BQ24640 Datasheet, PDF (20/25 Pages) Texas Instruments – High-Efficiency Synchronous Switch-Mode Super Capacitor Charger

bq24640
SLUSA44 – MARCH 2010
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A cost effective and small size solution is shown in Figure 13. The R1 and C1 are composed of a damping RC
network to damp the hot plug-in oscillation. As a result the over voltage spike is limited to a safe level. D1 is used
for reverse voltage protection for VCC pin. C2 is VCC pin decoupling capacitor and it should be place to VCC pin
as close as possible. The R2 and C2 form a damping RC network to further protect the IC from high dv/dt and
high voltage spike. C2 value should be less than C1 value so R1 can dominant the equivalent ESR value to get
enough damping effect for hot plug-in. R1 and R2 package must be sized enough to handle inrush current power
loss according to resistor manufacturer’s datasheet. The filter components value always need to be verified with
real application and minor adjustments may need to fit in the real application circuit.
D1
Adapter
Connector
R1 (2010)
2W
C1
2.2 mF
R2 (1206)
4.7 - 30 W
VCC pin
C2
0.1 - 1 mF
Figure 13. Input Filter
PCB LAYOUT
The switching node rise and fall times should be minimized for minimum switching loss. Proper layout of the
components to minimize high frequency current path loop (see Figure 14) is important to prevent electrical and
magnetic field radiation and high frequency resonant problems. Here is a PCB layout priority list for proper
layout. Layout PCB according to this specific order is essential.
1. Place input capacitor as close as possible to switching MOSFET’s supply and ground connections and use
shortest copper trace connection. These parts should be placed on the same layer of PCB instead of on
different layers and using vias to make this connection.
2. The IC should be placed close to the switching MOSFET’s gate terminals and keep the gate drive signal
traces short for a clean MOSFET drive. The IC can be placed on the other side of the PCB of switching
MOSFETs.
3. Place inductor input terminal to switching MOSFET’s output terminal as close as possible. Minimize the
copper area of this trace to lower electrical and magnetic field radiation but make the trace wide enough to
carry the charging current. Do not use multiple layers in parallel for this connection. Minimize parasitic
capacitance from this area to any other trace or plane.
4. The charging current sensing resistor should be placed right next to the inductor output. Route the sense
leads connected across the sensing resistor back to the IC in same layer, close to each other (minimize loop
area) and do not route the sense leads through a high-current path (see Figure 14 for Kelvin connection for
best current accuracy). Place decoupling capacitor on these traces next to the IC.
5. Place output capacitor next to the sensing resistor output and ground.
6. Output capacitor ground connections need to be tied to the same copper that connects to the input capacitor
ground before connecting to system ground.
7. Route analog ground separately from power ground and use single ground connection to tie charger power
ground to charger analog ground. Just beneath the IC use analog ground copper pour but avoid power pins
to reduce inductive and capacitive noise coupling. Connect analog ground to GND pin. using PowerPAD as
the single ground connection point to connect analog ground and power ground together. Or using a 0Ω
resistor to tie analog ground to power ground (PowerPAD should tie to analog ground in this case). A
star-connection under PowerPAD is highly recommended.
8. It is critical that the exposed PowerPAD on the backside of the IC package be soldered to the PCB ground.
Ensure that there are sufficient thermal vias directly under the IC, connecting to the ground plane on the
other layers.
9. Decoupling capacitors should be placed next to the IC pins and make trace connection as short as possible.
10. All via size and number should be enough for a given current path.
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