MAX8798ETX+ Maxim Integrated Products, MAX8798ETX+ Datasheet - Page 27

• Manufacturer Part Number: MAX8798ETX+
• Description: IC INTERNAL-SW BOOST REG 36-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8798ETX+

• Applications: LCD Monitor, Notebook Display
• Current - Supply: 400µA
• Voltage - Supply: 1.8 V ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-TQFN Exposed Pad
• Operating Supply Voltage: 1.8 V to 5.5 V
• Maximum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Maximum Power Dissipation: 2179.8 mW
• Minimum Operating Temperature: + 85 C
• Supply Current: 4 uA
• Input Voltage: 4V
• Output Current: 400mA
• Output Voltage: 18V
• No. Of Outputs: 2
• Power Dissipation Pd: 2.18W
• Supply Voltage Range: 1.8V To 5.5V
• No. Of Pins: 36
• Operating Temperature Range: -40°C To +85°C
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Careful PCB layout is important for proper operation.
Use the following guidelines for good PCB layout:
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Integrated 3-Channel Scan Driver for TFT LCDs
Minimize the area of high-current loops by placing
the inductor, output diode, and output capacitors
near the input capacitors and near LX and PGND.
The high-current input loop goes from the positive
terminal of the input capacitor to the inductor, to the
IC’s LX pin, out of PGND, and to the input capaci-
tor’s negative terminal. The high-current output loop
is from the positive terminal of the input capacitor to
the inductor, to the output diode (D1), to the posi-
tive terminal of the output capacitors, reconnecting
between the output capacitor and input capacitor
ground terminals. Connect these loop components
with short, wide connections. Avoid using vias in
the high-current paths. If vias are unavoidable, use
many vias in parallel to reduce resistance and
inductance.
Create a power ground island (PGND) consisting of
the input and output capacitor grounds, PGND pin,
and any charge-pump components. Connect all
these together with short, wide traces or a small
ground plane. Maximizing the width of the power
ground traces improves efficiency and reduces out-
put-voltage ripple and noise spikes. Create an ana-
log ground plane (AGND) consisting of the AGND
pin, all the feedback-divider ground connections, the
operational-amplifier-divider ground connections,
the COMP capacitor ground connection, the
______________________________________________________________________________________
PCB Layout and Grounding
Internal-Switch Boost Regulator with
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Refer to the MAX8798 evaluation kit for an example of
proper board layout.
BOOST and VL bypass capacitor ground connec-
tions, and the device’s exposed backside paddle.
Connect the AGND and PGND islands by connect-
ing the PGND pin directly to the exposed backside
paddle. Make no other connections between these
separate ground planes.
Place the feedback-voltage-divider resistors as
close as possible to the feedback pin. The divider’s
center trace should be kept short. Placing the resis-
tors far away causes the FB trace to become an
antenna that can pick up switching noise. Care
should be taken to avoid running the feedback
trace near LX or the switching nodes in the charge
pumps.
Place the IN pin and VL pin bypass capacitors as
close as possible to the device. The ground con-
nections of the IN and VL bypass capacitors should
be connected directly to the AGND pin with a wide
trace.
Minimize the length and maximize the width of the
traces between the output capacitors and the load
for best transient responses.
Minimize the size of the LX node while keeping it
wide and short. Keep the LX node away from the
feedback node and analog ground. Use DC traces
as shield if necessary.
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