LM3248 Datasheet, PDF (21/31 Pages) Texas Instruments – LM3248 2.7 MHz, 2.5A Adjustable Boost-Buck DC/DC Converter

LM3248
www.ti.com
SNVSA01A – JULY 2013 – REVISED SEPTEMBER 2013
To help minimize noise coupled back into power supplies:
• Use a star connection to route from the V_BATT power source to the LM3248 PVIN_BOOST pins and to
the PA device VBATT pins.
• Include sufficient decoupling capacitance, for both low and high frequency, at the PA V_BATT connections.
• Route traces for minimum inductance (short, wide connections) between supply pins and bypass capacitor(s).
• Route traces to minimize inductance between bypass capacitors and the ground plane.
• Utilize necessary power supply trace inductance(s) to reduce coupling between function blocks.
• Inserting a ferrite bead in series with the V_BATT power supply trace may offer a favorable tradeoff between
board area and additional shunt bypass capacitors, by attenuating noise that might otherwise propagate
through the supply connections.
4. LM3248 RF Evaluation Board
VBATT
C34
1 nF
C1
10 nF
L1
1.0 µH
U2
C27
1 µF
Lo ESL
PVIN_ SW_BOOST
C2 _BOOST
10 µF
C28
10 µF
SW_
BUCK
GPO1
GPO2
GPO3
DAC
BB or
RFIC
ENABLE
LM3248
BOOST GATE
FB_
BUCK
MODE
ACB
VCON _{PGND SGND BGND}
VBATT_PA
L3
1.5 µH
C19
330 pF
VOUT
C7
10 µF
C8
3.3 nF
VCC_PA_3G
PA
C20
2.2 µF
VCC_PA_2G
C3
C5
4.7 µF 3.3 nF
PA(s) VBATT_PA
Figure 29. Simplified LM3248 RF Evaluation Board Schematic
LM3248 PCB Placement Considerations
1. Input Capacitor C2 should be placed closer to LM3248 than C1.
2. Optional to add a 1 nF (C34) capacitor on the input of LM3248 for high frequency filtering.
3. Bulk Output Capacitor C7 should be placed closer to LM3248 than C8.
4. Connect GND terminals of C7, C8 and C34 directly to System RF GND layer of phone board.
5. Connect GND terminals of boost output C27 and buck input C28 capacitors to copper PGND island on the
component side. This island is then tied to the system ground layer through several vias.
6. Connect bumps SGND_BOOST(B2), SGND_BUCK(C3), DGND(E3), and BGND (F3) directly to system
ground layer.
7. Small high-frequency filtering capacitors (e.g., C34) work better when they are connected to system ground
instead of the PGND island. These capacitors should be 0201 (0603 metric) or 01005 (0402 metric) case
size for minimum footprint and best high frequency characteristics.
Copyright © 2013, Texas Instruments Incorporated
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