LM3697_15 Datasheet, PDF (24/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
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8.2.2.3 Output Capacitor Selection
The LM3697's inductive boost converter requires a 1-µF (X5R or X7R) ceramic capacitor to filter the output
voltage. The voltage rating of the capacitor depends on the selected OVP setting. For the 16 V setting a 16-V
capacitor must be used. For the 24-V setting a 25-V capacitor must be used. For the 32-V setting, a 35-V
capacitor must be used. For the 40-V setting a 50-V capacitor must be used. Pay careful attention to the
capacitor's tolerance and DC bias response. For proper operation the degradation in capacitance due to
tolerance, DC bias, and temperature, must stay above 0.4 µF. This might require placing two devices in parallel
in order to maintain the required output capacitance over the device operating range, and series LED
configuration.
8.2.2.4 Schottky Diode Selection
The Schottky diode must have a reverse breakdown voltage greater than the LM3697 device’s maximum output
voltage (see Overvoltage Protection (Inductive Boost) section). Additionally, the diode must have an average
current rating high enough to handle the LM3697’s maximum output current, and at the same time the diode's
peak current rating must be high enough to handle the peak inductor current. Schottky diodes are required due
to their lower forward voltage drop (0.3 V to 0.5 V) and their fast recovery time.
8.2.2.5 Input Capacitor Selection
The LM3697 device's inductive boost converter requires a 2.2-μF (X5R or X7R) ceramic capacitor to filter the
input voltage. The input capacitor filters the inductor current ripple and the internal MOSFET driver currents
during turn on of the internal power switch.
8.2.2.6 Application Circuit Component List
COMPONENT MANUFACTURER
L
C_OUT
C_IN
Diode
TDK
TDK
TDK
On-Semi
VALUE
10 µH
1.0 µF
2.2 µF
Schottky
PART NUMBER
VLF302512MT-100M
C2012X5R1H105
C1005X5R1A225
NSR0240V2T1G
SIZE (mm)
2.5 x 3.0 x 1.2
0805
0402
SOD-523
CURRENT/VOLTAGE RATING
(RESISTANCE)
620 mA/0.25 Ω
50 V
10 V
40 V, 250 mA
8.2.3 Application Performance Plots
V_IN = 3.6 V, full-scale current = 20.2 mA, LEDs are WLEDs part # SML-312WBCW(A), Typical Application Circuit , T_A = 25°C
unless otherwise specified. Efficiency is V_OUT × (I_HVLED1 + I_HVLED2 + I_HVLED3)/(V_IN × I_IN), matching curves are (ΔI_{LED_MAX}/I_{LED_AVE}).
Three String, L=22µH, 500kHz
92%
90%
88%
86%
84%
82%
80%
78%
76%
74%
72%
70%
2.5 3 3.5 4 4.5 5 5.5
VIN (V)
C002
Top to Bottom: 3x3, 3x4, 3x5, 3x6, 3x7 (LEDs)
Figure 16. Boost Efficiency vs V_IN
Three String, L=22µH, 1MHz
92%
90%
88%
86%
84%
82%
80%
78%
76%
74%
72%
70%
2.5 3 3.5 4 4.5 5 5.5
VIN (V)
C002
Top to Bottom: 3x3, 3x4, 3x5, 3x6, 3x7 (LEDs)
Figure 17. Boost Efficiency vs V_IN
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