MAX1645B Datasheet, PDF (29/32 Pages) Maxim Integrated Products – Advanced Chemistry-Independent, Level 2 Battery Charger with Input Current Limiting

Advanced Chemistry-Independent, Level 2
Battery Charger with Input Current Limiting
D3 is a signal-level diode, such as the 1N4148. This
diode provides the supply current to the high-side
MOSFET driver.
The P-channel MOSFET P2 delivers the current to the
load when the AC adapter is removed. Select a
MOSFET with an R_DS(ON) of 50mΩ or less to minimize
power loss and voltage drop.
Inductor Selection
Inductor L1 provides power to the battery while it is
being charged. It must have a saturation current of at
least 3A plus one-half of the current ripple (∆I_L):
I_SAT = 3A + ¹/2 ∆I_L
The controller determines the constant off-time period,
which is dependent on BATT voltage. This makes the
ripple current independent of input and battery voltage
and should be kept to less than 1A. Calculate the ∆I_L
with the following equation:
∆I_L = 21Vµs / L
Higher inductor values decrease the ripple current.
Smaller inductor values require higher saturation cur-
rent capabilities and degrade efficiency. Typically, a
22µH inductor is ideal for all operating conditions.
Table 9. Component Suppliers
COMPONENT
Inductor
MOSFET
Sense resistor
Capacitor
Diode
MANUFACTURER
PART
Sumida
CDRH127 series
Coilcraft
Coiltronics
D03316P series
UP2 series
Internal Rectifier IRF7309
Fairchild
Vishay-Siliconix
FDS series
Si4435/6
Dale
IRC
WSL series
LR2010-01 series
AVX
TPS series,
TAJ series
Sprague
595D series
Motorola
1N5817–1N5822
Nihon
NSQ03A04
Central
Semiconductor
CMSH series
Other Components
CCV, CCI, and CCS are the compensation points for the
three regulation loops. Bypass CCV with a 10kΩ resistor
in series with a 0.01µF capacitor to GND. Bypass CCI
and CCS with 0.01µF capacitors to GND. R7 and R13
serve as protection resistors to THM and CVS, respec-
tively. To achieve acceptable accuracy, R6 should be
10kΩ and 1% to match the internal battery thermistor.
Current-Sense Input Filtering
In normal circuit operation with typical components, the
current-sense signals can have high-frequency tran-
sients that exceed 0.5V due to large current changes
and parasitic component inductance. To achieve prop-
er battery and input current compliance, the current-
sense input signals should be filtered to remove large
common-mode transients. The input current-limit sens-
ing circuitry is the most sensitive case due to large cur-
rent steps in the input filter capacitors (C1 and C2) in
Figure 1. Use 1µF ceramic capacitors from CSSP and
CSSN to GND. Smaller 0.1µF ceramic capacitors can
be used on the CSIP and CSIN inputs to GND since the
current into the battery is continuous. Place these
capacitors next to the single-point ground directly
under the MAX1645B.
Layout and Bypassing
Bypass DCIN with a 1µF to GND (Figure 1). D4 protects
the device when the DC power source input is
reversed. A signal diode for D4 is adequate as DCIN
only powers the LDO and the internal reference.
Bypass LDO, BST, DLOV, and other pins as shown in
Figure 1.
Good PC board layout is required to achieve specified
noise, efficiency, and stable performance. The PC
board layout artist must be given explicit instructions,
preferably a pencil sketch showing the placement of
power-switching components and high-current routing.
A ground plane is essential for optimum performance.
In most applications, the circuit is located on a multilay-
er board, and full use of the four or more copper layers
is recommended. Use the top layer for high-current
connections, the bottom layer for quiet connections
(REF, CCV, CCI, CCS, DAC, DCIN, V_DD, and GND),
and the inner layers for an uninterrupted ground plane.
Use the following step-by-step guide:
1) Place the high-power connections first, with their
grounds adjacent:
• Minimize current-sense resistor trace lengths and
ensure accurate current sensing with Kelvin con-
nections.
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