LM3697_15 Datasheet, PDF (22/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
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Typical Applications (continued)
8.2.1 Design Requirements
For typical LM3697 white LED applications, use the parameters listed in Table 24.
DESIGN PARAMETER
Full-scale current setting
Minimum Input Voltage
LED series/parallel configuration
LED maximum forward voltage (V_f)
Efficiency
Table 24. Design Parameters
EXAMPLE VALUE
0.0202 A
3V
7s3p
3.5 V
80%
The designer needs to know the following:
• Full-scale current setting
• Minimum input voltage
• LED series/parallel configuration
• LED maximum V_f voltage
• LM3697 Efficiency for LED configuration
The full-scale current setting, number of led strings, number of series LEDs, and minimum input voltage are
needed in order to calculate the peak input current. This information guides the designer to make the appropriate
inductor selection for the application.
The LM3697 Boost converter output voltage (V_OUT) is calculated as follows: # series LEDs × V_ƒ + 0.4 V.
The LM3697 Boost converter output current (I_OUT) is calculated as follows: # parallel LED strings × full-scale
current.
The LM3697 peak input current (I_{IN_PK}) is calculated as follows:
I_{IN_PK} ! V_OUT u I_OUT y Minimum V_IN y Efficiency
V_OUT 24.9 V 7 u 3.5 V 0.4 V
I_OUT 0.0606 A 0.0202 u 3
I_{IN_PK} ! 0.629 A 24.9 V u 0.0606 A y 3 V y 0.8
8.2.2 Detailed Design Procedure
8.2.2.1 Boost Converter Maximum Output Power
The LM3697 devices maximum output power is governed by two factors: the peak current limit (I_CL = 880 mA
min.), and the maximum output voltage (V_OUT). When the application causes either of these limits to be reached
it is possible that the proper current regulation and matching between LED current strings will not be met.
8.2.2.1.1 Peak Current Limited
In the case of a peak current limited situation, when the peak of the inductor current hits the LM3697 device's
current limit, the NFET switch turns off for the remainder of the switching period. If this happens each switching
cycle the LM3697 regulates the peak of the inductor current instead of the headroom across the current sinks.
This can result in the dropout of the boost output connected current sinks, and the LED current dropping below
its programmed level.
The peak current in a boost converter is dependent on the value of the inductor, total LED current in the boost
(I_OUT), the boost output voltage (V_OUT) (which is the highest voltage LED string + V_HR ), the input voltage (V_IN),
the switching frequency (ƒ_SW), and the efficiency (Output Power/Input Power). Additionally, the peak current is
different depending on whether the inductor current is continuous during the entire switching period (CCM), or
discontinuous (DCM) where it goes to 0 before the switching period ends. For CCM the peak inductor current is
given by:
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