LM3697_15 Datasheet, PDF (23/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

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LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
I_PEAK =
I_OUT x V_{OUT +}
V_IN
x 1 - ^{VIN x efficiency}
V_IN x efficiency 2 x ¦_SW x L
V_OUT
(5)
For DCM the peak inductor current is given by:
I_PEAK =
§ · ^{2 u IOUT}
u
© ¹ _{¶´SW u L u efficiency}
V_OUT - V_IN u efficiency
(6)
To determine which mode the circuit is operating in (CCM or DCM) it is necessary to perform a calculation to test
whether the inductor current ripple is less than the anticipated input current (I_IN). If ΔI_L is less than I_IN then the
device is operating in CCM. If ΔI_L is greater than I_IN then the device is operating in DCM.
§ ^{IOUT u VOUT}
© ^VIN ^{u efficiency}
V_IN
> _{´¶SW u L}
u
1
V_IN u efficiency _·
V_OUT
¹
(7)
Typically at currents high enough to reach the LM3697's peak current limit, the device is operating in CCM.
Figure 14 and Figure 15 show the output current and voltage derating for a 10-µH and a 22-µH inductor. These
plots take Equation 5 and Equation 6 and plot V_OUT and I_OUT with varying V_IN, a constant peak current of 880 mA
(I_{CL_MIN}), 500-kHz switching frequency, and a constant efficiency of 85%. Using these curves can give a good
design guideline on selecting the correct inductor for a given output power requirement. A 10-µH inductor will
typically be a smaller device with lower on resistance, but the peak currents is higher. A 22-µH inductor provides
for lower peak currents but a larger sized device is required to match the DC resistance of a 10-µH inductor.
0.100
0.095
0.090
0.085
0.080
0.075
0.070
0.065
0.060
0.055
0.050
0.045
0.040
0.035
0.100
22
0.095
24
0.090
26
0.085
30
34
0.080
38
0.075
0.070
0.065
0.060
22
0.055
24
26
0.050
30
0.045
34
0.040
38
0.035
VIN (V)
C052
Figure 14. Maximum Output Power (22 µH)
VIN (V)
C001
Figure 15. Maximum Output Power (10 µH)
8.2.2.1.2 Output Voltage Limited
In the case of an output voltage limited situation (V_OUT = V_OVP), when the boost output voltage hits the LM3697
device's OVP threshold, the NFET turns off and stays off until the output voltage falls below the hysteresis level
(typically 1 V below the OVP threshold). This results in the boost converter regulating the output voltage to the
programmed OVP threshold (16 V, 24 V, 32 V, or 40 V), causing the current sinks to go into dropout. The default
OVP threshold is set at 16 V. For LED strings higher than typically 4 series LEDs, the OVP has to be
programmed higher after power-up, Software Reset, or HWEN reset.
8.2.2.2 Inductor Selection
The boost circuit operates using a 4.7-µH to 22-µH inductor. The inductor selected must have a saturation
current greater than the peak operating current.
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