User Guide 035
UG035.0
June 15, 2015
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3
PCB Layout Recommendation
The PCB layout is very important for normal functioning of the
ISL78171, to ensure the system works with low EMI. The main
power loop is composed of the input capacitor, boost inductor,
the output capacitor, the LX pin and the PGND pin. It is necessary
to make the power loop as small as possible and the connecting
traces among them should be direct, short and wide. The
switching node of the boost converter (LX pins) and the traces
connected to the node are noisy, so keep the low level signals
away from these noisy traces. The input capacitor should be
placed as close as possible to the VIN and GND pins. The ground
of input and output capacitors should be connected as close as
possible. The heat of the IC is mainly dissipated through the
thermal pad. Maximizing the copper area connected to the
thermal pad is preferable. In addition, a solid ground plane is
helpful for better EMI performance. It is recommended to add at
least 6 vias ground connection within the pad for the best
thermal conduction. Keep the PGND plane and AGND plane
separate and connect them at the thermal pad.
EMI Considerations
The LX node switches at the VIN potential so its capacitance to
GND should be made minimized. This is achieved by keeping the
LX copper island as small as possible and by opening the copper
GND area directly below it. This prevents injection of the
switching frequency noise directly into the GND, and reduces the
conducted EMI for the system. The evaluation board also has an
input LC filter option on it ie., L2/L2A and C
23
. These
components are not populated on the PCB but can be soldered in
for the purpose of EMI evaluation if needed. The snubber R
3
and
C
21
helps to reduce the peak voltage seen on the LX pin.
FIGURE 2. ISL78171 BLOCK DIAGRAM
40V, 6 x 50mA*
GENERATOR
V
IN
COMP
+
-
+
-
CH0
CH5
V
IN
= 4.5V TO 26.5V
FET
DRIVER
REG
ISL78171
RAMP
COMP
IMAX ILIMIT
PWM/OC/SC
PWM BRIGHTNESS CONTROL
DEVICE CONTROL
FAULT/STATUS
IDENTIFICATION
REGISTERS
HIGHEST VF
STRING
DETECT
DC BRIGHTNESS CONTROL
VDC
SENSOR
FPWM
LED PWM
CONTROL
PGND
CONFIGURATION
+
-
AGND
REFERENCE
SMBCLK/SCL
SMBDAT/SDA
GM
AMP
+
-
+
-
LOGIC
OVP
LX
OSC AND
= 0
DC
OC, SC
DETECT
FAULT/STATUS
REGISTER
TEMP
FAULT/STATUS
REGISTER
PWM
RSET
+
-
OC, SC
DETECT
SMBUS/I
2
C
INTERFACE
EN
*V
IN
> 12V
AND PWM
CONTROL
LOGIC
OVP
FAULT
L=15
u
H
DIODE, 60V, 3A
SCHOTTKY
FAULT/STATUS
REGISTER
