MAX1644 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
istry independent and, since it is defined as an SMBus
slave device only, the smart battery charger is relatively
inexpensive and easy to implement.
Selecting External Components
Table 9 lists the recommended components and refers
to the circuit of Figure 1; Table 8 lists the suppliers’
contacts. The following sections describe how to select
these components.
MOSFETs and Schottky Diodes
Schottky diode D1 provides power to the load when the
AC adapter is inserted. Choose a 3A Schottky diode 3A
or higher. This diode may not be necessary if P1 is
used. The P-channel MOSFET P1 turns on when VCVS >
VBATT. This eliminates the voltage drop and power con-
sumption of the Schottky diode. To minimize power loss,
select a MOSFET with an RDS(ON) of 50mΩ or less. This
MOSFET must be able to deliver the maximum current
as set by R1. D1 and P1 provide protection from
reversed voltage at the adapter input.
The N-channel MOSFETs N1 and N2 are the switching
devices for the buck controller. High-side switch N1
should have a current rating of at least 6A and have an
RDS(ON) of 50mΩ or less. The driver for N1 is powered
by BST; its current should be less than 10mA. Select a
MOSFET with a low total gate charge and determine
the required drive current by IGATE = QGATE · f (where f
is the DC-DC converter maximum switching frequency
of 400kHz).
Table 8. Components Suppliers
COMPONENT
Inductor
MOSFET
Sense Resistor
Capacitor
Diode
MANUFACTURER
PART
Sumida
Coilcraft
Coiltronics
Internal Rectifier
Fairchild
Vishay-Siliconix
Dale
IRC
AVX
Sprague
Motorola
Nihon
Central
Semiconductor
CDRH127 series
D03316P series
UP2 series
IRF7309
FDS series
Si4435/6
WSL series
LR2010-01 series
TPS series,
TAJ series
595D series
1N5817–1N5822
NSQ03A04
CMSH series
The low-side switch N2 should also have a current rat-
ing of at least 3A, have an R_DS(ON) of 100mΩ or less,
and a total gate charge less than 10nC. N2 is used to
provide the starting charge to the BST capacitor C14.
During normal operation, the current is carried by
Schottky diode D2. Choose a 3A or higher Schottky
diode.
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 MOS-
FET with an RDS(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 1/2 of the current ripple (∆IL).
ISAT = 3A + 1/2 ∆IL
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 ∆IL
with the following equation:
∆IL = 16Vµs / 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.
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, respectively. 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
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