LM3697_15 Datasheet, PDF (33/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

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10 Layout
LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
10.1 Layout Guidelines
The LM3697 device's inductive boost converter sees a high switched voltage (up to V_OVP) at the SW pin, and a
step current (up to I_{CL_BOOST}) through the Schottky diode and output capacitor each switching cycle. The high
switching voltage can create interference into nearby nodes due to electric field coupling (I = CdV/dt). The large
step current through the diode and the output capacitor can cause a large voltage spike at the SW pin and the
OVP pin due to parasitic inductance in the step current conducting path (V = Ldi/dt). Board layout guidelines are
geared towards minimizing this electric field coupling and conducted noise. Figure 57 highlights these two noise-
generating components.
VOUT + VF Schottky
Voltage Spike
(VSPIKE)
Pulsed voltage at SW
Current through
Inductor
IPEAK
IAVE = IIN
Affected Node
due to Capacitive Coupling
2.7 V to 5.5 V
Cp1
L
Lp1 D1 Lp2
SW
IN
CIN
LM3697
PWM
HWEN
OVP
Current through
Schottky and
COUT
Parasitic
Circuit Board
Inductances
Up to 40V
COUT
Lp3
LCD Display
SCL
SDA
HVLED1
HVLED2
HVLED3
GND
Figure 57. LM3697 Inductive Boost Converter Showing Pulsed Voltage at SW (High Dv/Dt) and Current
Through Schottky And COUT (High Di/Dt)
The following list details the main (layout sensitive) areas of the LM3697 device’s inductive boost converter in
order of decreasing importance:
1. Output Capacitor
– Schottky Cathode to COUT+
– COUT− to GND
2. Schottky Diode
– SW pin to Schottky Anode
– Schottky Cathode to COUT+
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