BQ24100 Datasheet, PDF (20/26 Pages) Texas Instruments – SYNCHRONOUS SWITCHMODE, LI-ION AND LI-POL CHARGE MANAGEMENT IC WITH INTEGRATED POWERFETS

bq24100, bq24103
bq24105, bq24108
bq24113, bq24115
SLUS606C – JUNE 2004 – REVISED SEPTEMBER 2005
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PCB LAYOUT CONSIDERATION (continued)
to provide an effective thermal contact between the IC and the PCB. Full PCB design guidelines for this
package are provided in the application report entitled: QFN/SON PCB Attachment (SLUA271). Six 10-13 mil
vias are a minimum number of recommended vias, placed in the IC's power pad, connecting it to a ground
thermal plane on the opposite side of the PWB. This plane must be at the same potential as V_SS and PGND
of this IC.
• See user guide SLUU200 for an example of good layout.
WAVEFORMS: All waveforms are taken at Lout (IC Out pin). V_IN = 7.6 V and the battery was set to 2.6 V, 3.5 V,
and 4.2 V for the three waveforms. When the top switch of the converter is on, the waveform is at ~7.5 V, and
when off, the waveform is near ground. Note that the ringing on the switching edges is small. This is due to a
tight layout (minimized loop areas), a shielded inductor (closed core), and using a low-inductive scope ground
lead (i.e., short with minimum loop) .
Precharge: The current is low in precharge; so, the bottom synchronous FET turns off after its minimum on-time
which explains the step between ~0 V and -0.5 V. When the bottom FET and top FET are off, the current
conducts through the body diode of the bottom FET which results in a diode drop below the ground potential.
The initial negative spike is the delay turning on the bottom FET, which is to prevent shoot-through current as the
top FET is turning off.
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