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3
Circuit Design and Layout Guideline
3.1
Optional Components
3.2
Layout Tips
Circuit Design and Layout Guideline
The bq24720/21 EVM has all components required for robust operation on a wide range of different
operating conditions and ac adapter/load transients. However, some of its components might not be
required, or can be of distinct value, depending on the application condition. See the bq24720/21
application notes for details.
Component
When it must be added to application circuit
D2/R8
Turn-on time for the bypass switch Q1 must be increased.
D3/R12
Turn-on time for the ac switch Q2 must be increased.
D6/R24
Turn-on time for the battery switch Q5 must be increased.
C17/C19
Switching time for the ac switch Q2 must be increased.
R16/R17
(1)
Switching time for the switch Q3 must be increased.
R19
Switching time for the switch Q4 must be increased.
C22/C24/C28/C29
These capacitors are always required for PWM ripple current filtering. The value may be decreased as long
value
as the low current ripple, low transient voltage spike, and loop response stability can be assured.
(1)
Use bootstrap resistor R16 to adjust the switching time of the high-side switch. Do not use high-side and low-side gate resistors.
The high-side gate resistor R17 must be lower than 5
Ω
to assure the break-before-make.
1. The control stage and the power stage are layed out separately. At each layer, the signal ground and
the power ground are connected only at the power pad with a single point underneath the IC.
2. the ac current sense resistor must be connected to the ACP (pin 4) and ACN (pin 3) by Kelvin contact.
The area of this loop must be minimized. The decoupling capacitors for these pins should be placed as
close as possible to the IC .
3. the charge current sense resistor must be connected to SRP (pin 20), SRN (pin 19), SYNP (pin 22)
and SYNN (pin 21) by Kelvin contact. The area of this loop must be minimized. The decoupling
capacitors for these pins should be placed as close as possible to the IC .
4. The decoupling capacitors for the VCC (pin 12), VREF5 (pin 11), REGN (pin 28) should be placed
underneath the IC (on the bottom layer). Make the interconnections to the IC as short as possible.
5. The decoupling capacitors for BAT (pin 18), IOUT (pin 17) must be placed close to the corresponding
IC pins. Make the interconnections to the IC as short as possible.
6. C17 and C19, if used, must be placed as close as possible to Q2 (AC switch) .
7. The decoupling capacitor (C15) for the charger input must be placed close to Q3 drain and Q4 source.
SLUU237 – January 2006
bq24720/21 EVM (HPA104) for Multi-Cell Synchronous
21
Switch-Mode Charger With System Power Selector and SMBus SBS-Like Interface