4.9 × 3.8 effi ciency comparison of different inductors is shown in MSS5131 100 to 390 5.1 × 5.1 × 3.1 Figure 2. Sumida CDC4D20 100 to 680 4.8 × 4.8 × 2 www.sumida.com 100 Toko D53LC 100 to 680 5.2 × 5.4 × 3 COILCRAFT 1812FS-104KLB www.tokoam.com COILCRAFT LPS4012-104MLB TOKO A915AY-101M TDK VLCF4020T 100 to 330 4 × 4 × 2 TDK VLCF4020T-101MR26 www.component. COILTRONICS SD3812-101 95 COILTRONICS SD52-101 tdk.com Coiltronics SD3812 100 to 330 4 × 4 × 1.2 www.cooperet.com SD52 100 to 330 5.6 × 5.2 × 2 EFFICIENCY (%) Murata LQH32M 100 to 560 3.2 × 2.5 × 2 90 www.murata.com LQH43M 100 to 680 4.5 × 3.2 × 2
Capacitor Selection
850.01 0.1 1 10 100 The small size of ceramic capacitors make them ideal for ILED (mA) 3595 F02 LT3595 applications. Only X5R and X7R types should be VIN = 45V L = 100μH 10 LEDS used because they retain their capacitance over wider voltage and temperature ranges than other types such
Figure 2. Effi ciency Comparison of Different Inductors
as Y5V or Z5U. Typically, 10μF capacitors on VIN and VCC are suffi cient. The output capacitor used across the 3595f 9